S64 a 4 ’ ala he a ls heh AO NPL PP DD, M4 Pea Yala be Ea ‘ ulcer pitsas a} MON 4 i} HHA RUNS Cah} NMRA R YONA SAAN AHEM TARA SDHC VRAIN Minh ic hnl SSH NGA IRAN H MET HRI Te OT AMWHANRI HT PO A DN weet 4 ih ie i Fe! } J Way) i ati) ath Hh) iy ‘) a iy Nh, atk va SSS S2etsey SS nen y =S= He Mh ai atest ait ) hi AM Win AS, Ni uy ; Varna ity y HG by ps i} iA Wie iyi NY ih | ¥ i uy OCH We NMAC NEN Pata ay MOR AP WiaTe ye al INNO ep ea hate Rie Wh vai utah i) ' Ay ENS, aN a Weed CORPO acon CaO ARSE RANGA ai as Dae Ra ; MMH Rott AE AM Dh AANA Wat ay ao ih), eh) Ms : Vial ‘y Sa er oe eee ese pero ens Ss See ea eh eniy) ‘ts iii iit ss pre eS = a aac aes stSe= ae St eee ae oe Seo =< Fee Tee Se st RAN : Ai i 4 ih ( << =. ee SES ae es —— at OO 7 Ht i} ' ee Se ee ee * a8 a gs sae a Se - Se Vine ARNG ey NAW} = Sega E= Sapa ee ees ah tar = aol ae, —— a ee a ao ree = vy hy) q { Aa ss ay iy sh Aah | it f SF et ; Py i Sa RSs Mi ay! ih) i nN Ni fond Hn an ne AH ARS RUAN NG RMA UNL bli URNA Eis ee ene repo ue SOE Bn ae Se SPA, at Tt ZF Se Ss =o = a SS iste stee SIGS , Rava \ i it ¥ oS soe - = sae Ss ae Sate a SIe5 poh we ies “25 ae etree Ky: H De Nl NR aliens iis bib i] ain vet NG | i I pe = 5 ak See nen Ss eet hatter NG HANOI SE eats Se pa ea a tee ae —— pee a ene — Sa Sa EEL I ae aS } i SS eS ae Fan ape = a2 : =e SS eH = = mee FS —_= SSS ea == eSe> ’ te Wala) aM) ERD} tei : bith Ht a0) HH 4 oy i) — TUM): oy Mi ty) at x, = = -= Se oe ih ohh! WN aoe SN <= Soe a eee =e =, Sahm a LD aS é = : = aS eo Fae a5 = Wyaeo Mra a: i, Natit hh ancy a ts aban i} = i i Mh { thy W) iN ai ANA { 7 aay) 3 2a Sas = — at ots, a2 ane eta : == eee SE eg See e == = : = = = = 2 SAE Sass : : = ——— = = a a = SSS aS eee = ate = = Aaa Wal chy YG ~ Ege erece: a Ny Ls ih nr v GH ‘ ; Sa eeru) BA A aT et iis rae =, ath Wot NAN aN i SAAN} a h) i, DAN FUSE SCC SE XS. ARABAAAAAAAR ARR RRRARAR — ARR | annapnnnnnn= PAAAA AANA al : - I - fam, —~'—}, ; 2 RRARAAARAAAAA | a |. PARRA AAA AAA AAA PNANAA ra nlamaae PR RAaa ea ~ ~~ '¢ ¢ C C € €€ C C ( ( 1 ( ( ( (( ( ( t AAA RREREAF AAA Oxnx~ \N\ ANON ne ANA { nen ae ee Ee AAA A . } i SSN Na ZS 7 net BULLETIN OF THE ILLINOIS STATE LABORATORY OF NATURAL HISTORY URBANA, ILLINOIS, U. S. A. Wet. VT, MAY, 1908 ARTICLE I. THE PLANKTON OF THE ILLINOIS RIVER, 1894-1899, witTH INTRODUCTORY NOTES UPON THE HYDROGRAPHY OF THE ILLINOIS RIVER AND ITS BASIN. PART II. CONSTITUENT ORGANISMS AND THEIR SEASONAL DISTRIBUTION. BY. Cie ROPOID Pm’. BULLETIN OF THE ILLINOIS STATE LABORATORY OF NATURAL HISTORY URBANA, ILuinors, VU. S. A. Wow. VII. ARTICLE I, THE PLANKTON OF THE ILLINOIS RIVER, 1894-1899, WITH INTRODUCTORY NOTES UPON THE HYDROGRAPHY OF THE ILLINOIS RIVER AND ITS BASIN. PART II. CONSTITUENT ORGANISMS AND THEIR SEASONAL DISTRIBUTION. By) CA KOrROED, Pu. THE ILLINOIS PRINTING COMPANY DANVILLE, ILLINOIS > A sae = ey = : re « re i ae ee _ Se run a CONTENTS. - 1 [ECAP DO RIGIDTOVE ER laser ee UA apace EAE, ca aac a Distribution of Collections by Months :(Table)..... 02.500 0.0....2%. Mal Seeteeeee Merete een ay! Sim eats eye to enh Os Sites Shows dh awe sd ERC RMOMPE OM CMM CI LSet apeere Aa lover aid tani cas nee Ms oc ale alae orcucw eke MB eet ss OLS ee aye tetera cas 2 MESO, Fac cd SIS 8 ee ee SG eas anda ons pire Composition«ol the Planktoniecas 4 $s... 25+ wi cede cis bueesee es Comparison of Fresh-water and Marine Plankton................... ARENAS GOSTOIE ISHN EZ Ez ee Orc Re eee oe ec Constituent Groups of the Annual Plankton of the Illinois River (Table) Discussion of the Statistical Data of the Species composing the Plankton OnernciiieipissRaver til S94 1899) aw. ca gd we wees eee shia eo PUNTO TION Ack aie he 5 Oe ee gee ee tee | BA NCUETR OSES 0s Ba eee: GPS se RO en Mn (SIE EMMA BI AS SEE eS od te eA PE a MPS UGS TONES DECLEST) xttaer A emo oe me's thd Ladl etl ott saa oS we OE Rei Moronpilny Coconuts feat og ch toce a cco eeciwi-cs sccc POOLS ont hen tad DS CHGS HOMOn DECI SHS acae Se See ais neces NAS Duclos Sle be Bo BS tcillariace cen ec. Ae eee ey ra RE NaS ce ad « aha oct ton Ue. Bactors controlling Diatom) Production: ......20¢.:.+ ds: sec. se Diagram showing the seasonal distribution of diatoms, total plankton, nitrates, and thermograph and hydrograph of IIli- HOISMNIMeD Ab lA ValA Tt Orel SOCK Mer kids cis mpcstuaies Alone «alee rises WISE HSI ONEOUNS PECIEG Se Cyt oc mne Seine bio nviaht At sts oe le earn RE coe os Ciscoe ee eas ee cht at Alaa ale Gs AD we ie Ak nee Bae 3 Sees IOUsOn. SMECIEGIE, (eae rere eh eRe Te Ae oa ly | DISS rAVEs ROE NAAUIE 3 ee a 1 eee cd ep ee a oe J PUROL REE AO LEE Need ase mere em Senay ORC RT eae na eS a a MMs GIO po Oars Ne ere hey ara aa eens Pas ee Ua Ree cob dooce whe cies Be Location and Amplitude of Pulses of Chlorophyll-bearing Organ- ism Shiiet Herd linGis: waver (Chaplet g cc.a4 ce ees» couees ac ee ee DWASCUSSIONLOL SPECIES T Acer en se ai oe ee ce cio hae Be JE NATO OLSL6 2, ec se Rae ee Se He en me Pm Pee PC CUSHIOE OMS PECICS yess Lark otss Fh ues ar rn Vlora Ue aise Pe eco arpa eriste ele Shae i hk & eugene os Neo ehatiars dinate Syd SSE HOS CUS SOMO by OPE CICS. 1 ae OR alee eerie Sate Sante SES ce SYOICTO AOE S05 Nester ee Bes RPS Pee eM ae Ra Gee \ OBIE SS Biche te ane ord ele ee cA ee AR Se Sa a DASCHSSIONNO lS PECIOS a ney ena iel a Sug. ella a, exe Goh Cs Res SEP iets SBS HOT EIEN Sg Sts ch RT ORS EC eae a A Mic eUSStOiy OP ODECIES yao Matalin ae nasa os Secs tee hour ee LP OSRERESER. hate Wig i eee tho aS RC RO ae aa 34 35-43 (OG =) (Sra ney): eee ee RAD Recor Mr eR URS FG Ee eR Up a aie a a Bio Sco. ¢ Platyhelminthes: sic acide soe eee aie eee eee ee MMambella ria <.eec.2 fos Ake Gene eae et ed ce Oe ee ee AWie’ss a oF 6-2 Rae a ote or reer Pai eee a a OE TS SAS aeons wt Cestodai orarsergens eae epee es ANI chee ae tote ua ca alata aioe INGIM ETAT Saar c oe SNAG eee Ci Seo seo CSI eee INematelmaimGhes 83 collections. The optimum temperatures plainly lie near the max1- mum, that is, about 80°, and the season of growth approximates that of the land flora, being limited to the months of April-Septem- ber. This isa variable organism, and a number of species have been described in the genus in recent years. Many of these occur in our waters, but no attempt has been made to separate them, since they are based on minute characters. Mallomonas plossli Perty. and M. producta Zach.—These two forms will be treated together, as in my opinion they are merely divergent variants—perhaps seasonal—of a single species. In 1898 M. plossli was found but three times—in June and July—and M. producta eight times—from May through September. In 1897 the latter only was recorded, and in September and October. In 1896 M. plossli appeared in July and M. producta in April and August. In 1895 M. producta alone was recorded, and that in November. The data are hardly sufficient for generalization, but so far as they go they indicate that producta is more prevalent in late summer and autumn and flossli in early summer, the more attenuate form (producta) in the warmer season. Butschh (80-’89) has intimated that there may be some genetic connection between Mallomonas and Synura_ uvella. Certain features of its occurrence in our plankton lend their support to this view. Synura in our waters is a winter planktont, with December and February or March pulses. Mallon:onas is a summer planktont, making its first appearance during the time of the decline of Synura, and when many of the colonies of the latter are breaking up into their individual zodids. Again, the differences-in structure and size between the two genera are quite superficial, and might result from the growth attending the free life of a Synura zoéid and its prepara- tion for sporulation. It is a noticeable phenomenon that the pro- portion of sporulating individuals of Mallomonas in the plankton is exceptionally large among all plankton organisms. “Free cells”’ of Synura are plainly referable to that genus by their resemblance, and by the fact that they are often united in clusters of several ind1- viduals forming fragments of disintegrating colonies. It may be that some reproductive phase, as conjugation, intervenes between the free-cell condition of Synura and the Mallomonas stage, and that the relatively smaller numbers of the latter are due to the in- frequency of this process. While the features of seasonal distribu- Gi) 84 tion, structure, and sporulation thus suggest the possibility that Mallomonas is a free zodid stage leading to sporulation in Synura, they do not demonstrate it, and the genera must stand im statu quo until breeding experiments shall clearly demonstrate the full life- cycle of Synura. Pandorina morum Bory.—Average number, 6,957. In 1898 this organism was about half as abundant as Eudorina, but in 1897 it more than equals it. On account of the small size and the motility of the colonies many of them escape through the silk, so that it is not so adequately represented in silk-net collections as Eudorina. It is probably the most important quantitatively of the Volvocide in our plankton. It occurs from April to October, with a few spo- radic appearances in March and up to January. Its greatest growth occurs from May to October. There is no predominant vernal pulse in 1898, but a series of smaller ones culminating May 3 (48,400), June 14 (60,000), July 26 (63,200), and August 30 (3,200),—all upon declining floods (Pt. I., Pl. XII.) and coincident with pulses of other Volvocide—Eudorina and Gonium. In 1897 its seasonal dis- tribution was also similar to that of these genera, exhibiting a max- imum pulse August 31 of 638,000 at 80°. In 1896, a year of inter- rupted hydrograph (Pt. I., Pl. X.), Pandorina attained no marked development. Its optimum temperatures lie at and above 60°, and its larger pulses appear during the season of maximum temper- ature, that is, at about 80°. Pandorina does not attain any marked autumnal growth, but declines in September, and as a rule dis- appears in October. The period of its growth thus lies within that of the land flora. As in Eudorina, so also here, parasitism by Dangeardia mammul- lata is of frequent occurrence. Pandorina is an important element in the food of summer rotifers such as Brachtonus. Peridinium tabulatum Ehrbg.*—Average number, 3,875,769; silk, 3,711. This is a perennial planktont, having been found in every month of the year. Its principal development is, however, reached during warmer months, from May till September. In 1897 the maximum pulse of 172,800 was on August 10,and in 1898 one of 66,800 fell on July 26, thetemperatures being 81° and 89° respectively. The only exception to this predominance in warm months is an 1so- lated pulse of 2,400 which developed on the declining flood of Febru- ary, 1899 (Pt.I., Pl. XIII). Theabsenceofany autumnal development a a 85 of this species is noticeable. -Its optimum temperatures lie close to the summer maximum (80°), and though perennial, its occurrences at other seasons than late spring and summer are irregular and its numbers few. Its seasonal distribution in German lakes, as re- ported by Apstein (96), is similar to that in the Illinois River. The Peridinide play but an insignificant part in the plankton of the Illinois River. Phacus longitcaudus Ehrbg.*—Average number, 61,153; silk, 3,031. This species in 1898 made its first appearance in the planktonon March 23 and continued till November 15. The species is small enough to escape through the silk net, and the data from such collections do not fully express its seasonal fluctuations. There is no marked vernal pulse, and there are traces of but a few small ones during the summer, the largest in 1898 being one of 35,200 on September 27. The distribution in previous years is much the same. A well-sus- tained development throughout the warmer months—save when rising floods, as that of May, 1898, reduce the numbers—indicates that the optimum temperature for the species approaches the summer maximum (80°). There are almost no occurrences below 45°. This is the most abundant member of the genus 1n our plank- ton, but it is not quantitatively an important element therein. Phacus pleuronectes Nitzsch.*—Average number, 450,000; silk, 298. It is less abundant (from one fifth to one tenth) than P. longicauda in the catches of the silk net but apparently much more abundant in the filter-paper collections, which may be due in part to its smaller size and greater tendency to escape through the silk in the collections of the net. Its occurrences are even more closely limited to summer months—from June till September. There is no vernal development, and the largest numbers occur during the period of maximum heat. Pulses are but feebly defined. It is also a summer planktont. Phacus pyrum Ehrbg. was found but once—on August 10, 1897. Phacus triqueter Ehrbg. occurred in small numbers during July and August, 1897. Platydorina caudata Kofoid.—Average number, 17. In 1898 this interesting new genus of the Volvocide was found in the plank- ton only in the latter part of July. In 1897 it was much more abun- dant (average number, 21,963) and ranged from July 14 to October 86 12. There was a pulse on July 21 of 18,400 and another on Septem- ber 7 of 600,000. In previous years the occurrences were scattering, but confined to July, August, and early September. It is evidently a summer planktont, whose optimum temperature lies near the max- imum attained by our waters. No record of occurrence below 60° was made. The smaller and younger colonies escape readily through the silk net. Its pulses in 1897 coincide very closely with those of Gonium, Pandorina, Eudorina, and Pleodortina. Pleodorina californica Shaw.—Average number, 11. In 1897 this species, in common with other members of the family, was much more abundant than in any other year of our work, stable conditions of low water with the accompanying sewage contamination seeming to favor its development. The earliest record for P. californica in the plankton is May 18, 1896, at 71°. This was a year of lower water and higher temperatures than usual in spring months (Pt. L, Pl. X.). In other years P. californica did not appear until June or July. It continues into September, the latest record in 1895 being October 2. In 1897 there were pulses on July 21 (5,600) and September 7 (4,000). The occurrences at other seasons are too scattered to trace the seasonal fluctuations, but there is a well-de- fined predominance during the period of maximum heat. This is evidently a summer planktont, whose optimum temperature les near 80°. Pleodorina illinoisensis Kofoid.—Average number, 6,917 in 1897. This is somewhat more numerous than the preceding species, and its range of occurrences is quite similar. Its maximum pulse in 1897 (180,000) is on August 31, a week earlier than in other members of the family. These pulses of the Volvocide occur (Pt. I., Pl. XLIV.) in a depression of nitrates and just prior to the volumetric pulse of September, 1897. This pulse is doubtless built up partly at their expense. Their decline in numbers corresponds with its rise. This is also a summer planktont, and was not recorded below 71°. Salpingeca brunnea Stokes.*—This species was not recorded in 1898. Average number in 1897, 1,887,356. It occurred on May 25 and July 21, dates of culmination of pulses of Melosira granulata var. spinosa. In August-September a pulse occurs, culminating September 7 at 47,250,000—a week after the culmination of a Mel- osira pulse. In 1896 (silk collections only) it was present through- eee 87 out most of the summer, attending only approximately the sup- pressed and interrupted pulses of Melosira in that year of disturbed hydrograph. It has been recorded from the latter part of April till the middle of September, and, as a rule, above 60°. This beau- tiful little choanoflagellate is sessile upon the filaments of Melostra, principally upon the variety spinosa, and but rarely upon MW. varians or other planktonts such as Pediastrum. It is often associated with Bicoseca lacustris and is usually found upon the sides of the filaments, the bowl of the transparent brownish lorica being closely sessile upon the diatom. In one instance a lorica was found upon the corner at the end of the filament. The lorica had adapted itself to this novel situation by an angular indentation fitted upon the corner of the diatom. Syncrypta voluox Ehrbg.—Average number, 625. This species” has a definite and somewhat unusual seasonal distribution. In 1898 it was found from March 1 to April 12, and reappeared Novem- ber 8, attaining a maximum of 13,500 on December 6, and of 43,000 on January 1, declining then to 800 and rising on February 14 to 4,800, and subsequently disappearing in the flood waters of March. It was not recorded 1n 1897. In1895 it appeared September 27 and continued for a month, reappearing in February and March, and not occurring after April 10. It has attained its largest develop- ment at minimum temperatures under the ice—43,000 January 3, 1899, at 32.7°. The greater part of its occurrences in 1898-1899 le very near this temperature, and but three in all the years lie above 50°. It is par excellence a winter planktont, or at least a cold-water one. Its occurrences in 1895-1896 lie near the beginning and the close of the seasonal pulse of Synura. In 1898-1899 the pulses of Syncrypta coincide in location with or immediately follow those of Synura. The resemblance of Synerypta to small colonies of Synura is striking, and this fact combined with the relation of their seasonal fluctuations raises the query if Syucrypta may not be an encysting stage of the Synura colony. Its life history should be fully worked out. Synura uvella Ehrbg.—Average number of colonies, 8,463. The seasonal distribution of this chrysomonad flagellate is some- What similar to that of its near relative Syncrypta. It is a perennial, though predominantly cold-water, planktont. It appears 88 in the December plankton of 1894, but was exterminated from the channel plankton taken in the following February by the stagna- tion attending the long-continued ice blockade. It reappears in April, and again disappears promptly, but does not return until September 12, and not in numbers until October. There are pulses November 20 (506,800) at 42.8°, and December 30 (362,520) at 36.5°. The December pulse is followed by a decline, with a rise during February to a well-sustained maximum during March, approaching 400,000, and at from 35° to 48°. The decline follows in April, and there are only isolated occurrences 1n small numbers at irregular intervals during the summer. Continuous occurrence begins again in September, and numbers rise rapidly in October. There is a pulse of 542,699 on December 3 at 32.2°, and another on March 22, 1897, of 159,500 at 43.8°. Symnura is very rare indeed in the summer of 1897, and in the prolonged low water, sewage contamination, and higher temperatures of the unusual autumn of that year it does not reappear continuously until October 26, at 59°, and does not exceed 1,000 until December 7, at 32°. There is a low maximum of 98,700 on December 14 at 36°, followed by a decline during the rising flood of January—March, 1898. The slight cessations in the flood invasion (Pt. I., Pl. XII.) in January and in the second weeks of February and March produce prompt responses in immediate rise in numbers in Synura. Finally, a low maximum of 320,600 is attained upon the crest of the March flood, on the 29th, at 49°. This is followed’ by a decline during April and a few scattered appearances during the summer. Synura returns at the end of October and rapidly mounts to a pulse of 1,999,500 on November 29 at 35° with the first decline of the November overflow (Pt. L., Pl. XII.). A second pulse of 2,764,800 on December 20 at 33°, under the ice, gives way to a decline to 51,600 towards the end of January, 1899, during rising water. On February 14 another pulse (348,800) appears at 32.5°, under heavy ice, and declines again in the sudden flood of the last days of February, but recovers quickly with a maximum pulse of 898,800 on March 7 at 32.8°. Within a fortnight this falls to the low level of 9,800, but its further history was not followed. From these data it is evident that in our waters at least Synura is limited to the months from October to April, except isolated and irregular occurrences of small numbers during the summer. Its 89 optimum temperatures lie below 50°, and its greatest development has taken place in minimum temperatures under the ice. Rising floods and disturbed hydrographic conditions tend to reduce its numbers or- to suppress its development, while declining floods initiate increase in numbers and favor the appearance of pulses. A “late” autumn delays the appearance of Synura. Not only are colonies of Synura found in the collections, but at times large numbers of free cells make their appearance. These are released by the breaking up of colonies,and occur in all degrees of isolation. It seems to be a natural phenomenon, and occurs most abundantly with or immediately after the crest of the pulse. Thus the pulse of December 29 (1,999,500 colonies) was attended by 21,600,000* free cells on that date. A week later there were 1,693,500 colonies and 57,600,000 free cells. There are in the rec- ords several instances of meteoric increases of free cells at other times than at those of apparent pulses. It does not seem possible from the data at hand to determine whether this is due to environ- mental influences or to the accidents of collection and subsequent handling. In the discussions of Mallomonas and Syucrypta, sugges- tions have been made that these organisms may be stages in the lite cycle of Synura. Synura is the largest and by far the most important synthetic organism of the winter plankton. It shares appreciably in the winter volumetric pulses—as, for example, those of December, 1898 (Pt. -1., Pl. XIT.). Its fluctuations do not seem to produce any marked effect upon the nitrates, possibly because the latter are present 1n excess of the needs of Synura. In the winter of 1898 nitrates are high, 1.25 parts per million with the pulse of 1,999,500 colonies on November 29, but decline rapidly to .1 on December 13 with a fall of Synura to 78,000. On December 20,Syunura rises to 2,764,800, but the ni- trates rise only to .35. It is evident that the nitrates are not the only factor regulating the fluctuations of Synura. Marsson (00) reports Synura as abundant in the winter plankton of lakes about Berlin, and Brunnthaler (’00) finds it in the winter plankton of the Danube. There is, however, no recorded instance in which Synura forms so prominent a part of the plankton of a body of water as it does of that of the Illinois River. It may be that a closer analysis than has yet been given the potamoplankton of other streams will reveal its prominence there also. It is present (Kofoid 90 05) in the summer plankton of the Great Lakes at temperatures 15° to 20° below the summer maximum of the Illinois River. Trachelomonas acuminata Schmarda.*—Average number, 1,094,- 615; silk, 873. This species appears in the plankton in April or May and continues into October or November. There is no vernal pulse, and the data are too irregular to trace the seasonal fluctuations. The greater numbers occur during the period of maximum heat. Excepting a single occurrence in February, this species has been found only above 40°, and its period of continuous appearance from May to October lies above 60°. It is evidently a summer plank- LOC. Trachelomonas hispida Stein.*—Average number, 1,002,115; silk, 1,251. This is a perennial organism, found in every month of the year but in larger numbers during the warmer months. It was more abundant than usual in the winter of 1897—98 following the low water and unusual development of the previous fall. There are no large pulses in 1898, but in 1897 there is indication of a vernal max- imum on April 27 and an autumnal one of 85,500,000 on September 7. The data are too irregular to trace the seasonal fluctuations in detail. There is no doubt, however, from the evidence at hand that this is a predominantly warm-water planktont similar to the other members of the genus. Frachelomonas volvocina Ehrbg.*—Average number, 17,672,692; silk, 7,162. This is the most abundant species of the genus and is found throughout the year in almost every collection. It is most abundant from May to October, during the period of maximum heat. There are no well-defined vernal or autumnal pulses, but recurrent maxima during the summer are to be found in both 1897 and 1898. There are four such pulses in the former year, and in the latter five, as follows: May 17 at 64° (14,400,000), June 21 at 77° (147,600,000), July 19 at 84° (86,400,000), August 9 at 83° (252,000,000), and- October 4 at 71.5° (11,700,000). The periods of greatest growth thus le above 60° and the optimum is near 80°. None of these pulses coincides with a volumetric maximum of the silk:net catches (Pt, de (Ply Sane) ney, usually follow these maxima at intervals of one or two weeks— a phenomenon often observed in other synthetic species. It may be explained by the decrease in animals which feed upon the organ- isms in question. These volumetric pulses are predominantly ot animal in their composition, and when they decline the organisms upon which the disappearing animals were feeding have an oppor- tunity to multiply with less decimation in their ranks. This species is one of the most abundant of the synthetic organ- isms in the summer plankton, and next to Euglena is the foremost among the synthetic elements of the food cycle of the plankton. The presence of many hght-colored or even colorless forms (forma hyalina K1.) justifies the suspicion that members of this genus, like those of its near relative Euglena, adopt holozoic nutrition in the presence of abundant organic matter suitable for food. This species, as well as the others above listed, is exceedingly variable in the proportions of the lorica, in its color, and in the development of the neck. It is very desirable that its life history and the full limits of its variation be determined before many more new species are proposed in the genus. In addition to the forms above listed,the following have been noted as present in small numbers in the summer plankton, viz.: T. armata Ehrbg., T. caudata Ehrbg., T. torta Stokes, T. urceolata Stokes, and T. volvocina var. rugulosa Kl. Uroglena americana Calkins.—This species was found in small numbers in July and September, 1897, and in January, 1899. Uroglena radiata Calkins.—This species was found in January, 1896; in April and May, 1897; and in March and April, 1898. There was a vernal pulse of 15,279 on April 29, 1896. Uroglena volvox Ehrbg.—This species was found sparingly in the spring plankton in 1896. Uroglena is one of the few organisms which the usual method of plankton collection and preservation fails to keep in fair condition for subsequent indentification. The gelatinous matrix is easily crushed, and debris adheres to it so as to obscure it beyond recognition. Judging from the frequency of Uroglena in the living plankton it is very probable that the genus is much more abundantly represented in the Illinois River than the data at hand indicate. The genus seems to prefer the cooler waters of autumn and spring to those of midsummer. Volvox aureus Ehrbg.—This species was found from March to August, but in small numbers and irregularly. Volvox globator L.—This was somewhat more abundant than the previous species, and was found more frequently, especially during 92 1895 and 1896. It occurred from the first of May till the end of August, but always in small numbers. It 1s occasionally abundant in backwaters where there is much vegetation. In addition to the Mastigophora above listed there were many individuals belonging to unidentifiedspecies. They were asarule the smaller forms, which are not readily identified in preserved material and under the conditions of plankton enumeration. They consti- tute about twenty-six per cent. of the total Mastigophora enu- merated. In silty planktons their number is relatively somewhat larger on account of the difficulties attending the determination of species in such material. These unidentified flagellates occur in every collection, and are somewhat more abundant in the summer months. RHIZOPODA. Average number, 55,364, including filter-paper collections; 23,826 without them. This group of Protozoa is numerically of less importance than the ciliates or flagellates, but its quantitative significance is greater than the numbers of individuals indicate. This is due to the relatively large size of the R/iuzopoda, and also to the fact that plankton collections afford only an irregular and in- complete record of the rhizopodan fauna of any body of water, and give but an imperfect idea of the part which these organisms play in the total economy of the lake or stream. This results from the fact that they are as a rule largely bottom or shore-loving species, and are generally either adventitious or temporary constituents of the plankton. The seasonal distribution of the total RKhizopoda in the Illinois River gives evidence of the adventitious or temporary nature of the contributions of the group to the plankton. There are pulses in 1898 on January 25 (66,388), February 22 (141,524), August 23 (36,800), September 27 (59,200), and November 15 (42,000), all of which appear on rising water and are largely adventitious, their presence in the plankton being due to the disturbances of currents, waves, and the like. There are pulses on May 10 (49,800), June 28 (37,000), and July 19 (28,800) which cannot be traced to any general hydrographic condition. These, as will be suggested in the discussion of the seasonal fluctuations of individual species, are probably due to the temporary adoption of a limnetic habit on the Sam = < oe) part of some of the rhizopods, or to the appearance of limnetic forms, varieties, or species—according to the systematic value placed upon these eulimnetic individuals. I am inclined myself to regard them as seasonal forms of species which are predominantly of the bottom or littoral fauna, which have multiplied rapidly under the stimulus of abundant food. Owing to this fact, to the storage in their tissues of the products of metabolism, such as gas and oil vacuoles which tend to lighten their specific gravity, and to the frailer structure of their shells under conditions of rapid multiplication, they abandon their customary benthal or littoral habitat and assume temporarily a limnetic distribution in the plankton where they con- tinue to find abundant food. Their appearance here under ‘these circumstances is a result of their physiological condition, and with its cessation they decline, as shown by their pulse-like occurrences. Whatever the systematic valuation placed upon these limnetic forms may be, there is no doubt of their occurrence. They have appeared in every year of our operations, but were most prevalent in 1897, a year of most stable conditions, and also in the quieter backwaters, and on the declining spring flood or June rise when hydro- graphic conditions are less catastrophic than those of early flood stages. In 1897 there was a pulse of 68,400 (silk-net only) on August 8 and another of 1,268,400 on September 7, both in stable conditions and almost exclusively of limnetic types, differing in this respect from the pulse of 141,524 on February 22, 1898, which was pre- dominantly of an adventitious character, resulting from the flood @ecnat period (Pt. I:, Pl. XII.). The contrast in.the numbers of Rhizopoda in the plankton during warm and cold seasons of the year is very striking in 1897. The average per m*, per collection from May 1 to October 1, that is, above 60°, is 161,045, omitting all filter-paper collections, while in the seven months of lower tempera- tures this average is only 4,771. During the warmer period the June rise was the only hydrographic disturbance (Pt. I., Pl. XI.) to which any adventitious increase might be attributed. This con- trast is less evident in 1898, when the summer hydrograph was more disturbed. These larger numbers during warmer months may be attributed in part to the greater numbers of the Riizopoda in their littoral habitat, and in part, doubtless, to the fact that at low water the shore and bottom fauna are brought into more intimate relation with the plankton, and in the river the disturbance of these regions 94 by current, waves, seines, boats, and fish make relatively larger contributions at low-water stages to the diversification of the plankton. In addition to these factors, however, there is abun- dant indication that many individuals assume during the warmer months a eulimnetic habit, and that some of the Rhizopoda become, for the time being at least, typical, though temporary, planktonts. It naturally follows that in so far as the plankton is concerned, the Rhizopoda exhibit a seasonal preference for the warmer months above 60°. Maximum numbers were attained only at the higher temperatures save in those instances where they attend winter floods. Ina measure the seasonal distribution of the Rhizopoda in the plankton reflects that of the group in its normal habitat; but at the best the picture is incomplete. The Rhizopoda have important relations in the economy of the plankton. They feed upon diatoms, desmids, the smaller alge, and even the chlorophyll-bearing Mastigophora such as Trachelomonas and Carteria. Their occurrences in the plankton do not exhibit any striking correlation with those of the groups named. The great pulse of September 7, 1897, for example (Pl. II.), lies in a depression of the diatoms and coincides with pulses of Chlorophycee and Mastigophora, and that of August 10 (68,400) exhibits a similar relation, the diatoms rising the following week as the Riizopoda fall. In 1898 the pulse of Riizopoda on June 28 of 37,000 (Table I.) culminates a fortnight after that of the diatoms and Chlorophycee and a week after that of the Mastigophora. It thus 1s intercalated between the June and July pulses of these chlorophyll-bearing organisms (PI. II.). The Rhizopoda pulse of July 19 (28,800), on the other hand, occurs with the coincident pulses of the three groups named (Pl. IJ.). The immediate diluent effect of flood waters upon the plankton combined with their tendency to increase the number of adventitious Rhizopoda results at times in the intercalation of their pulses with those of the chlorophyll-bearing organisms whose relative numbers are reduced by the dilution. The data evidently do not afford any adequate solution of the inter- calations of the Riizopoda with other organisms. The Rhizopoda are very frequently found in the digestive tract of limnetic rotifers, but I have never noted the Entomostraca feed- ing upon them. They are important elements in the food of young a ee 95 fish (Forbes, ’80) such as the Catostomide and some of the Szlu- ride and minnows. I have found them in great abundance in the _ intestine of the adult gizzard-shad (Dorosoma), and in the contents of the digestive tract of the German carp (Cyprinus carpio). In the pages which follow, the seasonal distribution, or occur- rence in the plankton, of thirty-one K/uzopoda is discussed, and the presence in the plankton of the Illinois of twenty-eight other rhizopodan forms which have been recognized by other writers as of specific rank is noted. This by no means exhausts the rhizopo- dan fauna of the environment which was the field of this investiga- tion. A continued study of the plankton itself would doubtless greatly extend the list of adventitious forms from the shore and bottom, and a more careful analysis of the variants, especially in the Dzifflugia globulosa-lobostoma group, would still further increase the richness of the fauna from the systematic point of view. Hem- pel (99) lists sixteen species from this locality, and Penard (’02), in discussion, remarks: “Une pareille pauvreté dans une région riche en organismes de toute nature, est une impossibilitié maté- mele; However, neither Hempel’s paper nor the present one pretends to give a full account of all the RKiizopoda of the region. He dealt largely with plankten collections, and the present paper deals with them exclusively. There is but little in plankton literature which gives with any fulness the seasonal distribution of the Rhizopoda, or indicates that they are of any considerable importance in the economy of the plankton. The importance which they acquired in the plankton of the Illinois is no doubt in part due to the nature of the environ- ment with which we are dealing. The somewhat sporadic and meteoric character of their appearances in our waters leads to the inference that full seasonal analyses of the plankton of other bodies of water at brief intervals may reveal a greater prevalence of the Rhizopoda in the plankton than has hitherto been detected. DISCUSSION OF SPECIES OF RHIZOPODA. Ameba limax Duj.—This was frequently abundant in the water- bloom of midsummer, but was not identified in the plankton collections. Ameba proteus Rosel.—Average number, 342. The individuals here assigned to A. proteus include those taken in our plank- 96 ton which belong to the type of A. radiosa Ehrbg., a type which presents no distinctions sufficiently well-defined to separate it spe- cifically from the first-named form. It seems probable that A. radiosa includes small individuals of A. proteus which are not, at the time of observation, creeping upon a substratum; that is, they are limnetic, floating free with filamentous pseudopodia character- istic of that condition. Verworn (’97) has shown that A. proteus takes the radiosa form in weakly alkaline solutions. Pond water rich in algee may have an alkaline reaction (Knauthe, 98) 1n bright sunlight. Larger individuals, distinctly referable to the A. proteus type when taken in the plankton, possess at times the slender pseu- dopodia of the A. radiosa type as well as the blunter ones charac- teristic of the A. proteus form. I see no valid reason for separating the two as distinct species. Most of the Ameba recorded from the plankton collections belong to the A. proteus type, the smaller ones belonging to the radiosa type probably escaping through the meshes of the silk net. This species was found in 30 of the 180 collections examined, being observed in all months of the year except May, November, and December. The conditions attending its occurrence suggest that it is not, habitually at least, an active planktont at all seasons of its occurrence, but rather a tycholimnetic member, an invader from the littoral or bottom fauna, or a temporary accession during the warmer months. In the first place, both the number of occur- rences and the numbers of individuals found are small, and the seasonal distribution, plotted from the data of the collections of the five years, is exceedingly irregular. Furthermore, 17 of the 30 occurrences happened on rising floods, when the fauna of the bot- tom and shore of both the river and its tributaries is most mingled with the plankton. Further evidence of the agency of floods in introducing Ameba into the plankton is brought to light by a com- parison of its occurrences in 1897 and 1898. As shown by Plates XI. and XII., Part I., the hydrograph of 1897 is much less irregular than that of 1898, the latter year exhibiting repeated fluctuations in level due to floods. As a result we find Ameba occurring rela- tively (to the number of collections) almost twice as often in 1898 as it did in 1897. It may also be significant that Ameba was not found in November and December, months of unusual stability in river levels. There is, however, a suggestion in the data of distri- Srvc _ ww 97 bution (see Table I.) that Ameba may become an active member of the plankton during the warmer seasons, like other Riizopoda, as a result, perhaps, of the formation of gas or oil vacuoles in its proto- plasm. Of the 30 occurrences, 21 fall between April 19 and Octo- ber 17, with water temperatures of 58° and 56°, respectively. Of these 21 occurrences in warm waters but 8 accompany flood inva- sions, while all of the 9 occurrences during the colder months are in connection with such disturbances. Finally, the maximum num- ber per cubic meter (6,400) was found July 21 1in clear waters, free from the debris of flood invasion. In conclusion, it seems probable that Ameba in warmer seasons of the year (above 56°) may adopt a limnetic habit. There is, however, the possibility that local and minor disturbances of the water due to current, waves, etc., are the occasion of its presence 1n the plankton in the absence of flood conditions. Jennings (00a) reports both A. proteus and A. radiosa in the open water of Lake Erie. The range of temperature of river water in which Ameba was found was from 32° to 89°—the full extremes observed by us in the river at Havana. The temperature at the maximum occur- rence, July 21, 1897, was 82°. It is perhaps significant that 14 of the 30 occurrences of Ameba were between June 21 and Sep- tember 6, the period of maximum heat, the river averaging almost 80°—apparently the optimum temperature for the occur- rence of -Ame@ba in the plankton in this locality. The relative numbers of individuals found in the various collections of the five years are too irregular to suggest any conclusions as to a seasonal Bey cle. Ameba verrucosa Ehrbg.—Average number, 19. This species was found but three times in the plankton, once each in May, August, and September, occurring but singly, and in each case in flood waters. It is apparently a tycholimnetic member of the plank- ton. The temperature limits of its recorded occurrence in the plankton were 58° and 82° respectively. Arcella. This genus is represented in the plankton by four species and two varieties which, like most of the Rhizopoda, are exceedingly variable, grading in some instances into each other by occasional 98 individuals which present intermediate characters. The majority of the individuals were taken in a living condition, though many empty shells were found. The conditions of the examination of the plankton and the opacity of many of the shells made it impossible to distinguish the dead shells in all cases. The records include many dead shells. Arcella costata Ehrbg.—Average number, 48. For the purposes of this paper I have included here all those individuals which possess an angular or ribbed shell. Leidy (79) refers such forms to: A. vulgaris. Individuals of this type are rare, occurring infrequently and in small numbers. It was recorded but 18 times in the 180 collections, and the largest number per cubic meter was only 1,187. As in the other species of the genus, the warmer months are favored, fourteen occurrences falling in June-September in water at 70° or above. The other four records are one each in April; October November, and December. The seasonal range of this form in the plankton thus falls in the main within the period of the maximum abundance of A. vulgaris, of which species it may be but a variant. Arcella discoides Ehrbg.—Average number, 972. This prevalent Species is not in all instances easily separated from A. vulgaris. Indeed, even Leidy (79) states that it graduates into A. vulgaris, and that he views it as the variety of this species 1n ‘‘which the shell presents a greater proportionate reduction 1n height compared with the breadth.’ In the enumeration of our plankton catches, the larger, flatter, and unornamented individuals have been referred to this species. Both the brownish and the hyaline forms should probably, for reasons hereafter given, be included here, and they are so grouped in the present discussion. Thus considered, A. discoides is the most abundant member of the river plankton be- longing to this genus, including two thirds of all the individuals observed. This species occurred in almost two thirds of the collections, hav- ing been. recorded in-115-of the 180, and more frequently and im larger numbers in the latter half of the five years than it was in the earlier period. This is in part explained by the unusual fluctua- tions of the river levels in 1898, during the maximum summer occurrence of the species. Like the other species of the genus, A. discoides has a period of maximum occurrence in the latter part of summer, as is Shown in Table I. Of the 115 occurrences, 55 were in Bri 99 June—-September, in water at or above 70°, while in the remaining eight months there were but 60 occurrences. This contrast is heightened by the ratio of occurrences to the total number of collec- tions, which in the period from June to September inclusive is 55 to 68 and in the remainder of the year only 60 to 112. The num- ber per cubic meter is also higher during this warm period, averag- ing for a single occurrence 1,376 to 1,028 for one in the remainder of the year. The average for the colder months falls to 850 if the large accessions attending the floods of February and November are omitted in the totals. The same causes efficient in determining the summer maximum in other K/izopoda of the plankton are doubtless operative here, and as in A. vulgaris the impetus of the summer in- crease is carried over into the autumn, causing a slight increase in numbers as compared with the numbers at corresponding temper- atures in the spring months. It seems probable that high temper- atures favor its occurrence in the plankton, not, however, directly, but because of greater abundance of food under those conditions, greater metabolism, and the storage of the products as oil or gas vacuoles which tend to lower the specific gravity and thus to bring the animal into the plankton. The adventitious occurrence of A. discotdes in the plankton is shown by the fact that 45 of the 115 occurrences are with rising flood waters. The greater part of them lie in the colder months; in fact, nine tenths of the occurrences between October and May are correlated with flood movements. For reasons above given, how- ever, A. discoides may be regarded as temporarily adopting a lim- netic habit during warm months as’a result of its physiological condition; at least many individuals of the species exhibit this habit during the warmer months. The data do not indicate that the open water is at any time the center of distribution of the species. There are no indications of recurrent pulses in the species and, as might be expected in case of adventitious planktonts, but little evidence of a characteristic seasonal distribution. There is some evidence that the summer is the period of most active multiplica- tion, and that an exceedingly transparent and hyaline form other- wise resembling A. discoides is the young of this species. In 1898 separate records were kept of the two types with the result that they were about equally abundant—24,159 and 26,387 for the brown and hyaline types respectively. (8) 100 With but few exceptions the seasonal distribution exhibited by the hyaline form was very similar in time and numbers to that of the brown form. Both occurred more frequently and in larger numbers in the warmer months, and irregularly and in small numbers in the colder waters. Both entered in larger numbers with flood waters. The differences though slight are suggestive. The hyaline form was less frequent than the brown both in occurrences and numbers dur- ing cold weather, and summer floods sometimes brought a rela- tively larger number of the hyaline type. These are conditions that might be expected if the latter is only the young (that is, the daughter organism occupying the new shell after fission of the oc- cupant of the old) of Arcella discoides. In warmer months food is more abundant and, presumably, fission more frequent. For this reason the young individuals abound at that time. Owing to the difference in the specific gravity of the two, the hyaline type 1s more readily transported by flood waters. Though not con- clusive, the data here presented seem to favor the view that the hyaline form is only a stage in the life history of the individual Arcella discoides. The species A. artocrea Leidy and A. polypora Penard occur also in our waters, but were included with A. dzscoides in the enumera- tion. Typical representatives of these species are not, however, present in any numbers Arcella mitrata Leidy was found but once—on Aug. 1, 1895, in small numbers, at 78.5°. Arcella stellata Perty.—Under this designation are included only those individuals which have well-defined prolongations on the margin of the shell. Only a single occurrence in small numbers (48 per cubic meter) was recorded for the typical A. stellata — July 29, 1895, at a temperatune ol 75-9 -. Arcella vulgaris Ehrbg.—Average number, 1,098. This species is somewhat more abundant than A. discoides, but occurred in fewer collections. It is a somewhat common planktont, whose seasonal distribution exhibits some irregularities attributable in part, as in the case of other members of the genus, to flood conditions. It was found in 61 of the 180 collections examined, and in approximately one third of those made in each year, excepting in 1894, when it was not recorded, and in 1898, in which year it was found in about DONS eee ey is: -— 101 half the collections, the river levels for this latter year being subject to more than the usual disturbance. Arcella vulgaris is found throughout the whole year, with a marked predominance of occurrences during the warmer months, June to September inclusive, for during this period,in which a total of 68 collections were made, this species was found in the plankton 34 times. If the month of October be included, the ratio is 44 oc- currences in 83 collections, while in the remaining 97 collections, from November to June, only 17 occurrences were recorded. Of the 10 occurrences in October, 7 were in water at or above 55°. The season of frequency in the plankton thus ranges from June through October. In both frequency of occurrence and in numbers of individuals (see Table I.) there is an apparent maximum in August, preceded by an increase in June and July and followed by a decline in September and October. Arcella vulgaris thus seems to be a late summer planktont. The continuance into October may in part be due to the temperature conditions above cited, and perhaps also to constant seining of the river by fishermen in the low-water stages at that time, causing repeated disturbances of the bottom and shores, where Arcella habitually lives. This maximum frequency of Arcella during the warmer months in the plankton -1s, however, probably due to the formation of gas or oil vacuoles in the plasma under the conditions of higher temperatures. Their flotation is thus facilitated, and they become, in a way, semi-active but temporary planktonts. That floods are also in part responsible for the presence of Arcella in the plankton is evident from the fact that 32 of the 61 occurrences come with rapidly rising waters, or shortly after rapid rises, during the interval of rapid decline. The larger numbers of individuals also appear in flood-waters, occurrences of more than 1000 per cubic meter happening 10 times with floods to only 4 in more stable eanciions. The maximum occurrence, 25,272 per cubic meter, came with the flood of February, 1898, indicating the presence of this species in large numbers, even under winter conditions, in some local environment tributary to the flood plankton. The average number per cubic meter in the 61 collections con- taining Arcella was 1,260; and the maximum, 25,272,as above noted. This species occurred in only 10 collections in stable conditions of the river, when the temperature of the water was below 55°. The 102 average number of individuals in these cases was, however, only 230 per cubic meter as against 1,443 when the temperature was above 55°, or, if below, when floods prevailed. The seasonal and numerical distribution of occurrences and individuals alike point to the agency of floods and higher temperatures in the introduction of Arcella into the plankton from its usual habitat, the bottom and the shore. This species occurred in water ranging in temperature from 32° to 89°. Being a bottom form, the plankton data do not afford a satisfactory basis for determining its true seasonal distribution and optimum temperature. The maximum number found, 25,272, was in water at 32°; but this was an isolated occurrence in a flood, and serves only to illustrate the irregularity of distribution in the plankton of tycholimnetic organisms. Centropyxts aculeata Stein.—Average number, 570. This species has appeared in collections in every month of the year, but its sequence is frequently interrupted and its numbers are quite irregu- lar. Practically without exception all the larger occurrences attend rising flood waters. It is evidently adventitious at all seasons of the yeas. Centropyxts aculeata var. ecormis (Ehrbg.) Leidy.—Average number, 604. In former years this species was less frequent than the preceding species. Its appearances in the plankton tend to coincide with those of C. aculeata (Table I.), and are doubtless due to the same causes. Thus in the February flood of 1898 there is a pulse of 12,636 of C. aculeata and one of 9,477 of var. ecornis. C. levigata Penard seems to be identical with this variety. The data concerning both C. aculeata and its variety ecornis are too irregular to throw any light on the seasonal cycle of these adventi- tious planktonts. Cochliopodium bilimbosum (Auerbach) Leidy.—Average number, 1,384. This species was found in the plankton during 1898 in irregular numbers in 27 of the 52 collections. The distribution of the occurrences affords indubitable proof of their close dependence upon flood waters. In 15 of the 27 cases Cochliopodium appeared with a rising river, and in all but 6 cases, in periods of considerable movement in river levels (cf. Table I. with Pl. XII., Pt. I.), such as the rising flood of January and February and the repeated minor 103 fluctuations of August and the following months. The year 1898 was one of unusual irregularity in the hydrograph (Pt. I., Pl. XI1.), especially at the lower stages of the river, at which times this rhizopod appeared most frequently. Its maximum occurrence, 20,898 per cubic meter on Jan. 25, accompanied a rise of 0.6 of a foot in 24 hours. At other times the numbers range from 100 to 8,000 per cubic meter, their irregularity affording additional ground for regarding this species as an adventitious planktont. Cochliopodium was present in water ranging from 32.1° to 89°, the maximum number observed being found in water almost at the freezing point, when the river was full of running ice. That this is the optimum temperature for this organism is not, however, to be inferred, since, as has been shown above, this species is adventi- tious in the plankton. Plankton collections do not afford adequate data for determining the seasonal cycle of the organisms habitually living upon the bottom. This species was not found, though careful search was made for it, in the winter collections of 1899. Its absence from the records of years previous to 1898 may in part be due to a failure to observe it 1n the silt-polluted collections in which it is most apt to occur. Cyphoderia margaritacea Ehrbg.—Average number, 198. This species has occurred in every month but February. In 1898, the majority of the occurrences and three fourths of the numbers ap- peared between May 1 and October 1 at temperatures above 60°. It was never abundant at any time, though there is this indication of its increased numbers during the warmer season. It is not an im- portant element in our plankton. Apstein (96) found it somewhat irregularly in the plankton of German lakes. In our waters it exhibits no marked dependency upon floods for its presence in the plankton, though it is probably capable of assuming the limnetic habit in the warmer season. Cyphoderia trochus Penard appeared occasionally with the pre- ceding form, from which it is distinguished by its conical horn on the fundus and by its larger scales. Difflugia, This genus is the most abundant one of the Riizopoda in the plankton of the Illinois River, and is a factor of quantitative 104 importance in its economy. It includes a number of forms notorious — for their variability and for the difficulty with which specific dis- tinctions can be applied. I shall discuss the species as they were enumerated, and shall correlate my work with Penard’s (’02) recent elaborate analysis of the species so far as I can with the aid of my notes in the absence of the collections. Opinion as to the validity of the species is expressly withheld excepting in those instances in which it is formally stated. Difflugia acuminata Ehrbg.—Average number, 315. This spe- cies has occurred in every month of the year and in 83 out of 180 collections. In 1898, two thirds of the occurrences and three fourths of the individuals were taken between May 1 and October 30, at temperatures above 70°. In this year there are six recurrent pulses from June to November, but all but one of these are found on rapidly rising flood waters, and they bear no constant relation to the pulses of diatoms previously noted, with which in some instances they are intercalated, though this is not regular or constant. Similar tendencies to appear with floods and in greater numbers and more frequently in summer can be detected in records of other years. It was more than twice as abundant in 1896—a year of interrupted hydrograph (Pt..I., Pl. X.)—as in 1898. This is one of the larger and heavier rhizopods, and its occurrence in the plankton 1s doubt- less adventitious, due to floods and currents, and its greater numbers and frequency in the summer may result from its greater abundance at that season in its natural habitat, the shore and bottom, and perhaps, also, from its lighter specific gravity during the warmer season. An illustration of this appears on the rising flood of June, 1897, when the maximum number recorded (10,000 per m.*) oc- curred. The shell of this species is exceedingly variable in size, constitu- ent particles, and proportions. A number of forms separated by Penard (’02) and others as distinct species were grouped under D. acuminata in the enumeration. The greater number of these belong to the type designated by this name by Penard (02). D. acuminata var. inflata Penard and the somewhat similar D. elegans Penard are not uncommon. JD. acuminata var. umbilicata Penard, D. elegans var. teres Penard, D. curvicaulis Penard, D. lanceolata Penard, and D. scalpellum Penard occur also, but are rare. Aiibeets ye ~ « 105 Difflugia bicuspidata Rhumbler.—Average number, 76. A sep- arate record was kept of this bicuspid type in the later years of our collections. Penard (’02) regards it as a synonym of his D. elegans, though it would seem to be as worthy of specific distinction as many other variants to which he accords this rank. It varies greatly in the relative development of the accessory “horn,” which is sometimes but a mere elevation near the base of the main horn. Individuals with equal and symmetrical horns represent the other extreme. Ina few cases tricuspid individuals have been seen, evidencing a tendency to vary towards the type found in D. varians Penard and D. fragosa Hempel. This form was about one fourth as abundant as D. acuminata, and eight of the ten occurrences fall between May and October, usu- ally with D. acuminata and presumably for the same reasons. Difflugia constricta Ehrbg.—Average number, 46. This species occurs irregularly at all seasons of the year without marked prefer- ence tor the warmer months, and often, but not always, with flood waters. It occurs throughout the whole range of temperatures, and the largest number (2,778 per m.*) appeared during the decline of the spring flood. Data are too infrequent to establish any seasonal routine. This species varies greatly, and is connected by an unbroken series of variants with the genus Centropyxis. Penard (’02) also notes the existence of this connection, and states that after careful search he was unable to find any constant distinction which would suffice for its separation. In my enumeration only the elongated and smooth individuals were referred to this species. The spinose forms were referred to Centropyxis aculeata, and those similar in form to the spinose type; but those free from spines, to C. aculeata var. ecornis. Difflugia corona Wallich.—Average number, 36. In 1896, when the hydrograph was much disturbed, the average number was more than twice as great. This superb species was found in every month of the year except December, but never in large numbers. Its large size (200-300 «), and its heavy shell militate against its pres- ence in the plankton, and its occurrences are irregular and its num- bers few. There is no marked preference for warmer months, and four fifths of its occurrences are in rising flood waters. It 1s plainly 106 an adventitious planktont. The data are too irregular to trace its seasonal distribution. As a species it is as well defined as any in the genus. It 1s not in our waters connected by intermediate forms with other species. Its assignment to D. lobostoma by Schewiakoff (93) is not in my opinion justifiable unless we regard all forms of Dzffiugia as belonging to one species. Difflugia fragosa Hempel.—Average number, 25; in 1896 over 100. This species occurred in every month of the year but Febru- ary, though three fifths of the records and the majority of the in- dividuals were found between May and October at temperatures above 60°. The data are too irregular to trace the seasonal history of the organism, but they suffice to suggest the agency of floods at all times and of high temperatures during the summer, as factors in the occurrence of the species in the plankton. ‘The shell of this form is relatively to that of other species rather heavy, and this fact combined with the irregularity of its occurrence seems to justify the conclusion that it is largely adventitious at all seasons of the year. The species exhibits a great deal of variation in the development of the central spine—Hempel (’99, Fig. 1)—and in the number and arrangement of spines in the accessory circlet. The mammillate form of the central spine figured by Hempel is not usually present. Individuals in which the central spine is but feebly developed seem to connect this species with D. varians, recently described by Penard (’02). Otherwise, and in our waters, the species is well delimited. Difflugia globulosa Duj.—Average number. 7,194; in 1897, 47,329, the larger number in this year being in part due to a remark- able pulse of 1,240,000 early in September. This is the most abundant of all the rhizopods in our plankton, occurring most frequently and in largest numbers. It is found in every month of the year, and in1898 appeared in every collection except four in De- cember. With a few exceptions in the autumn of 1898 (Table I.), no large development (exceeding 10,000 per m.*) has taken place earlier than May or later than September—that 1s, at temperatures below 60°. The occurrences are most continuous and the numbers of individuals are largest during the warmer period between the months named. The largest pulse, that of 1,240,000 on September 107 7, 1897, was at 80°. A pulse of 48,000 on November 22 at 40° gives evidence of considerable range in adaptation to temperatures. In Table I. the seasonal distribution of D. globulosa is given in full. It differs from that of previous years mainly in the fact that the summer pulses do not here have the amplitude reached in other years; for example, in 1896 (252,000) and 1897 (1,240,000). It is characterized by considerable irregularity caused by somewhat abrupt pulses at irregular intervals. A comparison of these occur- rences with the hydrographic conditions (Pt. I., Pl. XII.) indicates that in the colder months increase in numbers in the plankton at- tends flood waters only, as, for example, in January, February, late October, and November. In the summer, pulses may also come with floods. For example, that of 252,000 on May 25, 1896, ap- peared on the upward slope of the June rise of the year, and that of 80,000 on June 28, 1897, came with the belated June rise of that year. On the other hand, some of the minor fluctuations appear on declining floods, and the maximum one of our records, that of Sept. 7, 1897, came in the midst of the most prolonged period of ssaple low water (Pt. I., Pl. XI.) found in the six years of our operations. From these facts it is evident that floods are efficient in increasing the number of D. globulosa in the plankton, and that the amplitude of the pulses to which they contribute is much greater in the warmer months (above 60°) than in the colder ones—as a result, perhaps, of the greater numbers present in their normal habitat, the shores and bottom, and also as a result of their readier flotation at this season. In so far as their presence is due to floods they are adventitious. On the other hand, it is very probable that they become temporarily eulimnetic in habit during the summer months. The evidence for this lies in their greater numbers in a period which is predominantly one of greater stability. Thus in 1898, in the 22 collections between May 1 and October 1, the average number present is 9,731, while in the remaining seven months of colder weather the number is only 5,200. Additional evidence arises from the fact that pulses of unusual magnitude have occurred quite independently of any factor such as flood or other disturbance which might cause their adventitious introduction into the plankton. Thus on Sept. 7, 1897, there is a symmetrical pulse whose rise and decline occupy four weeks, as shown in the following table. The total change in river levels in this period of four weeks (Pt. I., Pl. 108 ey Stage of Turbidity Silt . Date Number per m.3 F . river above (in meters) | (in cm.?) eenenr a. AMOS ts 2AM i oe eee | 4,800 pod melts 1.8 PNGGUSE Shoes aaoagoce sus] 112,000 aS) .19 1s September a. se sae | 1,240,000 as 5 ti September: (Ay ances | 106 , 000 | ao) 1.04 20 September 21 tes ae et 800 | oS) trace 2,0) XI.) was only a fall of .1 and a rise of .2 of a foot—changes due to wind and the operation of the locks in the dams at either end of the pool. The estimated percentage of silt is near the minimum—from | a trace to 5 per cent.—and the turbidity was no greater than is customary (Pt. I., Table III.) in-our waters during periods of abun- dant plankton such as this (Pt. I., Pl. XI.). Beyond the presence of these rhizopods there was nothing in the plankton to suggest that the bottom had been stirred up any more than usual. No environmental factor is apparent to which we can attribute this wave of Difflugia in the plankton. It is due, I believe, to their own physiological condition. This was a time of prolonged low water and great sewage contamination, and of remarkable development of water-bloom, chlorophyll-bearing flagellates, unicellular alge, and some diatoms,—all elements in the food of Dzfflugta. In the open water Difjlugia could find abundant sustenance and thus maintain itself there. It is not strange, then, that we find it in these warm waters, richly charged with its food, assuming for the time a eulim- netic habit, perhaps as a result of rapid growth and lighter shells, and of increased metabolism—with reserve products which lighten the specific gravity and so facilitate flotation. This species is found throughout the whole range of temperatures. There are indications that its optimum lies above.60°, and perhaps near the maximum, 80°. This may, however, be the result of the effect of temperature upon the food supply of the organism. In any case the plankton data can not suffice to follow the complete seasonal cycle of an organism which is either an adventitious or but a tem- porary constituent. i 109 The question of specific limits and variation in this organism is one of exceeding difficulty, and I see no satisfactory solution for it until some one attacks the problem by a study of the variation by modern quantitative methods, and endeavors by breeding under control to establish the limits of variation within the normal range of seasonal changes of the environment. When this is done, some more satisfactory criterion for species in this group of planktonts will be feasible than the present condition affords, in which slight differences from previous descriptions are held to be valid for specific distinctions. Thus, in recent years, species of plankton Difflugia have been described by Heuscher (’85) (D. urceolata var. helvetica) from Swiss lakes; by Zacharias (97) (D. hydrostatica) from Lake Plén; by Garbini (98) (D. cyclotellina) from Italian lakes; by Levander (’00) (D. lobostoma var. limnetica) from Finnish waters; and by Min- kiewitsch (98) (D. planktonica) from Russian waters. All of these forms occur in the Illinois River, and there are others equally worthy of specific designation in our plankton as yet undescribed. They occur most abundantly at the times of the pulses, especially of those in stable conditions. In my opinion they are all mere limnetic varieties of D. globulosa or D. lobostoma, the form of the shell and its constituent particles being modified by the habit of life in which these individuals of the seasonal cycle are found. They occur at times of abundant food, rapid multiplication, and limnetic environ- ment. Their shells are accordingly lighter, more chitinous and transparent, and the foreign particles adherent to them partake of the nature of those of the silt in suspension. This, however, is merely an opinion based upon an examination of the statistics of occurrences, and upon the work of plankton enumeration in which all individuals must be assigned tv some species. This is at least a different point of view from that of the systematist, who may, per- ' haps, lay more stress upon divergences from described types and less upon links connecting such variants. For the sake of genuine progress in the science it would seem to the writer extremely desir- able that more attention be given to the question of variation and less to the description of new species under criteria now in vogue. It may be desirable, indeed necessary, to distinguish such forms in the plankton. It would be both safe and conservative to designate them as forms, or, at the most, as varieties. 110 The location of the pulses of D. globulosa bears no constant rela- tion to.those of other organisms, owing, in part, at least, to the irregularities of the floods upon which some of them seem to depend. The great pulse of Sept: 7,.1897,-1s intercalated between two pulses of diatoms and other chlorophyll-bearing organisms, and some others bear a similar relation to their food supply, while some co- incide with an increase in these synthetic organisms (cf. Table I. and: Ply I): Difflugia globulosa and the following species were reported by Smith (94) in the plankton of Lake St. Clair; by Jennings (’00a) in that of Lake Erie; and were common in the plankton of Lake Michigan (Kofoid ’95). Dzfflugza of the forms included here under D. globulosa and D. lobostoma have been reported by many authors from various European lakes and rivers, but in no reported instance do they reach the numbers or importance in the plankton that they do in the Illinois. Full records of their seasonal distribution may, however, bring such importance to light. Difflugia lobostoma Leidy.—Average number, 1,158. In the total of all collections it is about one fifth as abundant as D. globu- losa. Like that species it occurs throughout the whole year in almost every collection (Table I.), and the fluctuations in its occur- rence follow very closely those just described for D. globulosa in the direction of their movement. The amplitude of the pulses is less, as a rule, and their culminations and limits are coincident, or at least approximate. Thus, on Sept. 7, 1897, D. lobostoma attains only 24,000, and the pulse of D. globulosa on June 28 (80,000) is attended by one of 96,000 in D. lobostoma in the next collection, on July 14. There are in this species also the same influx into the plankton with floods, and increase in numbers at temperatures above 60°. There are 954 per collection per cubic meter below this temperature to 1,436 during the warmer months in 1898. There are also pulses during the warmer -months, in stable conditions, coincident with those of D. globulosa. Similar causes presumably contribute to these results in both species. Difflugia lobostoma is also exceedingly variable in proportions, in the texture of the shell and the degree of incision, and in the num- ber of lobes about the mouth. Two, three, and even four have been noted, and they vary greatly in depth, in regularity, in perfection of their development, and in the structural border which sometimes - 1 ial forms their margin. Chitinous, brownish, or more or less trans- parent shells are abundant when pulses occur. Forms which connect this species with D. globulosa have been observed. In- cluded with D. lobostoma are forms which have since been described by Penard (02) as D. gramen, D. gramen var. achlora, and D. lithoplites, though I have not found in the Illinois plankton any of the last-named with the peculiar tipped horns found by Penard upon many individuals of his species. Diffiugia pristis Penard (?).—A small Dzfflugia was found occa- sionally in the filter-paper collections in the colder months, but only from November to March. It was often dark, or even blackish, resembling in this respect Penard’s D. pristis. Individuals not thus darkened approach more nearly D. fallax Penard and D. pulex Penard. Difflugia pyriformis Perty.—Average number, 368. This species occurred in every month except January, but generally in small numbers and irregularly. The largest number taken—12,000, on May 25, 1896—came with the flood at that time (Pt. I., Pl. X.), and all the large occurrences of 1898 came with rapidly rising water (cf. Table I. and Pt. J., Pl. XII.). There are no indications of pulses during stable conditions, and we must conclude that the species is purely adventitious in our plankton. It is one of the largest species with a heavy shell, and its flotation 1s impeded thereby. This species is exceedingly variable. The following varieties or variants, given specific rank by some writers, have been noted, and are included with D. pyriformis in the enumeration: D. pyrtformis var. nodosa Leidy, D. pyrifornis var. claviformis Penard, D. pyriformis var. venusta Penard, and D. pyriformis var. lacustris Penard. A more slender and smoothly contoured form than the last is not uncommon. D. capreolata Penard and D. bacillifera Penard were also found, but are rare. Diffiugia rubescens Penard was taken but once—on May 25, 1896. Difflugia tuberculosa Hempel was also found but once in the planktons enumerated, though Hempel (’99) reports it as appearing occasionally from August to November in 1895. Diffiugia urceolata Carter was taken only in April and May, 1896, in small numbers at temperatures of 66°-80°. 4 Dinameba mirabilis Leidy was found in the plankton but once—Apr. 12, 1898, in small numbers, at 52°. Euelypha alveolata Duj. was found in small numbers in the plankton, but only on Nov. 1, 1898, and March 14, 1899, at tempera- tures of 45° and 36°. Euglypha ciliata Ehrbg. appeared in the filter-paper collections in 1897, in July, August, and November, in small numbers at tem- peratures ranging from 80° to 48°. This is said by Penard (02) to be predominantly a sphagnum species, but widely distributed elsewhere in small numbers. Euglypha levis Perty.—This minute rhizopod was found in the filter-paper collection of Oct. 4, 1898, at 72°. Nebela collaris Leidy was found only once—on June 25, 1898, at sae. Pontigulasia incisa Rhumbler.—This curious rhizopod occurred in the plankton in July and August, 1895, and again in August and September, 1897, at temperatures of 75°— 85°. Both occurrences were in stable conditions, and the temporary adoption of the lm- netic habit is suggested by their appearance at these times. Two other records in 1897—on March 22 and November 9, at 44° and 50°— extend the seasonal range of the species. These occurrences attended rising water and were apparently adventitious. Trinema enchelys (Ehrbg.) Leidy.—Average number, 158. This little cosmopolite rhizopod of the sphagnum fauna was found but eight times in the plankton. The individuals observed were all dark- ened by the granular food vacuoles to such a degree that structural details were obscured. It was noted only in the somewhat turbu- lent years of 1898 and 1899, though on account of its small size and the obscurity of its structure it may have been overlooked in prev1- ous collections. The few occurrences are insufficient to establish . any seasonal routine. They were at both extremes of the tempera- ture range and in all seasons but spring, with a predominance in late summer and fall. The species is evidently adventitious in the plankton, as shown by irregular distribution and small numbers, and by the fact that its occurrences coincide in all instances but one with rising water. 143 HELIOZOA. The Heliozoa of the plankton of the Illinois are few both in number of species and of individuals. They apparently play but a small part in the economy of the plankton. The average number for 1898 was but 4,883. Their occurrences are confined in the main to midsummer and early autumn. But four species were identified, though several others remain undetermined for lack of sufficient material, especially of the living forms. Apstein (’96) reports Heltozoa in considerable numbers in German lakes, with maxima in July-August. It is probable that these delicate forms are fre- quently crushed in manipulation or hidden in silt in our collections. DISCUSSION OF SPECIES OF HELIOZOA. Actinophrys sol Ehrbg.—Average number, 62. This species occurred irregularly from April to the early part of November at temperatures above 46°. It was recorded most frequently in the latter part of the summer, the largest number (28,000) appearing sept. 7, 1897, at 80°. Actinospherium eichhornu (Ehrbg.) Stein.—Recordeda few times, from July to October, at maximum temperatures (75°-80°), but always in small numbers. Endophrys rotatoriorum Przesm.—This heliozoan (?) has been recently described by Przesmycki (01) as parasitic, during a part of its existence, in Philodina and Hydatina. A parasite resembling this parasitic stage of Endophrys was observed by me in a bdelloid rotifer (Rotifer tardus) on several occasions, but it was never abun- dant, nor was its connection with any free-swimming condition noted. The heliozoan affinities of this organism seem very ques- tionable. = Nuclearia delicatula Cienk.—Average number, 4,760. This species in 1898 appeared first on June 21, attained a pulse of 78,400 on August 9 at 82° and another abrupt one of 65,600 on September 27 at 73°, and made its last appearance October 25 at 48°. Occur- rences in previous years are confined to midsummer. Its optimum conditions of temperature obviously le near the summer maximum, and its lower limits near 50°. Its appearance in the plankton 1s not traceable to flood conditions, and it is apparently eulimnetic in our waters. 114 Hempel (99) reports Raphidiophrys pallida Ehrbg.and R. elegans Hertwig and Less. in the plankton of Quiver Lake adjoining the river, and I have found an undetermined species of Acanthocystis and a small heliozoan resembling Nuclearia in the river plankton. SPOROZOA. Triactinomyxon sp.—In the plankton collections of each year there have been found free limnetic spores which unquestionably belong to that highly aberrant and peculiar group of organisms described by Stolé (’99) as Actitnomyxidia and regarded by him as Mesozoa, but later referred by Mrazek (’00) Caullery and Mesnil (704), and Leger (’04) to the Myxosporidia. The organisms de- scribed by Stolé were parasitic in fresh-water oligochetes, and it is not improbable that the hmnetic spores taken in our plankton collections are derived from parasites in some of the numerous aquatic oligochetes, or other invertebrates, found along the bottom and shores of the stream. The species here referred to Triactinomyxon differs in some details from T. 1gnotum Stolé. It was found in the course of the six years at least once 1n every month of the year, but most regularly in May—September, and rarely and in small numbers in the colder months. Its transparency and long, slender, radiating, tripod-lke arms give it a typically limnetic habit. Actinomyxidia, gen. et sp. indet.—Clusters of eight, or less, cylindrical spores radiating from a common center and bearing a marked resemblance in structural features to those of Tr1actinomyx- on, but lacking any anchor-like projections, were found sparingly in the plankton in June—September. The distinctively limnetic habit of these spore stages in the life- history of these parasites is unique among the Sporozoa, and has not, to my knowledge, been before noted. Many of the rotifers of the summer plankton, especially Brachi- onus and an occasional Asplanchna, have been heavily parasitized internally by small sac-like bodies, often pear-shaped, with the smaller end attached to the lorica, or of spherical or flattened form. They occur in such numbers at times as to be a menace to the rotifer population. They are usually most abundant in any given species at the time of, or subsequent to, its maximum occurrence. It ~~ =a tek was not unusual to find as high as ten or fifteen per cent. of the individuals parasitized, and a number of empty loricee bearing addi- tional testimony to their destructive agency. Bertram (792) describes these structures as “ parasitische Schlauche” in the body cavity of rotifers, and Przesmycki (01) works out their life history, and describes the organisms as Dime- rium hyalinum, but does not designate their systematic position or affinities. There are, however, marked suggestions of sporozoan affinities in the organism found in the rotifers of the Illinois plankton, which seems to be identical with that described by Przesmycki (’01). Obviously it is difficult to take a census of such internal para- sites. A record was kept, however, of the number of parasitized individuals in each species of rotifer, and references will be made to these results in the discussion of the hosts. Dimerium appeared in both summer and winter rotifers, and its seasonal distribution naturally depends upon the number of available hosts. It was in consequence most abundant during the midsummer and autumn months. CRETATEAS Average number, 15,812,346, including filter-paper collections. If these be excluded and the silk catches only averaged, the number will fall to less than a tenth of this sum. The ciliates are found in the plankton of the Illinois throughout the whole year, and as a whole they do not exhibit any common seasonal predominance. The analysis of the distribution of the individual species which follows, exhibits two diverse tendencies which affect the distribution of the totals. These are the vernal and autumnal pulses of the Tintinnide, represented by Codonella cratera and Tintinnidium fluviatile, and the autumnal-winter occurrence of a large number of species during the height of the sewage contamination and bacterial development. The dominant species in this ciliate wave are Carchesium lachmannt, Epistylis, Amplileptus, Lionotus, Plagiopyla. nasuta, Glaucoma scintillans, Stentor niger, and S. ceruleus. Some species, as Halteria grandinella, have a wider seasonal distribution, and others, as Vorticella, Trichodina, Zoéthamnium, Pyxtcola affinis, and many others, are adventitious in the plankton. Still others, as Rhabdo- styla, Cothurniopsis vaga, Opercularia, and similar peritrichan parasites, are passive members of the plankton. The actively (9) 116 limnetic ciliates are very few. As such we may include Codonella cratera, Tintinnidium fluviatile, and possibly Stentor niger. Car- chestum lachmannt and Epistylis enter the plankton only in the form of detached and often moribund zoédids, and thus are not typical planktonts, though of quantitative importance in our plank- ton in the colder months. A large number of species not here reported occur in our collections made elsewhere than in the river channel, especially in places where the decay of large quan- tities of organic matter is in progress. This is not a condition normally found in the open water of lakes, though it may occur along their shores, where vegetation is found, or in regions of sewage contamination. In the waters of the [llinois, on the other hand, the current, combined with sewage and industrial wastes and the organic detritus from the richest of fertile prairies, provides a suitable environment, even in the open water, for the support of a ciliate fauna of a magnitude somewhat unusual in fresh-water plankton. This fauna is present also in the back- waters, but is less abundant there than in the river itself. These species occur in greatest numbers of individuals 1n our plankton dur- ing the winter months at minimum temperatures, rising in November as the temperature falls below 50°, and declining again as 1t rises to this point in April. As shown by the bacteriological investigations of Jordan (00) and Burrill ('02 and ’04), the bacterial pulse attend- ing the decay of the sewage and wastes at Peoria does not reach Havana during the warmer months (see table on p. 231, Pt. I.), but when temperatures pass below 50° in November the increase in bacteria is marked. The decay is less rapid at low temperatures, and the process is still going on when the water in the channel passes Havana during the prevalence of low temperatures, and the ciliates that thrive in such an environment abound in the plankton at that time. The temperature limits of these ciliates of the period of bacterial development thus seem to lie between 50° and 32°. An examination of the plankton in the river at several points between Peoria and Havana at intervals throughout a year, will reveal how far the component species of this ciliate fauna are governed in their seasonal distribution in the plankton at Havana, respectively, by conditions of temperature and by the state of sewage contamination. The work of Roux (’01) upon the Czlzata about Geneva would seem to 117 indicate that many species of the fauna of stagnant water are more abundant in that region during the winter months. Owing to the difference in food conditions attendant upon the increase of sewage and bacteria during the colder months in the Illinois River, it is impossible to determine from the data at hand the relative efficiency of the two elements of temperature and food in regulating the seasonal occurrences of our ciliates. Here, as elsewhere, the disastrous effect of sudden floods can be traced. The number of ciliates (Table I.) drops as floods rise, and recovers as the waters fall again. For this reason the winter occur- rences of the total ciliates are subject to considerable disturbances in the winter floods of the several years. The combination of the two methods of collection and of the two groups of ciliates, typical and adventitious, causes further irregularities (Table I.) in the sea- sonal distribution of totals. In the Ilinois River, for reasons given above, the Ciliata occupy a place in the economy of the plankton of more than the usual im- portance. They feed principally upon bacteria, decaying organic matter, and the smaller alge, and are themselves eaten by the rotifers. I have found no evidence that they are utilized by the Entomostraca. They thus become active agents in the reduction of sewage and in the destruction of the bacteria of decay, in the purification of sewage-laden waters, and in the transfer of the matter in sewage to higher forms of animal life. The ciliates found in the Illinois include all the important species reported in the plankton of fresh water, and the list is somewhat larger than hitherto recorded in quantitative plankton collections in river or lake waters. These organisms escape readily through the silk net by reason of their small size, and in some instances the larger species, by reason of their mobility and flexibility, escape through the silk where less motile organisms of equal size are re- tained. By experiment I have found that well-shrunken silk bolting-cloth whose meshes average about 30-45 » will not retain Paramecium whose diameter is 40-70. It may be that supple- mentary methods of collection which will correct the error of leakage will show that the Cilzata are of wider occurrence in the plankton than has hitherto been found to be the case. 118 DISCUSSION OF SPECIES OF CILIATA: Amphuileptus spp.—Average number, 630. Amphileptus is a well- defined winter planktont in the river at Havana, and it affords a striking instance of the interdependency of organisms in the plank- ton. It feeds upon the heads of Carchesitum lachmannt, engulfing the head in situ and encysting during digestion. Such heads, joined to the colony or free in the plankton, have been found in our waters. Its seasonal distribution at Havana is almost identical (Table I.) with that of Carchestum, upon which it feeds. Thus in 1897-98 Car- chestum was continuously present in the plankton from October 26 to May 10, with a pulse on December 7 of 283,800, and one on February 8 of 197,600. Amphileptus appears October 26; continues, with interruptions, to May 17; and has pulses December 7 and January 25, the latter reaching 13,545. In 1898-99 both appear early in October and have coincident pulses on November 22 and January 24. In 1895-96 the interdependence is even more striking, Carchesitum reaching a greater development in this winter, with a pulse of 964,600 on November 27, and Amphileptus reaching 14,469 on this date and 14,835 a week later. Both species decline during the flood which follows, and rise during March to culminations, on thé 24th, of 104,535 and 3,636, respectively. In 1898, Amphileptus disappears on April 12 at 52°, save for an isolated occurrence May 17 at 64°. It does not reappear until October 18 at 52°. In 1897,1t reappeared October 26-at 59°" ana in 1895—96 its limits were 45° and 48°, with the exception of one occurrence, April 17, at 66°. Carchestum occurs irregularly and sparingly during summer months, and Amphlileptus was not taken in the plankton during that period. Its occurrence in the plank- ton is limited in the main to temperatures below 50°, but this limitation may be due primarily to the reduced numbers, at higher temperatures, of the organism upon which it feeds. It appears during the period of greatest sewage-contamination and bacterial development in the river at Havana. Roux (’01) finds Amphilep- tus most abundant in stagnant waters about Geneva in the winte months. Aspidtsca costata (Duj.) Stein.—Found in the plankton but once —-Jan. 11, 1898, at 32°. Bursaria truncatella O. F. Mutll—Average number, 23. This large ciliate was found in the plankton at irregular intervals and in 119 ‘small numbers. It was found six times in March; twice in January and April; and once in February, July, and November. Its ap- ‘pearance in the plankton is thus predominantly in winter months and at temperatures below 45°, though it occurs in the extremes of temperature conditions. Carchesium lachmanm 5S. Kent.—Average number, 26,546. This is normally an attached species, and its appearance in the plankton is due to the detachment of the heads. Small fragments of colonies are also found, but the greater number are isolated heads. The detachment seems to be a physiological process of the organism and not merely the result of accidents. It is thus a detached and an adventitious planktont. Many of the heads taken in the plankton are in a moribund condition. For example, in a pulse of March, 1896, the following proportions were recorded. Total Date Carchesium | Per cent. | Per cent. per m.3 normal | moribund 1896 INLEIRCIA IL e, 4. eae 60,420 BS) 45 UM, 3 8s CORO 104,535 | 48 52 “2s 2 ge eRe oa Asya =e 53 47 Ayorill Oo ¢.5:G.5 ca Renee ene 16,688 39 61 Enumerations were based on the total number of heads, both normal and moribund. The colonies are sessile, and adhere in vast numbers to any substratum furnishing a suitable place for attach- ment—submerged vegetation, brush, sticks, and fishermen’s nets. The latter sometimes become so clogged with Carchestum and floating mats of Crenothrix and Beggiatoa as to break down in the current of the river. How far the number of free heads in the plankton is an index of the development of the species in the stream can not be determined from the data at hand. This species has been taken in the plankton in every month of the year, but its occurrences between the early part of May and 120 October 1—that is, above 60°— are irregular and the numbers few (Table I.). It is thus predominantly a cold-water planktont. Winter collections in 1894-95 and 1896-97 were too few to trace its seasonal movements. In 1896-97 it appeared November 5, rose to a maximum of 964,600 on November 27, and declined in the December—January flood (Pt. I., Pl. IX.) almost to extinction, but recovered during its decline to a minor pulse of 16,160 on January 30. It again fell off in numbers during the floods of February (Pt. I., Pl. X.), but rose during the decline of March to a maximum of 104,535 on March 17. Numbers become smaller and occurrences irregular after May 1. In 1897, Carchesium increased rapidly in late October to a small pulse of 13,200 on November 2, with a decline in the following fort- night, and a pulse culminating December 7 at 283,800, with subse- quent decline. The fluctuations during 1898 may be followed in Table I. The numbers increase during the slowly rising flood of January to a maximum of 197,600 on February 8 at 32°, and decline again during the more rapid rise (Pt. I., Pl. X11) of the next thnee weeks. Stable conditions in early March bring about a pulse of 89,600 on March 15, and numbers decline again to 2,400 as the flood passes its maximum in the early part of April. As the levels fall another pulse of 99,200 appears April 26, from which a descent to minimum numbers—which prevail during the summer—takes place within a fortnight. The floods, especially sudden ones, seem thus to interfere with the appearance of Carchestum in the plankton, while gradual rises, as that of November, 1898, are not so detri- mental. The table of bacterial occurrences (Jordan, ’00) in the Illinois at Havana and Pekin given on p. 231, Part I., indicates that the bac- terial development consequent upon the sewage and industrial wastes of Peoria extends down the river to Havana during the colder months of the year. The occurrence of Carchesium in the plankton is thus coincident with that of greatest sewage pollution and bacterial development at Havana.. Carchesium is much more abundant in the channel of the river, where sewage pollution is greatest, than it is in the adjacent backwaters. It seems probable that the bacteria either directly or indirectly contribute towards its development, constituting, itmay be, an important element inits food. Flood waters, which dilute the sewage (cf. hydrograph and chlorine Wa in Pl. XLV. of Part I.) might for this reason tend to interfere with the development of Carchesitum, and thus cut off the source from which the plankton individuals arise. I am not able, however, to trace any close correlation between the fluctuations of the chem- ical matters indicative of sewage and sewage decay and those of Carchesium. In the stable hydrographic conditions of 1897 we find a symmetrical pulse of considerable dimensions rising from 2,200 on November 9 to 283,800 on December 7, and declining to 26,500 on January 11, 1898. Stable low water with an ice blockade (Pt. 1., Pl. XI. and XII.) characterize this season. No explanation for the fluctuation is suggested 1n the physical environment. The chemical condition of the water, was, however, greatly disturbed eer, Pl XLIV.). - The fivefold increase in free ammonia is indic- ative of approaching stagnation under the ice, and the threefold increase in chlorine marks the sewage concentration. Approaching stagnation might have caused the decline of Carchesitwm, or it may be a specific reproductive cycle of the organism which combines with the external factors of the environment to produce such a wave of occurrence. Chilodon cucullulus Ehrbg.—Average number, 102. This species was found in the plankton in January and February during the bac- terial increase. It was also found in July. It escapes through the silk net, and does not ordinarily appear in plankton collections, though abundant wherever decay is active. Codonella cratera (Leidy).—Average number, 101,024 or 452,500*. This is the most abundant of the ciliates in our plankton, consti- tuting about one third of their total number. It appears in every month of the year, and in 1898 it was recorded in every collection but one, that of December 13 (Table I.). It is sub- ject to great fluctuations in numbers, its maximum occurrences tend- ing to appear in April, May, or June, and again in September or October. Minimum numbers prevail during the winter, when many of the shells are empty, and the midsummer interval is subject to pulses of varying amplitude. Spring pulses were detected as follows: in 1895, on April 29 (16,324) at 64°; in 1896, on April 24 (562,152) at 72°;1n 1897, 0n April 27 (470,000) at 60°; and in 1898, on May 3(736,000) at 60°. These vernal pulses coincide with or approximate closely to the dates of the spring volumetric pulses. This somewhat remark- able approximation of dates near the end of April may be the result, 2 in part at least, of the dates of collection; but after allowance is made for this, the species still exhibits a seasonal cycle of remarkable regu- larity. The autumnal pulse is of less amplitude, and of less regu- larity in location as to time and temperature. In 1894 it appears September 4 (14,000) at 78°; in 1895, on September 12 (5,840) at 81°; in 1896, on August 29 (58,800) at 74° or October 14 (63,200) at 57°: in 1897, on October 5 (204,400) at 71°; and in 1898, on September 27 (92,800) at 73°. The midsummer pulses are, as a rule (Table I.), of less amplitude than the vernal or autumnal ones. In 1896 and 1898 exceptions to this statement appear in two large developments which follow in each case upon the decline of the June rise. In 1896 (Pt. I., Pl. X.) this pulse (152,400) came June 11, and’in 1898 (Pt. Ey PE XI) came (1,499,200) June 7 at 78° and exceeded in amplitude the re- corded vernal pulse. In both cases the pulse was recorded as occur- ring at an interval of a week after the crest of the June rise had passed. The character and sequence of these pulses is well shown in Tablet The occurrence of Codonella in abundance in the purer backwaters and in the plankton of our Great Lakes (Kofoid, ’95) indicates that it is not dependent upon the sewage bacteria directly for food for its development in our waters. The appearance of the greatest pulses during a period of considerable sewage dilution still further indicates its independence of sewage bacteria. A comparison of the fluctua- tions of the totals of the chlorophyll-bearing organisms with those of Codonella affords some evidence of a correlation between the two. Of 39 pulses which can be traced in our records in the chlorophyll- bearing organisms, 21 precede and 13 coincide with those of Codo- nella, while in the remaining 5 instances the multiplication of Codo- nella precedes that of the phytoplankton as a whole. Thus in the main the pulses of Codonella follow, or coincide with, those of the phytoplankton. The evidence of this sequence may be followed in Table I. by a comparison of the records of Codonella with those of the total phytoplankton. The sequence indicates that the food of Codo- nella may be found in the phytoplankton, and that these recurrent periods of growth have some connection with the conditions of nu- trition. The seasonal cycle of Codonella is closely followed by the other member of the family found in our plankton—Tintinmidium fluviatile. eo 123 Codonella occurs throughout the whole range of temperatures. The winter minimum and the decline during the maximum temperatures of summer, combined with the presence of vernal and autumnal, or late summer, pulses, indicate that the optimum conditions for this organism lie neither in winter nor in summer. The spring pulse was at temperatures of 60°-72°, and the autumnal one at a wider range of 57°-78°. Permanent increase in numbers does not begin (Table I.) until March 15 at 46°,and the permanent falling off is found on November 15 at 41°. The optimum temperatures in our waters thus lie near 60°-70°, and conditions favoring growth are limited to a range of 10°-15° upon either side of the optimum. This species readily escapes through the silk net on account of its smail size and its motility, and such collections give at the best in- complete evidence of its seasonal distribution. The amplitude of its fluctuations is thus reduced, and owing to the irregularity of the error arising from leakage, the reduction is not proportionally distrib- uted throughout the year. Tests made of the loss of Codonella by leakage through the silk indicated that but one was retained to twenty-fourfoundin the filtrate. Codonellawas counted in both the silk and filter-paper collections, with the result that in 1897 the totals for the year (omitting one date on which the filter collection contained an unusually large number of Codonella) showed one Codonella in the silk to twenty-five in the filter collection. In 1898, however, the ratio was one to four and a half. The error in the filter collection is large, but data seem to justify the conclusion that only a small proportion of the Codonella is retained within the silk net. The proportion for the whole period of collection by the two methods (August 3,’97, to March 28,99) is one to seven, if one date on which aberrantly large numbers appear in the filter collections be omitted. This species is a typical planktont, and is apparently the same as C. lacustris Entz, by which name it is designated by European writers. Leidy’s name, however, has priority according to the accepted rules of nomenclature. It is an exceedingly variable organism, at least in the form, proportions, and size of the shell, in the degree of its con- striction, and in the foreign particles which fill its matrix. The rings or bands which ornament the orifice vary in their number, width, and _ relative proportions, and in the perfection of their development. The intergradation which these variants exhibit is sufficient to my mind to make their elevation to specific rank unjustifiable. 124 Codonella is an important element in the food of many of the lim- netic rotifers, especially Asplanchna. Codonella is a common constituent in the plankton of our own Great Lakes (Smith, 94; Kofoid, 95; Jennings, 00a), and has been reported from most European waters. Apstein (96) finds in German lakes major pulses in spring and autumn and minor ones in midsummer. Lauterborn (’94) reports Codonella in the plankton of the Rhine, and Schorler (00) in that of the Elbe, but neither follows its seasonal history. Coleps hirtus Ehrbg.—Average number, 13. This species occurred in the plankton collections irregularly and in small numbers, princi- pally in autumn months during the height of the bacterial develop- ment. It escapes through the silk readily. Colpoda cucullus Ehrbg*.—Average number, 9,615. This species appears in the plankton principally during the colder months of bacterial predominance, from November to April, and occasionally during the summer. Cothurmopsts vaga (Schrk.) Blochmann was found in both 1898 and 1899 on Canthocamptus. Didinium nasutum (O. F. Mull.) Stein*.—Average number, 12,692. This species also is found in the plankton during winter months, especially in November and December during the bacterial increase. It was also found in midsummer. Epistylis spp.—Average number, 2,020. The free heads or frag- ments of colonies of one, or possibly of several, unidentified species of Epistylts, or it may be of Opercularta also, were associated with Car- chesium lachmanni in the plankton during the colder months, but in much smaller numbers (1 to 13 in 1898). Identification in most cases was impracticable, though in some instances EF. flavicans Ehrbg. was determined, and it seems probable that most of the winter forms at least belong to this species. Hempel (99) reports FE. plicatilis on snails, and various other aquatic animals have been found infested with colonies of undetermined species of Epistylis. The distribution of Epzstylis in the plankton (Table I.) is in its limits somewhat like that of Carchestum. It is more abundant and more continuously present during the period from November to June (at temperatures below 60°) than in the intervening warmer months. It is found throughout the whole range of temperatures. Its pulses coincide with those of Carchesiwm when thev occur, but they are not ue 125 always found in Epzstylis when they appear in Carchesium. This degree of similarity in the seasonal cycle of the two genera is indica- tive of their correlation with the same environmental factors, the principal one-of which is the increase in bacteria attending the colder months. Euplotes charon (O. F. Mull.) Ehrbg. was taken but once in the plankton—August 23, 1898. Euplotes patella Ehrbg*.—Average number, 2,888. It was found in small numbers and at irregular intervals from April to December throughout the full range of temperatures. It was most frequently taken in the summer. Glaucoma scintillans Ehrbg.*—Average number, 39,615. This species was taken in the plankton from the middle of October till the middle of April. It was present in larger numbers and more contin- uously in December and February. It is thus a member of the plankton during the time of bacterial increase. Halteria grandinella O. F. Miull.*—Average number, 255,769. The seasonal distribution of this species in the plankton does not show the limitation to the winter months noted so frequently in other ciliates. It was found in every month of the year but May, in largest numbers in July and August, and most continuously in December and January. The data are too few and irregular to determine any pre- dominance as to season or temperature. Holophrya simplex Schew. was found in small numbers in the filter collections of December, February, and March in the winter of 1896-97 at temperatures from 32° to 44°. Leucophrydium putrinum Roux.—Average number, 525. This species was recorded July—September, 1898, during the low-water period, at temperatures from 89° to 63°. It was described by Roux (99) from stagnant water, but in our plankton no conditions of stag- nation attend its presence, though sewage contamination is great and decaying organic matter abundant. Lionotus spp.—Average number, 94. With Amphileptus in the winter plankton there occur a number of other, smaller, gymnostome ciliates which in best-preserved specimens resemble Lionotus. A few occurring in March and April, 1898, were found to be L. jfasctola Ehrbg., and it is probable that most of the individuals belong to this Species, though exact identification is difficult with plankton mate- rial. The seasonal distribution of Lionotus coincides very closely 126 with that of Amphileptus. The species appear in November or De- cember and continue through March in temperatures below 50°, but the numbers retained by the silk net are too small to trace their sea- sonal routine. Their seasonal distribution in the plankton coincides with the period of greatest access of sewage and bacterial increase in the river at Havana. Roux (01) finds this genus well represented in the fauna of swamps, and most abundant in October and March. Loxodes rostrum Ehrbg. was identified but once—March 22, 1897, at 44°. Nassula rubens Perty occurred July 30, 1897, at 84°. Opercularia articulata Goldf.—This species is parasitic upon aquatic Coleoptera. In the plankton of June 28, 1897, eleven colonies or fragments of a colony were found, the largest with 115 zooids. Opercularia nutans (Ehrbg.).—Average number of zodids, 60. In the plankton this species was found attached to Alona affimis in January, 1898, and to Cyclops in April and August. Opercularia not specifically determined were found free in the plankton in June and July; in November, attached to Canthocamptus ; in January, attached to Brachtionus—and even to the eggs of this species. An unidentified form was also found upon Cyclops. Ophryoglena atra Lieberk.—Five irregular occurrences of this species in small numbers were recorded in 1899 from January to the middle of March. Paramecium spp.—Average number, 41. Paramecium was © found 18 times in the plankton. Two of these instances were in May and August at temperatures of 64° and 79°, and the remainder were between November 20 and March 30 at temperatures below 48°. Most of the occurrences are in midwinter at minimum temperatures under the ice. P. aurelia (O. F. Mull.) has been found in the river waters (Hempel,’99), but not all taken in the plankton belong to this species. Specific determinations are not easily made with accuracy in preserved plankton material. In our plankton, Paramecium is present principally during the period of greatest contamination by sewage. Plagiopyla nasuta Stein*.—Average number, 1,181,000 during the winter of 1898-99 from November 29 to March 28. This species was not recognized in the plankton of previous winters. It reaches a pulse of 11,520,000 on January 3, 1899, at 32.2° under the ice. 127 Levander ('94) finds it in numbers under the ice in Finnish waters. On account of its motility and small size it readily escapes through the silk net. Pleuronema chrysalis (Ehrbg.) Stein.—Average number, 9. Re- corded only in January, 1898, at minimum temperatures. Prorodon farctus Clap. and Lach.—-Only a few scattered occur- rences—from the last of September to the first of March at tempera- tures from 73° to minimum. An unidentified species of Prorodon was also found irregularly from November to April. Pyxicola affims 5. Kent.—Average number, 58. This species is usually attached to aquatic plants, especially to Lemna. It has been found in the summer plankton from June to August during maximum temperatures, especially in 1896, when recurrent floods brought much Lemna from the backwaters into the river. It was found October 18 at 52°, attached to Melostra varians. Rhabdostyla spp.—Average number, 110. Peritrichan ciliates re- ferred to this genus have been noted on Cyclops, Canthocamptus, Oligocheéta, and even in considerable numbers upon the body, append- ages, and eggs of Polyarthra platyptera. They have appeared thus passively in the plankton during winter months from December to March, especially in 1899. Stentor ceruleus Ehrbg.—Average number, 882. This species presents a characteristic seasonal distribution in our plankton. Its numbers are never very large, and its full cycle can not always be traced in the records. It is a planktont of the colder season in our waters. But three records—one Julv 28, 1896, at 82°, one August 3 of the same year at 80°, and a third, August 15, 1894, at 84°—lie outside of the period between September 1 and May 1. In 1898 (Table I.) the autumn cycle begins September 6 at 79°, but in both 1895 and 1897 the species does not appear until late in November or in December at 34° or below. In years prior to 1898 the numbers were small and irregular, but on January 21, 1898, the maximum number of 28,800 was reached at 34°, under the ice, during the slowly rising flood of that month (Pt. I., Pl. XII.). It accompanied an increase in Stentor niger, and there are indications elsewhere that the two species may fluctuate together. The high (Pt. I., Pl. XLV.) chlorine (38.), nitrites (.175), and free ammonia (4.6) at the season of greatest development in the plankton are in- dicative of conditions approaching stagnation. The appearance of 128 this species in stagnant water has often been observed. Roux (01) finds 1t especially abundant in September, October, and February in stagnant waters about Geneva. Stentor niger Ehrbg.—Average number, 3,124. In our waters this species also is a winter planktont (Table I.). There have been but four records of occurrence between May 1 and September 1. In 1895-96 the species appeared November 14 at 44° and reached a maximum of 68,635 December 18, after three weeks of minimum temperatures and approaching stagnation under the ice. Numbers declined in the December—January flood (Pt. I., Pl. X.), but rose again in March, as the flood declined, to 39,087 on the 24th at 40°. It disappeared from the plankton April 30 at 70° and did not re- appear until November 17, from which time it continued until March 22. In 1897-98 it returned September 21 at 71°, attained a maxi- mum of 42,000 November 23 at 43°, declined during December, and rose to 47,000 on January 21 at 34° under the ice, and in the con- ditions approaching stagnation described in connection with the dis- cussion of S. ceruleus. A decline in numbers continued until April 12 at 52°. Favorable conditions for growth are thus found in our waters between 32° and 50°, and the optimum seems to lie near 40° or below. . This species reaches its greatest development in our waters during the time of greatest sewage pollution and bacterial development. It is known as a bog-water species, and was found by Roux (01) in stagnant waters about Geneva during the colder months. Hempel (99) reports this species as S. zgneus (?), but from the descriptions of Roux (’01) I am inclined to consider it as S. mger Ehrbg. It may ‘be that both species are included in our data, but they are predomi- nantly of the mger type. They include also individuals of the black- ish variety S. igneus var. fuliginosus Forbes, which, it would seem from Roux’s description of these species, should be transferred to S. niger. The fuliginosus form was very abundant in the margins of Pine and Round lakes, Michigan (Kofoid, ’95), during the summer in surface temperatures of 61°-70°, where sewage contamination was but shght. Stentor polymorphus (O. F. Mull.) Ehrbg. was found sparingly in July and August during maximum temperatures. Hempel (99) reports S. barretit Barrett and S. roesela Ehrbg. from the river, but I have not identified them in the plankton collections. oe 129 Strombidium viride Stein was found in small numbers in January— March, 1899, at minimum temperatures. Siylonychia mytilus (O. F. Mull.) Ehrbg. was found in the plankton sparingly from September to February, and once in June. Tintinnidium fluviatile Stein.—Average number, 22,590 or 1,640,- 192*. This species is somewhat sharply limited to the warmer months in its seasonal distribution. In 1898 (Table I.) it makes its appearance April 4 at 49°, reaches a maximum of 720,000 May 3 at 60°, and has three decreasing pulses; one of 104,000 on June 14 at 80°, one of 95,200 on August 2 at 79°, and one of 22,400 on September 27 at 73°, and disappears from the plankton October 18 at 52°. The records in previous years are more irregular, though traces of vernal and midsummer pulses can be found in the records. Filter-paper catches indicate that only one in eighty of this species is retained by the silk. They also locate the pulses as approximately coincident with those of the silk collections. Apstein (96) finds Tintinmidium to be a spring planktont with its maximum in April in Lake Pl6n, while Seligo (’00) finds it in lakes near Danzig in the autumn, with a maximum in September. In our own waters in 1896 the autumnal pulse in August-September exceeds the vernal one. The gelatinous lorica of this species is subject to great variation in its size and proportions, and especially in the region about the aper- ture. A somewhat thimble-shaped form was described by Hempel (96) as T.2llinotsensts, the specific distinctions being based wholly on the lorica. This form intergrades with the typical lorica of T. fluviatile Stein, and should not in my opinion be given specific rank. Trachelius ovum Ehrbg.—Average number in 1895, 847. This species did not occur in 1898 but was rather common in November-— December, 1895, reaching a maximum of 10,695 on December 4 at 32.5°. Isolated appearances in small numbers in December and January of other years have been recorded. In our waters it is thus a winter planktont. Stagnation conditions under the ice were approaching (Pt. I., Pl. XLIII.) when the pulse of 1895 occurred in the Illinois River. Apstein (’96) found it, however, in Lake Plon with a maximum in May—June, disappearing in the summer and returning again in November. Trichodina pediculus Ehrbg.—Average number, 1; in 1897, 874. This species is normally found upon Hydra, on the gills and skin of 130 amphibians, and on young fish. It appears in the plankton during the summer months in every year except 1898, a single record only being made in that year. The earliest record was on June 11, and the latest on November 31. The whole temperature range is practically included in these occurrences, though the species disappears within a few weeks after the temperature falls below 50°. It usually appears in small numbers and irregularly, and no pulses like those of typical plank- tonts can be traced. A free life in the plankton is apparently not its usual habit. Zacharias (00) has recently called attention to its appearance in the plankton in German waters. Vorticella rhabdostyloides Kell.—Average number, 61. This little Vorticella is found attached in small clusters to Anabena spiroides and occasionally to other members.of the phytoplankton. It is some- what common in the waters of Lake Michigan, but is rare in spring months in the Illinois River. Vorticella spp.—Average number, 7,843. At irregular intervals from April to November isolated individuals and small clusters at- tached to bits of debris in the silt were taken in the plankton. They were most abundant at temperatures above 50°. The irregularity in their occurrences indicates that they are adventitious in the plank- ton. Identifications of plankton material are impracticable except in strongly marked species. Hempel (99) has found V. campanula Ehrbg., V. microstoma Ehrbg., and V. sumilis Stokes in the river and its adjacent waters. ; Zoothamnium arbuscula Ehrbg.—A few colonies were taken in August and September in 1896 in the plankton, probably adventitious during the disturbed hydrograph of that year (Pt. I., Pl. X.). The preceding list of 45 species does not complete the catalog of the ciliate constituents of the plankton, though it includes all of the species of quantitative importance during the years of our operations. The residium of unidentified ciliates, which, excluding the partial identifications in the above list, does not often exceed two per cent. of the total individual ciliates, includes principally isolated individ- uals of species difficult of identification or others whose preservation did not permit it, and a considerable number of small ciliates and of forms ectoparasitic upon Entomostraca and other planktonts. Most of these organisms are either adventitious or passive members of the plankton, and further study of the littoral region, of stagnating 134 waters, and of these parasitic forms will reveal the great richness of the ciliate fauna in this aquatic environment. SUCTORIA. Average number, 332. This class is not quantitatively im- portant in the plankton, being represented, in so far as our records go,only by adventitious or passive planktonts. No limnetic species has as yet been found in the Illinois. An examination of the littoral region during the prevalence of ciliates will probably yield a rich suctorian fauna. DISCUSSION OF SPECIES OF SUCTORIA. Acineta lingutfera Clap. and Lach.—This species is usually found on aquatic Coleoptera. A single occurrence of an unattached ind1- vidual was recorded June 21, 1898. Metacineta mystacina Ehrbg.—Average number, 301. This species occurred in the plankton from March till October in 1898 and in the winter months of 1899, at irregular intervals and in small numbers (Table I.).. Most of its occurrences attend flood invasions, and it is evidently adventitious. It is frequently attached in the plankton to minute particles of debris. This species varies greatly in the size of the lorica. Sand (01) gives the range in height as from 33-700. The variation in proportions has given rise to a number of descriptions of new species by Stokes (’88 and ’94) and Maskell ('87), but an examination of a series of individuals such as appear in the plankton shows that they intergrade so closely that specific distinctions can not be maintained for the variants. Meta- cineta appears throughout the whole range of temperatures, no seasonal predominance appearing in the records. Podophrya fixa O. F. Mull.—Average number, 12. This species is also adventitious in the plankton. It was recorded in March and september at 37° and 73°. Cysts were noted January 21. Tokophrya quadripartita Clap. and Lach.—Average number, 4. Adventitious in the plankton in March and November. - Hempel (99) finds it most abundant in May and June, associated with Epistylis plicatilis and Opercularia trritabilis on crayfish, insect larve, and turtles. Tokophrya cyclopum Clap. and Lach.—Found occasionally upon Cyclops during spring and summer. (10) 132 PiQ RAP, RAY: Spongilla spp.—Average number of spicules, 772. The identifi- cation of fresh-water sponges by isolated spicules is practically impossible, and, moreover, the sponge fauna of the Illinois River is as yet practically unknown. No attempt, therefore, was made to identify the species to which the spicules which occur in our plank- ton collections belong. They belong to the genus Spongilla in part, and were usually the simple sarcode forms, the gemmules or their spicules not appearing in the plankton. They occurred in all months of the year, and were found in 46 per cent. of the collections. They are adventitious, and their occurrence in the plankton is there- fore dependent in part upon hydrographic conditions. Records in December and January are few (3) and always occur on rising floods. In February and March, months of rising floods, they are increased (8 and 7), but decline again in April-June (3, 5, and 5), months of predominantly declining water and more stable conditions. In midsummer and autumn months (July to November) they again occur more frequently (8 to 12), probably as a result of proximity to the season of greatest growth and frequency of sponges in the river and its backwaters. Here also they occur most frequently in years of greatest hydrographic disturbance, as, for example, in 1898. The adventitious relation which they bear to the plankton is also seen in their erratic and irregular numbers. The maximum record (16,000 per m.*) was made June 28, 1897, on the rising flood; the next in size, on August 10 in stable low water. In both instances the plankton was probably taken from water in which as a result of some local disturbance the remains of some disintegrating sponge had been distributed. Living sponges are found in considerable abundance on submerged brush and timbers in the channel and backwaters during the summer months, and feed on the smaller organisms of the plankton, being one of its depleting agencies. CHLENTERATA. Hydra fusca L.—Average number, 39. Hydra occurred in about 16 per cent. of our channel collections—a percentage which would be considerably increased if the whole of each collection had been examined for it, or if backwater collections should be included. With one exception the 28 occurrences recorded, all fall in May—September 133 at temperatures rarely below 70°. The earliest record in channel waters was on May 1, 1896, at 68.75°, and the latest on November 15, 1897, at 47°. Of the 28 records in channel waters the months from May to September have, respectively, 6, 3, 10, 7, and 1 record, and there is 1 in November. Hydra is thus a late vernal and a summer planktont in our waters. Observations in the field and a cursory examination of the col- lections made in the backwaters have indicated that Hydra is often very abundant on the vegetation. It is also limnetic in habit, floating with the foot attached to the surface film and tentacles widely extended; or, without attachment, in the deeper strata of water. A similar limnetic habit was often observed in the case of Hydra in channel waters, especially on still warm days when the surface was unruffled. Hydra was generally more abundant in the plankton in May or in early summer. The maximum record in channel waters was 3,200 per m.? on July 21, 1897, the error of dilution being, however, large in this record. In Quiver Lake on May 8, 1896, a maximum record of 5,335 per m.* was made, the error of dilution being very small. This was during a vernal plankton pulse (8.14 cm.® per m.’) in these waters, when the food of Hydra was present in considerable abundance. Hydra viridis L. was seen frequently in spring-fed backwaters and in laboratory aquaria, but was never recognized in plankton collections made in channel or backwaters. The limnetic habit noted in H. fusca was not observed in the case of this species. PLATYHELMINTHES. TURBELLARIA. Numerically and from the volumetric standpoint the Turbellaria are not of great significance in the plankton of fresh waters as a rule. However, in some seasons and under certain conditions Stenostoma becomes very abundant, as, for example, in autumn months in back- waters, and generally where decaying vegetation abounds. In the autumn of 1895 the plankton in the relict pools of Flag Lake consisted almost entirely of Synura uvella, Stenostoma leucops, and Entomos- traca. 134 The average number in channel waters is 103 per m.*, and, as might be expected, their occurrences are erratic in seasonal distri- bution and their numbers are irregular. They occurred in channel waters in every month of the year and throughout the whole seasonal range in temperatures. The numbers in 1898 were larger and occur- rences more frequent in May, during the run-off ‘of the spring flood, and smaller and more erratic during the rest of the year. In the total of all collections enumerated the percentage of occurrences was highest. in June (60 per cent.), July (83 per cent.), August (48 per cent.), and October (47 per cent.), and lowest in colder months, when it rarely rises above 30 per cent. The numbers are also larger in the warmer months, a maximum record of 19,250 per m.* on September 4, 1894, following a slight rise in river levels at low stages. The adventitious character of the Turbellaria in channel plankton is sug- gested by the erratic data, but the adaptability, at least of certain species, to the limnetic habit under certain conditions is also indi- cated by the large numbers. The identification of the Turbellaria in plankton collections is not feasible in the course of the usual methods of examination of pre- served plankton. Accordingly no effort was made to identify the individuals occurring in our catches. Many of them were evidently rhabdoccele turbellarians, and of these probably many were Stenos- toma leucops. The genus Vortex was also represented. Mesostomum ehrenbergiu O. Schmidt was taken in small numbers on August 26, 1895, along the shores of the river in vegetation. This identification is that of Dr. W. McM. Woodworth (’97). Stenostoma leucops O. Schmidt.—Average number, 21. By far the greater proportion of the turbellarians in our collections probably belong to this species. The statements made regarding the group as a whole therefore probably apply to this species. TREMATODA. Many of our predaceous fishes and other aquatic vertebrates are infested to an extraordinary degree by flukes parasitic in the intestine or other viscera. This,in conjunction with the fact that the fish markets are located in house-boats along the stream and their refuse generally cast directly into the channel, is sufficient to account for the few adventitious adult distomes which have been noted in our plank- 135 ton collections. They have occurred singly in February and July, but were not identified. The free-swimming larval stages or cercaria of unidentified trem- atodes were also found singly in August, September, and October. Aspidogaster conchicola v. Baer, which occurs abundantly in the mantle cavity and pericardium of many of the Unionide (see Kelly, ’99), which form great beds on the river bottom, was taken in an immature condition in the plankton on June 27. Cotylaspis insignis Leidy, likewise a parasite of the Umonide, associated with Aspidogaster but confined principally to the mantle chamber, was taken in the plankton on February 4. CESTODA. Tetrarhynchus sp. was adventitious in the plankton on June 27, and doubtless of similar origin to the adult trematodes above noted. NEMERTINI. Fresh-water nemerteans were definitely identified as such in the plankton on only two occasions, ifaly, 23, 791304, and March 22, 1897. They were doubtless adventitious—from the shore or bottom, where they are most abundant. NEMATELMINTHES. NEMATODA. The free-living nematode worms are predominantly shore and bottom forms, living in the midst of the decaying organic matter of the bottom ooze. Ina habitat such as ours, where the quantity of this decaying matter is very great, the nematodes are correspondingly abundant, and, owing to the unstable hydrographic conditions, they find many opportunities of joining the plankton temporarily. Ac- cordingly we find that nematodes are met most frequently and in largest numbers in rising flood waters, when the bottom deposits of tributaries and the main stream are carried in channel waters as silt. Thus, in the month of March nematodes occurred in 13 of the 15 collections examined, with an average number per m.? of 465, while in August they were found in but 8 of 21 collections,and averaged only 186 per m.*. So, also, in the winter flood of 1895-96 nematodes were found in the plankton almost continuously till the middle of 136 April, while in the more stable conditions of the preceding year they were found in only one third of the collections. In 1897 most of the 31 collections examined were made in stable conditions, and nema- todes were found in but 5 of these, and 4 of these 5 were made in rising flood waters. In 1898, a year of greater hydrographic dis- turbance, nematodes occurred in 31 of the 52 collections, averaging 318 per m.? to 82 in 1897. Of the 31 occurrences in 1898 all but 6 were in recent flood waters. The hydrographic conditions attending the presence of nematodes in the plankton thus indicate that they are adventitious in the plankton. Further evidence of this is to be found in their erratic numbers. Thus, on February 20, 1896, none was re- corded, and on the 25th their numbers rose in flood waters to the maximum record for all of our collections—18,422 per m.? No effort was made to determine the species of these nematodes. A considerable variety of forms awaits the labors of some courageous systematist. ACANTHOCEPHALA. These worms are found abundantly in the Catostomide and other limophagous fishes of the Illinois River, and in many of the water- fowl which feed in its waters. A chance occurrence of a single specimen in the plankton on August 3, 1896, is probably to be ac- counted for as in the case of other intestinal parasites. ANNULATA. OLIGOCH ATA. The representatives of this order belong to the smaller aquatic species—generally littoral or limicolous forms found especially in decaying vegetation or among Lemnacee, and belonging principally to the family Natdide—and usually occur in the plankton in mutilated condition, since autotomy occurs when the preservative is added to the plankton. Specific identification of the fragments is therefore often impossible and usually of questionable certainty. I am indebted to Professor Frank Smith for assistance in such identi- fications as have been made. The following list (see Smith, ’00) gives the relative frequency of the species from which accessions to the plankton are made, with my notes on identified forms in the plankton. Dee, te NAIDIDA. Stylaria lacustris (L.).—Abundant. Taken in the plankton in April. Nats elingumts O. F. Mull.—Abundant. This species was identified in the plankton on April 29, 1895, during the decline of the spring flood. Slavina appendiculata (D’Udekem) (Nats lurida Timm.).—Fre- quent. Ophidonais serpentina (O. F. Mull.) (Nats serpentina O. F. Mull.). —Frequent. Dero limosa Leidy.—Abundant. Dero obtusa D’Udekem.—Abundant. This species was taken in the plankton in July and August, 1895, during the run-off of impounded waters from recently invaded back- maners. (cee Pt. I., Pl. TX.) Dero vaga (Leidy).—Abundant. Two individuals (Part I., p. 297) were found in channel waters in stagnation conditions under the ice on February 23, 1895, ata time when the plankton was almost entirely exterminated. Under normal conditions we have no evidence that this species 1s more abundant in stagnant waters. Dero furcata Oken.—Frequent. Pristina leidyt Smith.—Abundant. Pristina flagellum Leidy.—One specimen. Chetogaster limnei v. Baer.—Abundant. Chetogaster diaphanus Gruith.—Abundant. Chetogaster diastrophus Gruith.—This is apparently the most abundant species of the order in the plankton,—having been identi- fied in all months but January and May,—especially at times when impounded flood waters are drained off from backwaters, as, for example, in the March flood of 1895. AZOLOSOMATID. Atolosoma hemprichi Ehrbg.—Frequent. A‘olosoma tenebrarum Vejdovsky.—Abundant. A£olosoma sp.—Abundant. For reasons assigned above, the great majority of the oligochetes in the plankton remain unidentified and are included in our records 138 of total oligochzetes. These records throw some light on the condi- tions controlling the occurrence of oligocheetes in the plankton and their seasonal distribution. They occur in all months of the year and throughout the whole seasonal range of temperatures. They appear in the plankton most frequently and in largest numbers in disturbed hydrographic condi- tions. Thus, of the 31 collections made in 1897, only 6 contained oligochztes, and the average number per m.* was only 32. ‘Five of the 6 collections containing oligochaetes were made during the run-off of flood waters from impounding backwaters. In 1898, a year of much disturbed hydrograph (Part I., Pl. XII.), there were 52 col- lections, in 35 of which oligochetes occurred with an average number of 76 per m.* Over 50 per cent. of the non-occurrences of oligo- cheetes fall in the more stable conditions of January, July-August, and December. The seasons of run-off from impounded backwaters are in all years favorable to the occurrence of oligochates in the plankton. This is in sharp contrast with the nematodes, which appear with rising floods and access of tributary waters. The oligocheetes are thus largely adventitious, at times when run-off from vegetation-rich backwaters prevails, and when Lemnacee and Ceratophyllum are washed into the channel by hydrographic changes. ROTIFERA. (Plates III. and IV.) Average number, 592,416, of which 195,326, or 33 per cent., are eggs, free or carried externally by the parent. Records were kept of males, of females, of females with eggs, of attached and free, summer, winter and male eggs, and of parasitized and dead indi- viduals. Rotifers occur in every collection and at all seasons of the year. Numbers are uniformly low (below 75,000 per m.* and often below 15,000) during minimum temperatures from late in December till early in March. At other seasons of the year numbers fluctuate greatly, rarely reaching the level of the winter minimum except occasionally at the depressions between pulses. The curve of seasonal occurrence falls into the form of recurrent pulses (Pl. III. and IV.) previously noted for other organisms. Of these pulses the vernal one in April-May is uniformly high, attaining 3,954,920 per 1L3)9) m.?on April 24, 1896, 2,287,160 on May 25, 1897, and the maximum record of all years, 5,247,800, on May 3, 1898. Pulses in excess of 1,000,000 per m.* occur 14 times in our records: in July, August, November, and December in 1895; in April, 1896; in April, May, september, and October in 1897; and.in May, June, August, Sep- tember, and October in 1898. There is, apparently, in years or seasons best represented in our records, a tendency for a vernal pulse, often the maximum one of the year, to occur in April-May, and for an autumnal pulse of large amplitude to appear between the last of August and the middle of October. The pulses contiguous to these major pulses of the year are often of considerable magnitude; as, for example, in 1897, when the maximum of September 7 (5,121,- 000) is followed by another large pulse on October 12 (2,906,400), and in 1898, when the vernal pulse of May 3 (5,247,800) is followed by a June pulse, on the 21st,.of large amplitude (2,601,200). The recurrent character of the puises appears throughout maximum and minimum periods, and may be traced in Plates HII. and IV. Inthe period of 15 months from July, 1895, to October, 1896, there are 10 such pulses, and 6 months in which pulses do not appear. In the 21 months from July, 1897, to March, 1899, there are 18 pulses, and 3 months in which they do not occur. They often coincide with or approximate those of the Entomostraca (Pl. III. and IV.) and of the chlorophyll-bearing organisms (Pl. I. and II.). With the exceptions of the November—December pulses of 1895 at 33° (1,595,359 on November 27 and 1,636,640 on December 11) and the pulse of October 25 (1,048,620) at 48°, no pulse of con- siderable amplitude is found at temperatures much below 60° in channel waters. In the discussion which follows, 104 forms are listed, 6 belonging to the Rhizota, 6 to the Bdelloida, 91 to the Plowma, and 1 to the Scirtopoda. RHIZOTA. The Riizota by virtue of their fixed habit are represented in the plankton either by adventitious species, torn from their location on water plants or other aquatic substrata by disturbances in the water, or by colonial species with a free-swimming habit, such as Conochilus. As represented by the latter type they are of some quantitative im- 140 portance in the plankton, especially of the backwaters. Average number, 8,796. DISCUSSION OF SPECIES OF RHIZOTA. Apsilus lentiformis Metsch.—An Apsilus doubtfully referred to this species was taken December 25, 1895, and April 29 and July 23, 1896, at temperatures of 41°-78°,in each case with rising river levels. Conochilus dossuarius Hud.—Average number of females, 517. This species wes more than ten times as abundant in the collections of 1896 and 1897 asin 1898. Hempel (’99) reports it from January to September, with a maximum in March. In the plankton collections of 1896 I did not record it until June 11, at 73°. It reached a maxi- mum of 25,800 July 18, and another of 142,800 August 15, at 86°, and disappeared from the plankton September 30 at 58°. In 1897 it reappeared May 25 at 66° and reached greatest numbers September 7 at 80°, and was not recorded after the 14th. In 1898 (Table I.) it first occurred March 8, at 37°, and attained its greatest number, 14,400, on September 27 at 73°. In 1899 it returned towards the end of January, under the ice, and continued till the cessation of operations in March. It thus occurs in the Hlinois throughout practically the whole range of seasonal and thermal conditions, but not continu- ously. Colonies are of few individuals, and isolated individuals are often found in the preserved plankton. Females with 1-4 eggs were taken, and were most numerous during the rise of the pulse. About 4 per cent. of the females observed, were carrying eggs. Males were found on the decline of the pulse of July, 1896. Conochilus unicornis Rouss.—Average number of females 8,208; eggs, 100: Recorded trom Mareh’15, at 462" to july, 5a SOres pulse of 8,000 on April 26 and one of 392,000 on June 7 constitute the only fluctuations. It was not found in 1897, and only sporad- ically, during the summer, in 1896. Females with 1-3 eggs attend the rise of both pulses in small numbers. The colonies of this species also are composed of but few individuals. Conochilus volvox Ehrbg.—Average number of females, 129. A few large colonies were taken March 29 and April 5 at 49°. Megalotrocha alboflavicans Ehrbg.—Colonies of this species are found in numbers on Ceratophyllum in the backwaters, and in 1894, ala 141 when the vegetation was common along the margins of the stream, it was taken in the plankton occasionally. Megalotrocha spinosa Thorpe.—-Isolated individuals of this un- usual species were taken in small numbers in the plankton in August, 1896, at maximum temperatures, but no colonies were observed. This is one of the largest of the rotifers in the plankton, individuals measuring about 1 mm. in length. The species was described by Thorpe (93) from Chinese waters; was next reported by Weber (98) from the neighborhood of Geneva, Switzerland; and its occurrence in the Illinois is, I believe, the third record of its appearance. It affords another illustration of the cosmopolitan nature of the fresh- water plankton. In both Chinese and Swiss waters it was associated with M. semibullata Thorpe, also from Hong Kong and Brisbane. This latter species occurs in our waters also (Hempel, ’99), though it was not taken in the plankton with M. spinosa. BDELLOIDA. Average number, 7,807. They were less numerous in 1897, a year of more stable hydrograph, and fully twice as abundant in 1896, when river levels were much disturbed during summer months. In their seasonal distribution, save for the increase of Rotijer tardus in the winter of 1898, the bdelloid rotifers reach their greater numbers in the plankton in the period from March to November. There is a trace of a vernal pulse in April-May (Table I.), and some irregular summer fluctuations, attributable in the main to floods. Their tem- perature optimum seems (except in the case of R. tardus above noted) to lie above 50°. They are as a rule adventitious in the plankton, owing their presence in some cases to floods, though the vernal in- crease can not 1n most cases be attributed directly to this disturbance. The species are difficult to identify in preserved plankton material, and the list here cataloged is small. Examination of living plankton would considerably extend the list of forms. DISCUSSION OF SPECIES OF BDELLOIDA. Philodina citrina Ehrbg. was found in the plankton but once— September 14, 1897. Phuilodina megalotrocha Ehrbg.—Average number of females, 351. This species was found in the plankton (Table I.) from March 15, at 46°, to November 8, at 45°. The distribution in previous years fell 142 within these limits excepting a single record December 29, 1896, at 35°. The lower temperature limits are thus near 45°, and the num- bers are all small below 60°. The occurrences are never in very large numbers, and significant pulses do not appear—an indication that the species is adventitious in the plankton. The relative numbers in different years is suggestive. In 1896, with a total movement in river levels of 45.7 feet, the average number per collection is 770; in 1897, with a total movement of 44.8 feet, the number is 271; and in 1898, with 67.2 feet, itis 351. In 1896 a much greater proportion of the change in levels took place (Pt. I., Pl. XI.) during the summer, when P. megalotrocha is present. With this in mind, it is apparent that a disturbed hydrograph tends to increase the number of this species in the plankton. A comparison of the individual occurrences (Table I.) with contemporaneous conditions of the hydrograph (Pt. I., Pl. XII.) in 1898, and in previous years also, shows that most of the larger records were made in planktons from a rising river. For example, the largest record made—8,000 on September 27, 1898 —is on the crest of a slight rise (Pt. I., Pl. XII.). Some, however, appear in stable conditions, and may be attributed to the other causes of disturbance of the bottom and littoral fauna which tend to bring its constituents temporarily into the domain of the plankton. Rotifer neptumus Ehrbg.—Average number, 425. This species was found in the plankton in every month of the year but February, and thus throughout the whole temperature range. Between Novem- ber and March the records are scattered and the numbers small, while it is continuously present in larger numbers from March (50°) till late in October (50°-60°). The optimum temperatures thus seem to lie above 50° in our waters. The largest numbers recorded (22,224, April 29, 1896, at 72°, and 6,400, May 17, 1898, at 64°) attend the vernal volumetric pulse. Aside from this season, well-defined and symmetrical pulses are rarely traceable in the small numbers recorded. Some of the larger records, for example that of July 28, 1896 (10,200), attend rapidly rising water, but dependence generally upon this agency for presence in the plankton is less directly evident in this species than in the preceding. As also in-the case of R. tardus, the average number (246) in 1897, a year of more stable hydrograph (Pt. I., Pl. XI_), is greatly exceeded by that in 1896 (2,323), when the hydrographic conditions during summer were much disturbed(Pt. I., Pile Xe): 143 Rotifer spp.—Average number, 199. Some bdelloid rotifers un- identified because of the state of their contraction, or not even questionably referable to other species listed, are here included. It is quite probable that some individuals belonging to the genera Philodina and Callidina are among the number. The occurrences are irregular. They exhibit a distribution with respect to years similar to that noted in the two species just discussed. Vernal pulses are noticeable in 1896 on April 29 (19,446), on April 27, 1897 (28,800), and May 3, 1898 (3,200). Egg-bearing females were noted in the winter months of 1899, in December and March of the preceding win- ter, and in April, 1896. Individuals parasitized by Endophrys rotatoriorum Przesm. (?) were noted in April, 1896. Rotifer tardus Ehrbg.—Average number, 6,688. This is the most abundant of all the bdelloid rotifers in our plankton, outnumbering all the others in 1898 six toone. This was due to a sporadic and un- usual pulse of individuals in the plankton in midwinter under the ice in 1898. Owing to this, the average number in 1898 exceeds that in previous years. If, however, the large numbers in January and Febru- ary, 1898, be reduced to normal winter proportions—no record in 1896 in this season exceeds 7,000—the average for the year falls to about 3,500. The average of occurrences in the plankton for 1896, 1897, and 1898 would then be 5,201,1,254,and 3,500, which approxi- mates somewhat the ratios of the relative disturbance of the hydro- Peapm in these years (Pt. I., Pl. X.—XII.). The agency of flood water in affecting the numbers of this species in the plankton is to some extent indicated by this ratio. It is also apparent on compari- son of the seasonal distribution (Table I.) with the hydrograph for 1898 (Pt. I., Pl. XII.). The large numbers of January, February, and March appear in every case with rapidly rising water, and the same is true of the numbers on August 9 (12,000) and September 13 (17,- 500). Other disturbances than those due to floods, or other factors than disturbances in the water,must be invoked to explain such in- creases as one to 12,800 in April—May, 1898 (Table J.). This attends the vernal volumetric pulse (Pt. I., Pl. XII.), but does not conform to its proportions. It appears in the more stable conditions of de- clining flood, and no adventitious factor is apparent to account for its development to such numbers in the plankton. The winter pulse was attended by large numbers of ovigerous females, but none was recorded during this vernal pulse. A somewhat similar increase 144 in stable conditions was found in March and April, 1896, from 40°-72°. Temperatures of 50°-70° were several weeks earlier than usual this year, but the increase in R. tardus came at lower temperatures than in 1898. Asabovestated, this winter pulse, or, rather, sequence of three pulses (Table I.), culminating January 25 (89,397), February 15 (27,000), and March 15 (19,200), came with floods. No such increases attended the somewhat similar hydrographic conditions (Pt. I., Pl. XIII.) of 1899 nor the winter flood of 1896. There is nothing in the eviron- mental data to explain this unusual occurrence. An unusually large number of females with eggs still attached to the body were seen in the period from’ January 21 to April 12. Fifty per cent. were ovigerous, carrying a single egg. Numbers of similar free eggs were also noted. Rapid multiplication of the species at the time of these pulses is thus suggested, and these may be dependent upon favorable conditions of nutrition of whose nature no clue is suggested. The Species is in the main adventitious, with insufficient evidence of a par- tially limnetic habit at some seasons. The species occurs in almost every one of the plankton collections, and thus throughout the whole range of temperatures and environ- mental conditions. The largest numbers were taken during minimum temperatures under the ice; but large numbers also appear at other seasons, and no temperature optimum is definitely indicated, though in years prior to 1898 the larger numbers and more regular occur- rences are to be found in the period from March to November at temperatures above 50°. Rotifer vulgaris Schrank.—Average number, 275. This species has a seasonal distribution—though in smaller numbers and fewer occurrences—which corresponds somewhat closely with that of R. neptunius (Table I.). The same factors in the environment are pre- sumably operative in modifying its appearance in the plankton. Hempel (99) finds R. macrurus Schrank, Philodina macrostyla Ehrbg., and Callidina elegans Ehrbg. in the plankton of Quiver Lake, adjacent to the river. PLOIMA. Average number, 571,611, including eggs, which constitute about 30 per cent. These rotifers occur at all seasons and are found in every collection. They are quantitatively the most important order 145 of the Rotifera. They include about 97 per cent. of the individuals and almost all of the limnetic species. As a group they exhibit a seasonal routine which is a complex of the records of individual species, and as such it reflects to a remark- able degree a similarity to individual records, especially of the peren- nial species. In general the Plowma are less abundant in colder months, that is, below 50°-60°, than in the warmer ones from May to October. Midwinter numbers are nevertheless considerable,—5,000 —35,000,—and with the first rise of temperature in March we have, in 1898, a pulse of 175,000 which declines and again rises in a vernal pulse of April-May, which vies with an autumnal pulse for rank as the annual maximum. Following the vernal pulse there comes a series of summer movements which vary from year to year. In 1898 they grow smaller as the season wanes, rising againin September. In 1897 the autumnal pulse is the largest of the year and appears early in september. In 1895, on the other hand, it is carried into the last days of November. Numbers sink to the winter minimum shortly after the winter temperatures are reached. In a general way the direction of movement 1n the several parts of the seasonal curve of the total Ploima is much like that of the individual species of which it is composed. The differences le in the amplitude of the pulses and in slight changes in the locations of maxima and minima. There are, it is true, many exceptions to this sweeping general statement, but it is, nevertheless, both surprising and significant that the sum of so many complex records should still preserve the recognizable outlines of its parts. This is not due simply to the dominance of a few abundant species, but is a combination of many, as will be seen frequently in Table I., where species with insignificant numbers still show in their seasonal occurrences some correlation with the movement of the great mass of the totals. This similarity points to some common factor in the environment common to all of the species. It is to be found, I believe, in the food relations—in the wax and wane of the food supply. Most, if not all, ploiman rotifers are herbivorous, or at least omnivorous, and find their food to a large extent in the phytoplankton. I have already called attention to the recurrent pulses of the chloro- phyll-bearing organisms. These primarily, but combined with other and largely changing seasonal factors such as hydrograph and tem- perature, are the basis upon which the superstructure of the seasonal changes in the ploiman plankton are built. The correlation between P1Ooa1 4SPT IO ‘99USIINDIO SNONUTZUOS OLOUL JO WOs¥as dy} 0} JUONbasqns 410 z011d aUO poqeost ue st 4SIY B OJ UIATS sazEp OMY OY} Jo ‘AjaATJOadSoL ‘4s¥] IO JsIY 9Y} ‘oINYeU ILIIWIIS JO sa_qe} yUaNbesqns puv siy} UT» OZT OEE | OZ 99M | OOZ'STT | ST “AON | O2T'SZ6 | SZ “390 [008767 ‘T| Lz “3dag Ore $59 z ‘Bny | o9¢‘rF9 | or Am | gost O70'S6 |oe . | 000‘ TS FI “998d | 00S ‘8F 6 “AON | 008‘0S4‘T| ZT “390 jo00‘¢s¢0‘¢|Z “3dag 007‘ sor | 12 AML | Lost OF9'6TT | 9% ‘8nYy 090! #61 a Ammf | 9681 OSPF PLT‘ TI] TT “99d | £89°667‘T] LZ “AON $88'08 |z1 3deg jost‘co0'T| zt ‘8ny | oo4‘0e9 | 9 Amf | sost cps ‘19 Lt “PO 6FO'T6e | ST “sny FO8T ‘ON a1eq ‘ON a1eq “ON a1eq ‘ON a1eq ‘ON nae ‘ON a1eq reo X Ozs‘T8 | £4 ABW i00O'SZT | FT “G9q | Oze'sz | ZT “Uefl | 6681 QOG Deo TT aounl 000 cra cc Seale | 0STech loosen LTOWG) “Sie BUN eos |) OO TN Gee Neri LOST CHOOT WNGE os CUOECAS, GCE as OOF‘ cez | LT eunf| Itr‘sez | 8 AeW | srs‘trs‘s| Fz sdy zoe'st | 9 “Uefl |x968T Sg sto‘99g | 6c sdy S68T —— a FOS “ON aye q ‘ON a1eq “ON o4eq ‘ON o1eq “ON o1eq “ON a4eq Iva X ‘SOOW ONIGAIOXG ‘VWIOTG JO SaSTINg 146 147 the seasonal distribution of individual species and these recurrent plant pulses will be discussed in connection with the various species wherever the data are available. For the present it will suffice to call attention to such correlation as exists between fluctuations of the phytoplankton and the total Plowma. The table on the preceding page gives the location and amplitude of the maxima of the ploiman pulses, and a graphic presentation of the seasonal curve of distribution of the total Rotifera will be found in Plates III. and IV. On comparison of the plorman pulses with those of the chlorophyll-bearing organisms, graphically presented in Plates I. and II., it will be found that 15 of the 33 pulses of Plotma contained within the period covered by the plates coincide in location with the plant pulses; that 12 follow at the next collection, usually a week later, and 3 within a fortnight; while only 3 of the 33 exhibit no such correlation. The data suggest strongly the agency of the plant pulses in building up the Ploima, and that the food relations are fundamental in the fluctuations of these planktonts. DISCUSSION OF SPECIES OF PLOIMA. Anurea aculeata Ehrbg.—Average number, 1,839. In 1898 this species has a very well-defined and characteristic seasonal distribution (Table I.). It first appears March 8 at 37°, increases to a maximum of 45,200 on May 10 at 61°. then declines, and disappears June 14 at 83°. The curve of its occurrence in this year is a very symmetrical one. It reappears on December 27 at 32°, and there are scattered occurrences through the winter months of 1899. Records in other years suggest in the main a similar distribution. In 1896 it first appeared January 6, rose to a pulse of 6,550 on May 8 at 76°, and, on the decline of the June rise, there was a second and larger pulse of 29,600 on June 17 at 76°. It reappeared on Decem- ber 29, and in 1897 reached a vernal maximum of 22,400 on May 25 at 66°, then disappeared, and was not again noted in the following winter nor until March 8. In 1894 the last vernal record was made June 12, and on September 4, at 78°, there was an autumnal pulse of 13,825—a phenomenon not repeated in subsequent years. The normal course of its seasonal distribution in the river plankton seems to be as follows: reappearance in December when minimum temperatures have been reached; slow multiplication during the winter, and a well-defined pulse on the decline of the spring flood in (11) 148 April-May with the possibility of a second on the June rise; and prompt and complete disappearance when maximum summer tem- peratures are established. Low water in the autumn seems to inter- fere with an autumnal pulse. In 1894 there was a well-sustained rise in September (Pt. I., Pl. VIII.) and a pulse of A. aculeata. In 1896, however, no pulse occurred in the high water of the autumn. No midwinter occurrences followed the very low water of 1897. It is thus in channel waters a vernal planktont, with its temperature optimum near 70° but below the summer maximum. Hempel’s statement (99) that it is a “winter species’ is borne out by its presence from December through the winter, but its numerical distribution ranks it at once with the vernal organisms. Lauterborn (94) finds it abundantly in winter months in the Rhine, and Ap- stein (96) speaks of it as a “Sommerform,”’ absent from Lake Plon from November till March, and with maxima from April to July in different bodies of water where it continues through the summer and till October, and then disappears. Summer temperatures in these waters, however, are not recorded by him above 21° C. (69.8° F.), which is about the temperature at the time of the vernal maxi- mum in the Illinois, and at least 10° F. below that of the summer maximum in our waters. Jennings (94, ’96, and ’00) records it as abundant in the summer plankton of Lake Erie, Lake Michigan, and some inland lakes of Michigan. These waters also are somewhat cooler (5°-10° F.) than those of the Illinois River in midsummer. Temperature, it seems, must have a decided effect upon the seasonal distribution of this organism in our waters, though the chemical conditions and food supply may also enter as factors in the summer suppression of the species. Females carrying usually a single egg appeared in 1898 early in April, and were most abundant during the maximum of the pulse. On an average, less than a fourth of the females were ovigerous. Empty loricze appeared May 10 (4,800) and 17 (3,200) at the crest and decline of the spring pulse, and the same phenomenon of deca- dence was noted in previous years during this period. Outbreaks of parasites were not recorded for the species, and the decline is to be attributed to cessation of reproduction and to the death and destruction of the individuals by the more usual causes. This species is quite variable, but no effort was made to follow its seasonal history. The type form is by far the most abundant. FA fr 149 A. aculeata var. valga Ehrbg. was seen frequently. A. serrulata Ehrbg., regarded by Weber (’98) as a variety of A. aculeata, was recorded Jan. 24, 1899, and found by Hempel (’99) in December. It seems to be rare in our plankton. Forms approaching A. aculeata var. brevispina Gosse were also noted, but they, too, are rare, being recorded only in February and March, 1899. A. aculeata var. curvicormis Ehrbg. was noted April 29, 1896, at 70°. Anurea cochlearis Gosse.—Average number, 69,393, distributed as follows: A. cochlearts (sensu strictu) together with A. cochlearis var. macracantha Lauterborn, 9,421; A. cochlearis var. tecta Gosse, 15,432; and forms with posterior spine of intermediate length between cochlearts and tecta which include A. cochlearts var. stipitata Ehrbg., 44,540. Numerically this is one of our important species, containing over one ninth of all the rotifers in 1898. It is surpassed only by Brachionus bakert (with varieties included), Polyarthra, and Syncheta. Average number of eggs, 32,358. This is a perennial planktont, appearing in every month of the year throughout the whole range of temperature. Its entire absence in August, 1898 (Table I.), is not paralleled in any other year. In 1897, for example, there is a well-developed pulse of 45,600 on August 24. In1894, 1895, and 1896 there is a midsummer minimum of a few weeks’ duration in July, August, or September, but it is irregular in its location. While the appearance of sexual cycles was not traced by the records of males and winter eggs,—a matter of some difficulty and uncertainty in preserved plankton material,—the existence of such cycles is suggested by the recurrent pulses of occurrence in this species (Table I.). It is possible that the species is polycyclic in our waters. The pulses in 1898 are well defined, in fact, somewhat better than in previous years. The following table gives the num- bers in the pulses in the several years and the dates and tempera- tures at which the maxima occurred. All of the large pulses save those of November and December and one at the close of October (Oct. 25, 1898, 28,500) lie at tempera- tures above 60°. The vernal pulse of April-May is the largest and appears between 60° and 70°, and the amplitude diminishes as the period of maximum heat progresses, though in 1898 there was a recurrence of larger numbers as temperatures fell. The optimum 150 PuLses OF ANURZA COCHLEARIS. Year Date Temp. No. Date Temp. No. Date Temp. No. 1894 — = June 12 78° 1,344 ae 1895 | Apr.29| 64° 180,480 July 18 | 80° 17,805 1896 | May 8 76° 100,870 June 11 a> 95,200 july 2 81° 12,800 ‘ S 28 81° 17,600 1897 | May25 66° 620,800 July 21 82° 37,600 1898 | May10|] 62° | 1,145,600] June 21 77° | 372,800 | July 19 g4° 17,200 Year Date Temp. No. Date Temp. No. Date Temp. No. 1894 Sept. 4 78° Ueno) 1895 | Aug. 21 83° 17,805 Sept. 23 io ue ay al Nov. 20 44° 1120 1896 | Aug. 21 79° 5,600 Sept. 16 (pal 6,224 Dec. 29 B52 3,840 1897 | Aug. 24 78° 45 ,600 Octs, 5 70° 4,800 1898 | ———— | ———— ——— Sept. 27 ie 54,400 Nov. 21 40° 10,000 Oct 2'5 48° 28,500 : conditions seem thus to be found in the river at temperatures some- what below the maximum, between 60° and 70°. The phenomena of recurrent pulses are distinctly traceable in the seasonal distribution of this species, not only in 1898 (Table I) but also in preceding years. The large May and June pulses of 1898 appear on the declines of the spring and the June rise, respectively ; the pulse of September 27 is in a falling river; and that of October 25; on_a-slowly rising flood (Pt..1., Pl XU). Inviso7 (ec eer XI.) the first two pulses attend the spring flood and June rise in like manner, but the two subsequent pulses are in stable low water. In 1896 five of the seven pulses lie on the declines of the recurrent floods of that year and two in rising waters (cf. Pl. X. of Pt. I. and the table just given). In 1894 and 1895 the pulses appear either - in falling water or in the earliest stages of the rise. The number of pulses on declining waters is somewhat greater than the relative number of days of this condition would lead us to expect, and it seems probable that optimum conditions for the appearance of larger numbers of Anurea cochlearis are to be found in such hydro- graphic conditions. The run-off of impounded backwaters is one of the favorable phases during flood decline. On the other hand. wet “ED was Sil the distribution of the pulses with reference to the floods and the appearance of pulses during rising water suggest the operation of other factors than the one arising from contribution from back- waters. The pulse must be dependent to a large extent upon food supply of the organism, and a correlation between its periods of multiplica- tion and the pulses of its food, the chlorophyll-bearing organisms, is to be expected. A comparison of the seasonal distribution in 1898 (Table I.) and the pulses of chlorophyll-bearing organisms (PI. II.) reveals the fact that three of the A. cochlearts pulses coincide with those of the plants constituting their food, and the other three coincide in part only, the remainder of the chlorophyll-bearing groups reaching their culmination a week prior to that of the rotifer. In 1897 the three pulses of A. cochlearis which lie in the common period (Pl. II.) all culminate a week (in one case in part in fourteen days) after the maximum of the plants in question. In 1896, three pulses coincide and three follow in the subsequent collection; and in 1895, two coincide and two follow. Collections at daily intervals would be necessary to follow the correlation more accurately. It is probable from these juxtapositions and sequences in the A. cochlearts-algze pulses that we are dealing with a food relation. Multiplication of algae leads to increase of Anurea, which, in turn, reduces the algze,and then itself declines until the food planktonts again increase. Anurea cochlearis is exceedingly variable in the length of the posterior spine, in the development and degree of curvature of the anterior spines, in the arrangement of the areas of the lorica, and in the degree of its ornamentation by small spinules. The separation of these varieties where every individual must be assigned to some one of them,is a matter of some difficulty owing to the presence of intergrading individuals. The characters which signalize var. Iispida Lauterborn and var. trregularts Lauterborn are not quickly recognized under the conditions of rapid plankton enumeration, and no effort was made to trace their seasonal distribution in our plank- ton. Lauterborn’s var. macracantha was included with the type form—his var. typtca—in our records. These two include those individuals with medium-sized and longer posterior spines. In our waters the variety macracantha is relatively rare, at least as figured by Lauterborn (’98). Indeed, both the type and this variety const1- 152 tute less than a seventh of the total representatives of the species. Their distribution throughout the year (Table I.) accords with the | results obtained by Lauterborn (’98), who found that the average length of the posterior spine from January to May and from October to December was from 78 to 48, while from June to September it was from 28.5 to 21y. .In Table I. it will be seen that the longer- spined forms which I have referred to A. cochlearts var. macracantha and var. typica occur in the plankton from January to May 31, and then disappear, returning again, in small numbers, October 25. The short-spined variety referred by me to A. cochlearts var. stipitata and the spineless var. tecta are, on the other hand, continued during the summer. The natural result would be that the average length of the spines in the species as a whole would fall during the summer months. It, 1s) apparent that, this tendency on the parteoteae cochlearts to become shorter and smaller during the summer months does not bear out the contention of Wesenberg-Lund (798) that winter individuals are smaller and summer. ones larger among perennial rotifers. He reports.var. tecta as ‘‘die. Hauptform des Winters” in several Danish. lakes, and the variety with a long horn as a summer form, found in July—August. Of these varieties, macracantha, typica, and stipitata intergrade in our waters with numerous connecting links, while var. tecta is not connected with the other forms by many individuals with inter- mediate characters. Lauterborn (98) also notes the greater inde- pendence of this variety in the waters of the Rhine. In Table I. the seasonal distribution of these three varieties, the long-spined (typica and macracantha), the short-spined (st¢pztata), and the spineless (tecta) are given separately. It will be noted that the long-spined form has the distribution above mentioned, that var. tecta runs throughout the whole year, and that var. stzpitata is absent in midwinter and isa common summer form. The relative numbers of the varieties fluctuate in different years. For example,. var. tecta was relatively but one fourth as abundant in 1897 as in 1898. As shown in Table I., whenever coincidently present in the- plankton all the varieties respond to the causes which produce the rhythm of occurrence, the rise, culmination, and decline of the pulses being much alike in all of the varieties. About three eighths of the females noted in 1898 were ovigerous, carrying as a rule but a single egg. Instances of two eggs were 153 noted, but they are rare. The greatest proportion of egg-bearing females appears during the rise of the pulse, as 1s seen 1n the follow- ing table, which gives the data of the vernal pulse in 1898. From ANUR#ZA COCHLEARIS. | No. of | | Ratio of Date | ovigerous Total Total | eggs to No. of females females eggs | individuals dead oe lee ea | 800 DRGs We SO0s|< fF 2075 0 \jostill WO ite eae eae ea | 6,400 15,200 8 800 | Bale 7h} 400 PNG OTRO} sone) os even ew aaces | 45 ,000 137,800 65,000 | 1 Be 3,200 25S ee | 536,000 1,022,400 | 552,200 | 1 £185 | 9,600 JIL 597-1 OS rae eae | 489 , 600 1,145,600 | 643,200 | if S eei(htes | 99 , 200 UES? Ih AgeS ea eee | 110,400 434,800 | 160,000 | Lest 2ia7 ty 2 100, 008, WWeimeP eb teste ats zoe cet | 6,000 21200 | LOO | es 2 OAS 1,800 IMIG? Sh Shee see teee ee eee 3,000 11,200 S4OOR is Se 29 | 1,800 April 12 to the crest of the pulse on May 10 (not inclusive) the aver- age ratio of eggs to individuals was 1 to 1.87. From the.crest to the foot of the decline inclusive the ratio is 1 to 2.98. The number of empty loricie is given below, and it will be noted that on the week prior to the crest of the pulse there were 107 living to one dead;on the crest itself, one to twelve; while the week following the crest of the pulse there was an empty lorica for every 4.3 living females. Rapid multiplication thus attends the rise of the pulse and rapid destruction its decline. Parasites were very rarely observed in this species. The decline of a pulse is thus due to the cessation of reproduction and a relatively heavy death rate. Apstein ('96) finds that in Lake Plon Anure@a reaches its maxi- mum in July and is at its minimum in April. It is everywhere common in the German waters. A. tecta, on the other hand, was found only in the smaller lakes and in great numbers, replacing cochlearts in warmer months to some extent. Lauterborn (98) regards it as the most abundant rotiferin the Rhine. Our statistical records do not show that this is the case in the Illinois, for it is here 154 surpassed by several other species. Zimmer (’99) finds that this species is the most common winter rotifer in the plankton of the Oder, with a maximum in the spring and a predominance of var. tecta from July to September. Schodrler (’00) finds 1t to be the most common rotifer in the Elbe—from April to November; and Skor- ikow (’97) finds it in the Udy, in Russia, throughout the summer in great numbers, but surpassed by Syucheta, Polyarthra, and Brachionus angularis. The variety tecta greatly exceeds var. stipitata in these waters. Seligo (00) finds it throughout the year in Prussian lakes near Danzig, with a maximum in May. There are indications, in his data, of recurrent pulses during the summer, but his interval of collection is too great to follow their history. Burck- hardt (00a) finds it throughout the year in Swiss waters, with its single maximum in August. Jennings (’94, ’96, and ’00) reports it in the summer plankton of Lake Michigan and Lake Erie and of inland waters of Michigan. Anurea hypelasma Gosse.—Average number of females, 2,390; of eggs, 1,917. This species has a very definite limitation to a period extending from early in June to the first days of November. There are but two records outside of these limits—a single female and egg on Jan. 11, 1898, and another upon April 19 of the same year. The probabilities of occurrence in very small numbers at all tempera- tures 1s thus indicated. The following table gives the data of pulses and temperatures. All of the pulses save one occur at temperatures above 70°, and with this exception the species declines rapidly and disappears shortly after temperatures pass below 60°. It is plainly, in our waters, a summer planktont, with its optimum temperature close to the summer maximum. This species takes no share in the vernal pulse, and there is no satisfactory evidence of any fluctuation corresponding to it at any other season. There are three or four pulses in each summer, and the species is apparently polycyclic, for winter eggs were found in 1898 either at the maximum of the pulse or the week or fortnight following. Thus 24,000 winter eggs were recorded on Sept. 27, 1898, the date of the maximum of the Septem- ber pulse. The parthenogenetic eggs preponderate during the rise of the pulses in a very marked manner in this species. For example, in this September pulse 55,400 eggs were recorded during its rise to 500 during its decline. In like manner, in the case of the 185) Putses oF ANUR4ZA HYPELASMA. 9 First record Pulses Year Date Temp. Date emp, | No: TIDE, i sash ae ee eee June27:|) 280° June 27 B0° "|| 4;,200 ODF cin Ae Re a Pe eee June 28) 75° July 14 92 10, 400 | | LBOS sa 55 SAsegto cenit aire ieee June 14 83° June 21 lie T9600 Pulses Last record Mear Date Temp. No. Date | Temp: No. Date | Temp. 1896 | Aug. 15 PS Oey | | Seo Ol ee oe DBS) 74° 3,600 1897 Aug. 31 80° | 20000) |}Oct.~ 5 TS 23,200 | Nov. 2 SD 1898 | Aug. 16 77° | 16,000 | Sept. 27] 73° | 43,200 | Nov. 1 45° Octo 1s ony 13,500 August pulse 15,200 eggs were found on the rise to 4,000 on the decline. The location of the pulses of A. hypelasma is of special interest. It will be seen in Table I. that they occur in 1898 in the same col- lections in which the pulses of the other species of Anurea and many other rotifers occur, or in collections but a week removed. They coincide in general with dates of the ploiman maxima noted in the opening discussion, and exhibit the same correlation with hydro- graphic conditions and intercalation with the pulses of chlorophyll- bearing organisms which were noted in the general discussion and have been found in preceding species. The comparison with Anurea of the cochlearis group affords a curious instance of an entire sup- pression (Table I.) of one species of a genus (cochlearts) in the month of August and the occurrence of a normal pulse in another (ypelas- ma). Comparison of the distribution of cochlearis in previous summers would lead us to expect a cochlearis pulse in August, 1898, 156 but none appears in this interval, while hypelasma runs a normal course of recurrent pulses throughout the summer. This August pulse of hypelasma (Table I.) culminates August 16, just a week after the symmetrical and well-defined pulse of chlorophyll-bearing organisms (Pl. IT.) of August 9. With a single exception, all of the pulses of 1896 and 1897, indi- cated in the table, fall a week later than, or coincide with, the pulses of chlorophyll-bearing organisms, as in 1898. This species has not occupied a prominent place in the literature of fresh-water plankton. Weber (98) finds it rare in Swiss waters in the summer. Lauterborn (’93) classes it with the monocyclic summer forms in the plankton of the Rhine, though he states in a footnote that he had found winter eggs once in June. It is probably polycyclic in our waters. Skorikow (96) finds it in the summer plankton of the river Udy, in Russia, but it is not mentioned by other investigators of the potamoplankton of Europe. Apstein (’96) does not report it from Lake Pl6n. Asplanchna brightwellit Gosse.—Average number, of adults 2,079, of eggs, 396; averages in 1897, 16,161 and 2,156. This is a poly- cyclic perennial planktont in our waters. It has been found in every month of the year, but the greater numbers and more con- tinuous occurrences lie between May 1 and October 30. In 1898 (Table I.) all but 200 of the 108,120 recorded, lie within these limits, and all but 260 above 60°. In previous years approximately the same limits are found. The following table gives the data of pulses and temperatures. PuLsEs oF ASPLANCHNA BRIGHTWELLII. Year Date |Temp. No. Date |Temp.| No. USGA. £22: Se Re aero ey eecees ——— | —— | —— | —— | — | —— {posien! Wor, yom Ilana atte “June 19| 80° | 6,678 | TS9ON Gen ater tae eee ne eae May 1 | 70° | 25788) | June 27 802 1,600 USO 1s tis teehee ee, Oe een <= =" oa) = es 1898)... oe ey ee May 5 | 60° |20,800 | June 21] 77° | 1,100 157 PuLSES OF ASPLANCHNA BRIGHTWELLII—Continued. Year| Date |Temp.| No. Datem|emips|) No: Date |Temp.| No. me) faly 30°) 82° | 19,393" | ——-— fe9on | july 29 | 75° 1,344 | Aug. 12] 79° |118,206 | Nov. 14] 45° 125 Bier | | A 21 |- 79° | 1,200 1897 | July 21| 83° | 3,200 | Aug. 10] 81° | 5,200|Sept. 9| 80° [284,000 1898 | Aug. 2] 79° | 23,200 | Aug. 23| 81° | 4,000 | Sept.27| 73° | 6,400 It will be seen from this table that all the pulses save one, and that one (Nov. 14, 1895) poorly defined, lie between 60° and the maximum temperatures, indicating an optimum near the summer maximum. There is in this species no prominent vernal pulse such as that found in Anurea, and the highest numbers were reached _ during the height of the warm season. The evidence of the polycyclic character of the seasonal distribu- tion of this species is shown in the following table, which gives the occurrences of ovigerous females, males, and winter eges in 1898. It will be noted that ovigerous females are more numerous during the rise of the pulse; that the males appear just before, during; and after the culmination of the pulse; and that winter eggs are absent only during the rise of the pulse, and appear at or after its culmina- tion and during the decline. The data given afford a fine illustration of the seasonal distribution of polycyclic rotifers,and of the relation of the sexual cycle to the number and character of the representa- tives of the species in the plankton. The growth of the pulse results -- from a rapid succession of parthenogenetic generations in the course of about two weeks, and it culminates with or shortly after a pulse in the food supply. The decrease in food supply is attended by the appearance of males and winter eggs, a decrease in ovigerous females, and a decline of the species. With the recurrence of the food supply the parthenogenetic cycle again begins. The same course of events is run in each recurrent pulse. Food supply rather than temperature seems to be the determining factor in this rhythm. 158 ASPLANCHNA BRIGHTWELLII. Date Males ae hanes gs IME Beare 2, Sane Shs sepa ake Mace eee To 0 3,200 Bp LUO 5, siicysvemente,vss ere cueyetensh Mereweene ere 8,000 4,800 Seyi Ts, ate cat Rrseen ty poets arate meng ances Oe 1, 600 5,600 a2: a eR PE MNT Oe aa hy econ — 400 So Sia ale helee ay ak rcs meee td oar ME —_— 200 fl .Uhate mney Meeeer aaron ace oe tena er eee OP eh a 200 ee UAL gy. Aceieg ate cana avin malar cack cele uae — 0 SY Wali oat ha teme ait cues eineate ar ee —— 800 PE AS Ahe oi esata Se iet nyerinuastahe Shere eee ahi ae SSS 100 A calla re Shcye doe eh cr acer se auch nena deueun este —— 120 PLD hth ci ee Sette te dae cae eas —— 0 pe DO icc bin Mauaiameune aovyae Mayes =—— 40 210, Pe Perel Paar, eerste A nl ek anal 240 12,400 Pe NVORCA ES) Fam Ang es arr ne ena ee Lo 4,000 7,200 = OIE ci ptaas! Se eile ene te meee — 80 ; NGS Paes. shee cathe eierscs ee ereus ie otha eee ——= 0 DSi sgelteMene ts see SVS eae aad Pipe eps ele == 3 200 os, ied Ofes SheLeraue enc my eters a pera p ees — 1,600 DeplteMiber On amass wt aan ee ee —— 0 4 PSiggsssrant eove ciety sick ren — 0 i DON ake oreo a see e en ere == 540 a Zilicapatcs® Sea kio Nit se ee — 3,200 October. Aon Site Steep) eee — 500 + Lids Meat ors ena are ak seco aca ake ee — 1,000 Ovigerous females 12,800 8,000 4,000 O O 300 Winter eggs 159 An examination of the location of the pulses of Asplanchna brightwellit shows (Table I.) that in 1898 one coincided with the pulse of chlorophyll-bearing organisms (Pl. II.) and the remaining four followed it either in a week or fortnight. In previous years two pulses coincide with and five follow those of chlorophyll-bearing organisms, and a single ill-defined one (Nov. 14, 1895) precedes. This species is not wholly herbivorous in its feeding habits. Codonella, Difflugia, and even other rotifers such as Brachionus and Anurea,are frequently seen in the digestive tract. Diatoms, even Melosira and Peridiniude, as well as Pediastrum and other alge, are frequently taken as food. Inone instance a Daphnia cucullata 300 in length was seen in the stomach in a transverse position. It was fully a third the length of the animal which had eaten it. Asplanchna brightwella is reported by Skorikow (97) in the summer plankton of the Udy, in Russia; by Schorler (00) as spo- radic in the Elbe in June and September; and by Lauterborn (93) from the Rhine, where its cycle coincides with that of A. priodonta. Zacharias ('98) reports 1t in German reservoirs in June and August. It is a cosmopolitan species, but does not seem to have been found by other plankton investigators in European waters. Asplanchna ebbesborni Huds.—Average number of adults in 1895, 942. In 1898, only winter eggs of the species were noted in the plankton in February, June, July, September, and October, though adults were doubtless there. Adults have doubtless oc- curred sporadically in all other years, and in 1895 reach a pulse of 21,518 on July 6 at 81°, which was followed by the appearance of males and winter eggs. All records of adults lie between April 29 and September 14 and above 60°. This rare rotifer has not appeared in the literature of fresh-water plankton elsewhere to my knowledge. Hempel’s statement (99) that his record of its occurrence in the Illinois is the first for this continent must be modified, since Leidy (87) found it near Philadelphia. It is evidently a summer plank- tont in our waters, and the wide distribution of its winter eggs suggests that it, too, may be polycyclic; and their appearance in the plankton in large numbers with reference to the adults taken, leads to the further inference that its center of distribution is prob- ably not in channel waters, and that it may be predominantly limicolous species, or have its center of distribution in the quieter backwaters. 160 Asplanchna girodi de Guerne is reported by Hempel (’99) in the backwaters in April. Asplanchna herrickt de Guerne.—Average number, 15; in 1897, 295: in 1896, 317. This species was always rather rare in our waters, and is apparently a summer planktont. The earliest record is April 29, at 64°, and the latest, November 15, at 48°. There is an indication of a vernal pulse in April-May in 1896 and 1897, and the recurrence of the species at intervals of a few weeks during the sum- mer suggests a polycyclic habit similar to that of other members of the genus in our waters, but the data are insufficient to follow the cycles if such exist. Ovigerous females were present when numbers were greatest, and males and females with winter eggs were found at the time of the vernal pulse on May 25 (3,200) in 1897. Hempel’s statement (99) of its rarity in June and July is not borne out by the statistical records in these months in 1896, 1897, and 1898. This rotifer is abundant in the summer plankton of Lake St. Clair, Lake Michigan, and lakes of northern Michigan (Jennings, ’94 and ’96), and it may be significant that it reaches its greatest devel- opment in the Illinois in the spring at 60°-70° and not during the period of maximum heat. This is about the summer temperature of those northern waters. This species has not to my knowledge appeared in the literature of European plankton, though it is found in European waters. Asplanchna priodonta Gosse.—Average number, 441; winter eggs, 7. This species is much less abundant in our waters than its associate A. brightwelli1, being outnumbered by it five to one in 1898. It is in the Illinois River a summer planktont only, at least so far as the records go, though reported elsewhere as perennial. The earliest record in any year is April 29, 1896,at 70°, and the latest October 5, 1897, at 70°, when an unusual pulse of 22,000 was found. The records are too scattered to trace the seasonal history. There are only indications of recurrent pulses. In May, 1898 (Table I.), the best-defined pulse is recorded. The details, which conform in the main to the sequence noted in A. brightwellit of ovigerous females with summer eggs during the rise, with males and winter eggs at and after the culmination of the pulse, are given in the appended table. This is the only cycle found in this year. The presence of ovigerous females and winter eggs at other seasons as well, in other 4 wy 161 ASPLANCHNA PRIODONTA. [ | Females | Females |. Females Males | without | with sum- | with winter a fee eggs | mer eggs eggs = | ee ee —— 3,200 od oe 3,200 er ee oe 2 800 | 10,400 3,200 — 14,400 RR orl cre 120 {| 1,600 200 200 2,120 ee ee — = 1,600 400 — 2,000 years, leads us to infer that the species may be polycyclic in our waters. This limnetic rotifer figures largely in the fresh-water plankton of other localities, attaining a relative development greatly surpass- ing that thus far found in the Ilhnois River. Apstein (’96) reports it of irregular occurrence in the smaller lakes of Holstein, and Seligo (00) finds it perennial in Prussian lakes, with maxima in April and September. Wesenberg-Lund (’00) also finds it perennial in Danish waters, with sexual cycles in May and September. Marsson (’00), in waters about Berlin finds a great variation in the seasonaloccurrence, but the intervals of his collection—four to six weeks—were too great to follow seasonal distribution satisfactorily. Zacharias ('98b) finds it in the summer and autumn plankton in a number of German lakes and streams. Zimmer (’99) traces its appearance in the Oder _ from February to a maximum in May, from which time until the end of July it is “einer der haufigsten Planktonorganismen’”’ (!). It: then declines, but returns in small numbers in November. Schorler (00) records it in the Elbe irom April to October, with maxima in April-June and September. Burckhardt (’00a) finds, on the other hand, that in Swiss waters it reaches its greatest development from _ December to March with a maximum in January-February. There are also secondary maxima in May—June and in August. Lauter- born (’93) finds it to be a dicyclic perennial planktont in the Rhine, _ with maxima in April and September—October. A part of the great Variation. in the seasonal distribution of this species which is ap- parent in this survey of the literature may be due to insufficient _ collections or too great an interval between collections. The species 162 is probably a polycyclic planktont with its greater pulses in spring and fall. Asplanchnopus myrmeleo Ehrbg.—Taken in small numbers and irregularly from May to October at temperatures above 60°. Ascomorpha ecaudis Perty.—Found rarely in early summer, in temperatures above 60°. Brachionus. The discussion of the species of this genus in our plankton is fraught with great difficulty. The genus is represented in the Illinois River by a very large number of individuals (fully 25 per cent. of the total Plowma), and the species are, almost without exception, exceedingly variable. They are loricate forms, and the variations affect the proportions of the lorica and the development of its prolongations in spines, antlers, and various diversifications of its surface. . They are evident upon the most cursory examination in most cases, and have been utilized by systematists for the estab- lishment of species. For example, Weber (98) lists no less than 67 species of Brachionus, the most of which he regards as synonyms, and he includes only a part of the species. Fuller knowledge of the extreme variability in this genus has led the most thorough students of the rotifers to regard many of these so-called species as but varieties at the best, and to express their opinion with unmistakable plainness that descriptions of new species among rotifers should only be made after most careful determination of the variability of the organism (cf. Rousselet, 02, Jennings, 00, Wesenberg-Lund, ’00, and Weber ’98). For one not a specialist in rotifers, the attacking of the Brachtonus problem from the statistical standpoint is made difficult by the condition of the literature of the subject, owing largely to the sem1- tropical distribution of the genus; by the absence of any critical monograph of the whole genus dealing fully with the svnonymy of the subject; and by the necessity of establishing and maintaining constantly amid the ceaseless change of varying forms the same standards of distinction between the species or varieties into which all of the individuals enumerated must be assorted. Furthermore, these distinctions must be established before the plankton is counted; that is, before the limits of variation are fully appreciated. It is needless to say that my efforts are at best but approximations — A a he Wy = 1 163 to a satisfactory analysis of the genus in our waters. Brachionus contains by virtue of variation in the hard parts of its lorica most excellent material for the study of the problem of variation, and its rapid multiplication makes possible a correlation with seasonal and environmental changes not often afforded. Evidence has accumulated in the various papers of Schmarda, Ehrenberg, Barrois, v. Daday, Anderson, and others who have dealt with the microscopical fauna in tropical regions, that this genus attains its greatest development in the warmer waters. It is there- fore not strange that Skorikow (96) finds the genus well represented in the warm and shallow waters of Russia, and that the plankton of the Illinois River and its backwaters should contain a large and varied representation of the genus. For convenience in treatment I have arranged the individuals of Brachionus under the following species, without, however, intending to indicate thereby that they have equal claims for specific recogni- tion. The most of these include one or more varieties, and in desig- nating the varieties I have taken those forms—for example, in Brachtonus bakeri—whose descriptions most closely fit the predomi- nant varieties in our waters, designating them often without com- plete consideration of all synonymic possibilities. In some cases several possible varieties have been included under one head. The following is the list of species with the varieties which have been thus separately enumerated. Brachionus angularis Gosse, a ie var. bidens Plate = bakert Ehrbg. . “var. bidentatus Anderson Bs ie “ brevispinus Ehrbg. cluntorbicularts Skorikow melhemt Barrois and v. Daday obesus “‘ * s rhenanus Lauterborn tuberculus Turner budapestinensis v. Daday militarts Ehrbg. mollis Hempel “ pala Ehrbg. (12) 164 Brachionus pala var. amphiceros Ehrbg. ¥ i) Sdorcas Gosse s ios ks “forma spinosus Wierz. quadratus Rousselet urceolaris Ehrbg. a fi var. rubens. Ehrbg. * ‘“ bursarius Barrois and v. Daday vartabtlis Hempel Brachionus angularts Gosse.—Average number of females, 57,890; of males, 25; of summer eggs carried, 29,560; of winter eggs, 1,223; of male eggs, 54. Of the individuals, 13,973 belong to var. bidens and 43,942 to the type; of the eggs, 2,035 belong to the variety and 28,802 to the type. The combined statistics of the species will be discussed before the type and variety receive separate treatment. This species was found in every month of the year and throughout the whole range of temperatures, but the period of continuous presence and large numbers lies definitely between May 1 and November 1 and above 60°. In fact, in 1898, 98.6 per cent. of all the individuals were found between May 31 and October 4 and above 70°. Approximately the same conditions are found in previous years save in 1896, when an earlier spring (ct. Pl. X. and XII, Pt: I.) is attended by anveanlies appearance of this species. Temperature seems thus to have a very decided effect upon the seasonal distribution of the species, and may have something to do with its apparent absence in the cooler waters of our Great Lakes and of L. St. Clair, for in spite of all the work done upon rotifers in those regions by Jennings it has been found but once—by Kellicott (97) ina cove at Sandusky. This identification may be questionable, since he says “I at first took it for B. mollis Hempel.” Notops pelagicus, since described by Jennings (00), 1s found in the plankton of Lake Erie, and according to him this species is much like B. mollis in its appearance. In any event B. angularis is very abundant in our warm waters and practically absent in the more northerly waters of Michigan, whose summer temperatures are 10°-15° below that of the Illinois River and its backwaters. Brachionus angularts presents the usual phenomenon of recurrent pulses, but in spite of the large numbers they are rather less regular Bittner ee er un -~ 105 than usual—for example, than those of Anurea (Table I.). This irregularity is somewhat more pronounced in the separated records of the type and variety (Table I.) than in their combined statistics. This fact that their combined curve of occurrence is more regular than their separated curves constitutes, to my mind, evidence that we are dealing only with one genetic cycle, and that the variety does not belong to a fully separated genetic series. The following table gives the data of pulses and temperatures in the several years. PuLsEs OF BRACHIONUS ANGULARIS INCLUDING VAR. BIDENS. Date |Temp.| No. Date |Temp.| No. Date |Temp.| No. maa cae |e | Soe | ly 1S) 82812118 |e uly, <6 | 81° 309,196 | July 23 80° 100, 826 May 25| 70° | 67,600 | June17) 76° | 60,800 | July 10) 80° | 51,200 Se Zon eo Oe senor400 ume 2s 15°) 75.000) | July 21) “S32 "70,400 Waime” 7.| 78° 4,800 | June 28| 78° [544,000 | July 19) 84° (335,600 36,200 | 269,776 | 103,924 Date |Temp.| No. Date |Temp.| No. Date |Temp.| No. —— | —— | —— | Sept.17| 73° 1,272 | ——— | —— | ——— reed aes 08 5856000) |eAnie 291) 80°) 1050 735) ee] Aug. 8] 85° | 20,800 | Aug. 21] 79° | 29,600 | Sept.16] 71° | 5,051 Aug. 31 |- 80° |988,000 | Sept.14| 83° |368,800 | Oct. 5| 71° | 18,400 amo aio aos 000) |oept.. O1) 792 |£63 200° | Oct. 25), 48° >) 11,500 “— 27| 73° |494,400 Av’g | l4g6., 872 | 195 ,834 (eeres! 11,650 It will be noted that all the pulses with one exception lie above 70°, averaging in fact 78.25°, indicating an optimum temperature 166 near the summer maximum. The location of the pulses with respect to those of the chlorophyll-bearing organisms (PI. II.) shows in the main the same relation that has been observed in other ploiman rotifers. In 1895, three angularis pulses lie in the period common to both, one of these coinciding in location and two following at the next collection. In 1896,two coincide and five follow at the next collection or shortly thereafter. In 1897, four follow at an interval of a week or a fortnight, and one is located where data are incom- plete. In 1898, three coincide and three follow at a short interval, and one (June 7), a minor and ill-defined pulse, appears to le on the rise of the pulse of the chlorophyll-bearing organisms. In the main the dependence of these rotifer pulses upon the recurrent periods of increase in these primal links in the food cycle is suggested by this coincidence or sequence. The pulses of Brachtonus angularts co- incide in the main with those of the totals of ploiman rotifers (Vaile): There is no vernal pulse in the species at the time of the April— May volumetric maximum, and no large autumnal pulse. The pulses in August-September, at the close of our period of maximum heat, average much greater than those of other months, and still further indicate the relation of this species to the higher temperatures. The eggs are carried by the female attached to the posterior end of the lorica. Usually but a single summer egg is carried at one time, but often two, three, and even four, have been seen during the height of the period of rapid reproduction. The relation of the number of eggs to the pulses is obscured in this species to some extent by the fact that the eggs are similar to those of other Brachtio- nus and when detached cannot be identified with certainty. Records are therefore based upon attached eggs only. The number of these depends to some extent on the detachment in the processes of collection, killing, and subsequent handling. In a few cases de- tached male or winter eggs could be identified with some degree of probability by the constitution of the rotiferan plankton. An examination of the records of eggs (Table I.) will, however, suffice to indicate the prevalence of rapid reproduction during the rise of the pulses and the decline in the process during the fall of the pulse. Males, male eggs, and winter eggs were recorded in a number of instances at the culmination or during the decline of a pulse. For example, in 1898, they followed the pulses of August 16, September Vi Se Wt ie _ ae 167 6, and especially that of September 27, when they were found con- tinuously for a month. The separate records of the type and the variety (Table I.) contain in their seasonal distribution one point of special interest; namely, the appearance of the variety after the type has been present for some time. An examination of the records in the several years reveals the fact that var. bzdens is practically confined so far as large numbers are concerned to the months of July-September. This appears in 1898 (Table I.) and is equally evident in 1896 and 1897, but is less noticeable in 1895. The first large pulse is passed in each year before var. bidens takes any appreciable part in the genesis of the pulses. Even the second large pulse is not extensively con- tributed to by the variety in some instances. On the other hand, the later pulses in 1895 and 1897 were mainly of the variety. There is thus in this species some evidence of a tendency on the part of the variety marked by the development of a pair of posterior spines to appear an the latter part of the pertod of seasonal occurrence. The variety bzdens in our records includes individuals with well- developed spines (6. caudatus Barrois and v. Daday), but they are not to my mind worthy even of varietal distinction, since they intergrade so completely with var. bidens and are merely well-de- veloped examples of this variety, and I see no reason for giving the variety two names. Wesenberg-Lund (00) has expressed the opinion that the elonga- tion of structural processes which he has noted in summer planktonts is an adaptation on their part to the changes in the buoyancy of the water dependent upon changes in its specific gravity and, as shown by Ostwald (03 and’03a), in its molecular friction caused by seasonal fluctuations in temperature. It would seem that this tendency on the part of the spinous form of Brachionus angularis to appear in greater proportions in late summer at the period of maximum heat in our waters might be an illustration of Lund’s thesis and Ostwald’s theoretical considerations. The changes in temperature during the occurrence of the species are, however, not very great, though our incomplete records suggest (Pt. I., Table III. and Pl. X.—XII.) that August temperatures are higher on an average than those of July. The averages for June, July, and August are 77.75°, 81.03°, and 81.49°. In 1897, the dominance of the spinous type extends well into September, but it accompanies a period of summer heat (Pt. L., 168 Pl. XI.) prolonged for a fortnight into September, with river water at or above 80°. In 1898, it falls away in numbers more rapidly than the spineless form (Table I.) as temperatures fall in October, though this tendency is less marked in previous years. Brachionus angularts, as above stated, seems to be rare in the plankton of our more northerly and cooler American waters. It is also conspicuously absent from plankton of Swiss waters, as reported bv Weber (98) and Burckhardt (’00 and ’00a), and from German lakes examined by Apstein (96), Zacharias (’98), and Seligo (00), and from Finland waters examined by Stenroos (98). It was, however, found by Wesenberg-Lund (’98) in Danish waters, and in the Udy River, in Russia, by Skorikow (’97), whose statistical records show it to be the most abundant Brachionus in that stream, and outnumbered among the rotifers only by Syncheta stylata and Polvarthra. Schorler (00) finds it in the Elbe from April to July and most abundantly in June. Lauterborn ('98) reports it as perennial in the Rhine and polycyclic, with winter eggs in April, June, August, October, and November. This distribution is much like that in the Illinois River, and will probably be found in tem- perate waters wherever the seasonal cycle 1s thoroughly examined. Brachionus bakert Ehrbg.—Average number of females, including all varieties, 594; eggs,420. The following table, giving the average of each of the varieties in the several years, will serve to indicate their relative abundance, the totals showing the relative abundance of the bakert group in each year and of each variety in the total of all the collections. Though the species is greatly diversified by variation the number of individuals is much less than that of many other plankton rotifers in which variation is much less apparent. It will be noted that the species was apparently more abundant in the earlier years. This is only in part the result of the distribu- tion of the collections, as is shown by the fact that the numbers taken were much larger. Thus in 1898 the largest record is 7,600; in 1897 there are three occurrences in excess of this; in 1896, two; in 1895, three; and in 1894, four. The largest occurrence, 122,958, was on June 30, 1894. The largest numbers by far were recorded in 1894, a year of low water inspring. The hydrographic conditions of the following year were somewhat similar, but the development of B. bakert was much reduced, at least at the time of the collections. RS ie PGC Pal OCS TE IZ4E GICSC CIMT. Csr 7 £21 OLS ERS. DEN Sieh Gl) EAE AS) WK SOKO) TTA SSOKE™ |) te%oy1! 991 [eIOL Ott FOS ch OCOSST O66 0 4 ST 6S | Sot. APS) ASG. al00 CO. ATE ras 868T Gib Wilco C 99 = OES Nip “I8tzc 0 69 98 BECMOGT | eCOpen OT Same OAOm iver ica le iss o¢ LOST | TSO‘T | £4S6‘T |6zZ@ 919 901 |z6z OL 6L ssz | s9e | 79 One ao Lie 0 81 ef 9681 z oe9 807'% 992 |SOT‘T |Th {4S OF USGA ae 9% | 9FT | 19%. | 9ZT 008 0 0 Ve SST ; 816‘L1| LL9‘Lz loot‘ 90FO‘OTPL8‘Tl6zs‘¢| 9 691 | 6T 6T | £L6°%| 9S9°F| SL8'FELZz 6 | O 0 Ot POST i F } ; nee sa snpno4aqny amayjam adAy tuaypqg | snurds1aa4qg snunuays ae aan uno snsogo ae oue Ivo f | uorTzoo]JOD Jed IaquINN eseIaAy ] SOTJOIIV A ‘Taayvd SNONOTHOVAG 169 170 The reducing effect of the recurrent floods of 1896 may be traced in the smaller numbers recorded in this year; and the larger num- bers of 1897 may be referred to the more stable conditions then prevailing. The very small numbers of 1898 may also be due to disturbed hydrographic conditions of that year. The number is much smaller than in 1896, when the hydrograph was even more disturbed, but in this latter year there was more run-off of impounded backwaters during the occurrence of B. bakert,and this would tend to favor their appearance in channel waters. The occurrences and numbers of this species (as a whole) are everywhere somewhat irregular, so that pulses of occurrence are somewhat ill defined. Several such pulses are indicated in 1898, and others recur in the records of previous years. As suggested by the data of 1898 (Table I.), the several varieties share in these pulses. The evidence upon this point is much more striking in other years, when numbers are larger. For example, in the following table note the pulse of 26,800 on August 23, 1897. 2) < < = z LL Ss 2 Date Ss 3 = "Ss S Total z 8 : S S S 5 ‘'s = Ss s S Ss = oS ~ 8 = Is) S x Bs) S a Aug. 10 0 0 0 0 0 200 200 ao tatea OAD: ofc O L200 200 600 2,000 5,200 7,400 DS 0 7,800 1,800 3,400 2,200 11,600 | 26,800 28... O 200 400 200 O 1,000 1,800 In their location these pulses exhibit as a rule the same relation of coincidence or sequence to the pulses of chlorophyll-bearing organ- isms noted in some other species, and they frequently coincide with those of other Ploima, but not always. This is perhaps the most variable of the rotifers of the plankton. At least its variations affect the fixed processes of the lorica and are thus quickly and easily appreciated. The species, in common with a bore wt eT B. pala, B. angularis, and probably B. urceolaris, has a variety—in fact, several varieties—with two posterior spines which are usually symmetrically placed but not always symmetrically developed. The form without posterior spines (var. clumorbicularts Skorikow) inter- grades with these, and a series might be formed with complete intergradations linking this in turn with var. rhenanus Lauterborn, in which the spines are but slightly and often unequally developed. From this we pass, by a slight elongation of the posterior spines, to var. brevispinus Ehrbg., thence to the type in which the spines as figured by Rousselet (’97) are directed posteriorly with but shght curvature. From this we may pass toward variants in which the symmetry is preserved, but the spines are much elongated and curved outwardly. The anterior spines in such individuals are also more elongated and exhibit a similar outward curvature (var. melhemi Barrois and v. Daday). Extreme types of this curvature sometimes occur (B. falcatus Zach.). In another direction we find the bilateral symmetry of the processes, both anterior and posterior, to some extent lost as a result of differences in the curvature of the spines (var. tuberculus Turner). There are also differences in the surface markings of the lorica which have been utilized as specific distinctions. Kertész (’94) describes as B. granulatus a species with a minutely pustulate surface, and Turner’s B. tuberculus takes its name from this same feature. It seems questionable, however, if these surface markings are even of varietal value. Individuals without spines, in which the transverse diameter is relatively large (var. obesus Barrois and v. Daday), are also found. In assorting the individuals belonging to this variable group I have arranged them under the following heads: bakert O. F. Mull., bidentata Anderson (non bidentatus Kertész), brevispinus Ehrbg., cluniorbicularis Skor., melhemi Barrois and v. Daday, obesus Barrois and v. Daday, rhenanus Lauterborn, and tuberculus Turner. The number might have been increased. The individuals referred to var. melhemi include many if not all of the long-spined specimens such as Rousselet (97) has referred to the type, the latter designa- tion having been given to individuals intermediate between this and brevispinus. The variety tuberculus includes the asymmetrical individuals, regardless of the surface markings. I will now briefly compare the seasonal distribution of these varieties and note any peculiarities which mark them individually: hy2 Brachionus bakert O. F. Mull., type form.—Average number, 2. As shown in table on p. 193 (MS.), this form 1s much more abundant in previous years though it is relatively rare, ranking sixth in the list of seven forms recognized. The most of the records fall prior to the middle of August, and it seems to be an early rather than a late summer form. Brachionus bakert var. obesus Barrois and v. Daday.—Average number of females, 41; of eggs, 62. The proportion of egg-bearing to non-egg-bearing females—2 to 3 in all records—-s larger than in any other variety. It seems probable that the lateral expansion which marks this variety may be only the result of rapid reproduc- tion. In common with most of the other varieties this one occurs at the time of the pulses, but it is last in the list of seven, and the numbers are too small to trace its seasonal preferences with cer- tainty. Brachionus bakert var. bidentatus Anderson (non Kertész).— Found once—August 5, 1895, at 78°. Brachionus bakert var. cluniorbicularts Skor.—Average number of females, 90; of eggs, 95. This also was more abundant in all previous years. -This variety is, next to tuberculus, the most abundant of the varieties in our plankton. The two stand at opposite extremes of the series of varieties, the former being least modified, and the latter most, especially in the direction of asym- metry. It includes about one third of all the individuals of the species. The ratio in the grand total of females to eggs carried— 11,708 to 5,976—is somewhat less than the average in the entire - species. This variety is distributed throughout the whole seasonal range of the species with no marked predominance in any particular part of it. It is wholly absent in the early summer of 1897, but very abundant in late summer of that year, though not in other years. The autumn of 1897 was one of long-continued high temperatures (Pt. I., Pl. XI.), and under those conditions this variety constituted two thirds of the individuals belonging to the species. If we add to it the representatives of rhenanus, obesus, and brevispinus we have a total of 15,400 individuals with no posterior spines, or with spines but slightly developed, in contrast with only 2,200 with such well- developed spines referred to varieties melhemt and tuberculus. The conditions of temperature were those in which according to the IPS) hypothesis of Wesenberg-Lund (’00) we should expect a predomi- nance of the long-spined forms. Brachionus bakert var. rhenanus Laut.—Average number of females, 118; of eggs, 138; but more abundant in previous years. This is the third in numbers on the list of seven varieties, being surpassed only by clumtorbicularis and tuberculus. It includes about one sixth of the individuals referred to this species. It is found throughout the whole range of the seasonal distribution of the species and exhibits the same peculiarities noted in clumtorbicularts, to which it is very closely related. The proportion of females to eggs noted in this variety is very large; 5,284 to 5,485 in the grand total. Brachionus bakert var. brevispinus Ehrbg.—Average number of females, 795; of eggs, 390; but somewhat more abundant in previ- ous years. It was found throughout the whole seasonal range of the species, but not quite so abundantly in the latter as in the earlier half of the summer, resembling in this particular the type. The number of eggs carried in this species is in relation to the number of females less than usual—3,906 to 795. Brachionus bakert var. melhemt Barrois and v. Daday.—Average number of females, 49; of eggs, 49. More abundant in previous years, especially in 1894, when it constituted over a fifth of the individuals (25,764) in the largest pulse recorded for the species as a whole—122,958 on July 30. Inthe aggregate in all years it includes only about a ninth of the individuals referred to the species. This form was originally described from Syria, but it is found in great perfection in our plankton, even in the extreme type described by Zacharias (98b) as B. falcatus. It occurs throughout the whole seasonal range of the species, its distribution being somewhat similar to that of tuberculus. 1 do not find any constant tendency limiting its occurrence to any part of the seasonal range. Brachionus bakert var. tuberculus Turner.—Average number of females, 155; of eggs, 42; but very much more abundant in previous years, especially in 1894, when it constituted almost half (55,332) of the largest pulse of the species (122,958). This, the most divergent of all the varieties, constitutes over a third of all the individuals referred to the species. It occurs throughout the whole seasonal range of the species, though the larger numbers were found in 1894-97 in the earlier part or middle of the summer. I find nothing 174 in a comparison of the seasonal distribution of these more decidedly spinous varieties of B. bakert with that of the smoother forms, such as cluniorbicularis, which indicates any correlation with temperature conditions of a nature to support Wesenberg-Lund’s suggestion that the elongation of the processes of plankton organisms arises in response to the lessened buoyancy of the water during higher tem- peratures. Forms with and without such processes are found among the varieties of this species, and both occur indiscriminately through- out the whole range of seasonal occurrence, and, so far as I can see, the statistical data of their distribution with respect to temperature afford no evidence of a correlation of spinosity and high tempera- tures in this species. Other factors doubtless enter into this problem and obscure this response if it exists. B. bakeri is everywhere widely distributed in fresh water. Its occurrence in the plankton of open waters has not, however, been a matter of frequent note. In fact there is some reason to think that it is largely confined to shallow warm waters where vegetation is close at hand, or where at least the flagellates and smaller alge abound, as they do in water fertilized by decaying vegetation or other organic matter. There is, it seems, no reason for regarding this species as merely adventitious in our plankton. It bears all the characteristics of a true limnetic organism in our environment. Its presence in the plankton is not due to floods or other disturbances which might carry it from a littoral region into the open water. It exhibits characteristic pulses, and is found everywhere in summer in company with typical planktonts in open water. Zacharias ('98) records it in some German ponds and streams, and Weber ('98) in Swiss marshes in the warmer months. Stenroos (98) also finds it in the summer plankton of littoral and open waters in the shallow Nurmijarvi Lake in Finland. Jennings (’00) reports it as one of the commonest rotifers in East Harbor, Lake Erie, and in the swamps on the islands. In land-locked pools short-spined varieties were found, and in swamps the long-spined. Speaking of this difference, Jennings says ‘“‘ Possibly the different form found in these pools is due to the greater concentration of various salts in this water or to some kindred factor.” In our own region both varieties occur at the same time in the same environments, channel and backwaters alike, and such factors as Jennings suggests to explain the appearance of the varieties cannot well be operative here in > be Etre a 175 . channel waters. Schorler (00) reports the species as sporadic in the Elbe, and Skorikow (97) finds both B. bakert and its variety brevispinus sparingly in the Udy in summer months. This species in common with other Brachionide was infested by Bimerium hyalinum Przesm., and- occasionally by a filamentous fungus-like growth. Empty loricee were wont to appear with the culmination of a pulse and subsequently. No males were identified as belonging to this species, and attached male eggs were recorded only late in September, 1897, at the close of an unusual pulse. They were found on var. cluntorbicularts and rhenanus. Females with winter eggs were not at any time recorded for this species. It may be that some of the free winter eggs referred to the genus Brachionus (Table I.) belong to this species. The recurrent pulses are similar to those of known polycyclic species, and we may infer the probability of such a phenomenon in B. bakert, though conclusive proof of its - occurrence is not found in the statistical records. Brachionus budapestinensis v. Daday.—Average number of females, 4,211; of eggs (carried), 740. This is one of the most sharply defined species of Brachionus and a typical planktont of open waters. It has, moreover, a sharply limited seasonal distribu- tion in which it is apparently polycyclic. The appended table gives the dates and temperatures of appearance and disappearance and the pulses in the several years. In the main, the period of occurrence is practically from the end of June till the early part of October and above 60°. A record in May, 1896,and an isolated one in December of the same year, indicate an extension of this period, but such occurrences are rare and irregular and the numbers small. This abrupt decline in 1898 as Bemaperatures pass 60° (Pl: XII., Pt. I., and Table I.) is paralleled in previous years. The normal seasonal routine seems to be as follows: The species reappears in the plankton in May—June at 70°, rising slowly to its first pulse (average, 26,104) in July, witha larger pulse (average, 184,453) in the following month during the maximum heat, and a much smaller one (average, 10,044) in Sep- tember, followed immediately by an abrupt decline. The average temperature of the larger pulses lies close to the season’s maximum, while the latest pulse at the lower temperature(72.2°) averages but 10,044. These data all indicate that this is a midsummer planktont, with its optimum temperature near the summer’s maximum. 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WOT SD VelO09 Gal arse Ores ose ane @ OUiLy (sam caiman S68T 000'°¢S5S |, 0.08 | 2 3des So Weel On OOS Po s0%. |S 390 "009 Gre | ofZ |Fe Sny | O0% LE | ses WOSeATOE ee rT Ailes oes Se VOR : oze |e ‘29a | oor‘t |-.8s | og adesg ae ae cial oss | o¢ 3des | o00‘F o9L | 9% “BNY | 000‘ZS SM Gee SATIN TOYOIO) Een) Mr AIDEN Sah Nit} Miah Ay fo Pe OO *968T © ; els | €%. 400 3 er He | ie shaner a off | £¢ 3005 | cos‘h | 682 | Oz 3deS | L990‘eF 068 |. te shy | oTec¢ SOS el Cie STEVIE COS a |roireOUrN fs iecceue cats S681 oGL | Lp des] oor ooh |e 3de6°|———._ |. ——- "|| ———— | woz. | cos) koe sun | cee "ogeuni |. oa F68T a ‘duey) oajeq "ON 6[dwoep] eq ‘ON = |[dway| eq ‘ON [dwep) oeq |dwoy| aeq | rea plOddyY YSe’'T sos[ng plOd9Y ISAT “SISNGENILSAd VGN SONOIHOVAY AO SaSTNAg 1p) relation of hydrographic conditions to the relative development of pulses in different years is seen on a comparison of the record for me%ovand 1397, the former (Pt. I., Pl. X.) being a year-of recurrent floods and~the latter (Pt. I., Pl. XI.) one of stable conditions through the greater part of the seasonal distribution of the species in question. The average numbers in these two years were 3,105 and 31,306, respectively, and the average amplitude of the pulses 18,250 and 97,200, showing, respectively, a ten- or five-fold increase in the latter year. The extension of the heated term into September - in 1897, is reflected in the large September pulse (552,000) and in the extension of the period of occurrence into October. The locations of the pulses of Brachionus budapestinensis in 1898 correspond with those of the Plowma in general. They likewise coincide with or follow those of the chlorophyll-bearing organisms cere Pl Toand 1. with TiJ.and 1V.and Tablel.). Similar relations are apparent in 1896 and 1897 but are less evident in prior years. They suggest an interrelationship of the pulses in this species with the fluctuations in the food supply. Males, male eggs, and winter eggs were not recorded, but the recurrent pulses in this species are so similar to those in other rotifers in which the evidence of the occurrence of sexual reproduction at the culmination of each pulse has been found, that the inference may be made that this species likewise is polycyclic in our waters. Females carrying one or two summer eggs have been found in greatest abundance during the rise of the pulse, and only in small numbers, if at all, during its decline. This species is subject to some variation in the development of surface ornamentation, in the ratio of width and length, and in the curvature of the median spines. It is usually somewhat more slender than figured originally by v. Daday (’85) or even by Hempel (96), who described a form somewhat more slender than that figured by v. Daday, as B. punctatus. Shortly afterwards Skorikow ('96) described the same species as B. lineatus from Russian waters. The name given by v. Daday has priority, and as neither the Russian nor the American forms are to my mind well enough set off to merit even varietal distinction, I have used the name given by v. Daday, and have included under it both wide and narrow forms and those with incurved or outcurved median spines. The fact that their common record of seasonal distribution forms a seasonal curve of 178 typical character is corroborative of the view, though not conclusive, that we are dealing with a single species and not with several. This species has not been widely reported in the fresh-water plankton. It is evidently a planktont of warmer waters, and for that reason may have escaped notice, since the cooler waters have been the more thoroughly explored. Thus it was not found by Weber (’98) in Swiss waters in his thorough explorations about Geneva, nor by Jennings ('94, 96, ’00) in the Great Lakes or inland waters of Michigan. It has, however, been recorded by Skorikow (97) in the plankton of the Udy River, in Russia, where it was exceeded in number by only two species of its genus, B. pala and B. angularis, ranking tenth in numbers among all the rotifers. His data of frequency from July to October suggest several recurrent pulses. It has likewise been found by Lauterborn (’98) in the plankton of the Rhine, where he classes it with the stenothermal planktonts. Zacharias (98) finds it in ponds near Leipzig, and it was originally described by v. Daday (’85) from Hungarian waters, and again noted there by Kertész (94). Fuller exploration of the summer plankton in warmer regions will doubtless extend the record of its range. Brachionus militaris Ehrbg.—Average number of females, 147; of eggs (carried), 98. In previous years the species was much more abundant, the averages in 1897 being 1,412 females and 523 eggs, and in 1896, 1,288 females and 576 eggs. This greater development in years prior to 1898 is evident in many of the Brachionde. The following table gives the dates of first and last records in each season, and the location, temperature, and amplitude of the pulses in the several years. This is evidently a summer planktont with well-defined limits. These limits appear much less evident in 1898 (Table I.) than in prior years. In 1896 and 1897, for example, the species is almost continuously present in the plankton from the time of its first appearance until the last record for the season. All of the records save two lie above 70°, and the average temperatures at which the pulses occur are all at or above 80°. Its optimum thus lies near the summer maximum. The lower limits are not definitely established owing to insufficient collections in periods of rise and decline, but they seem to lie near 70°, with small numbers lingering to 60°. o8L Gr Ayn ol £7 At losis clawed kh tec 0 S681 a eGiMal cyee Wea Guall PP Pao 16 8T EG ” G Ayn Be) ia oun {i Ror oh cede. Oath DOGO 9681 008 eZ Aqn[ Auoeccsiop oO . S68T a oT Ami!) 8k) I eT Smiles wees es E681 eC o7eq ‘duo yp oct eeades" |"00¢ ¢ 008 |-0¢ “sny == OcT 209 |9@ 420 ob. | te des. | 000'r 008 | 1¢ “Ssny | 008 ‘7 ol8 | OT SnVy | 000‘'FZ | 000‘ 8t 08S | OF 3das == 007‘ Z 668 ST SUV OOG. OF6‘S Ory Wrath oy 064 | 02 3499 | 09F'T ole |G) des) 7eS T os) | be sony o8L |v 1des | szz‘7 | 082 | F “3009 O1T4s ‘-dway| eq ‘ON |dwoy| oq ‘ON |duoy| eq ‘ON pl0ody 4Se’T sos[nd PIOdOY ISALY eo “SIUVLITIN SONOIHOVAG AO SASTNd 180 This species has never developed large pulses in the channel waters of the Illinois. Hempel’s statement (99) that it is “the most abundant species of the genus” can apply only to certain collections in vegetation-rich backwaters, for in the river it is sur- passed in the totals of occurrences in the’statistical records by eight other forms of Brachtonus, namely, variabilis, pala, amphiceros, dorcas, rubens, budapestinensts, cluntorbicularis, and tuberculus. I found it in very great abundance in the July-August plankton of Crystal Lake, a shallow warm pond rich in vegetation,formed by damming a small creek tributary to the Wabash system, near Urbana, Ill. From the relatively small numbers, the shght ampli- tude of the pulses, and their somewhat irregular development I am inclined to think that the centers of distribution of this species are not in the open water of the river and its backwaters, but more in the vegetation of warm, shallow regions such as the margins of our bottom-land lakes. It is thus to some extent adventitious in our plankton. The pulses of this species are relatively so small that they do not contribute an appreciable amount to the total ploiman pulses, nor do more than 50 per cent. of their number coincide with such general pulses, though they are sometimes found during their rise. The greater part of them coincide with the pulses of chlorophyll-bearing organisms (Pl. I. and II.), suggesting a food relationship. This species is one of the best-defined in the genus, though in the character of its asymmetry it varies toward B. bakert var. tuberculus Turner. It exhibits some variation in the degree of asymmetry, in the curvature of the spines, and in the surface markings. The indi- cations of pulses suggest a polycyclic habit, but no evidence in the way of males, male eggs, or winter eggs was recorded which will substantiate the inference. A female carrying a winter egg was found Sept. 21, 1897, at the close of the period of occurrence. Fe- males with one, two, or three summer eggs were found throughout the summer and in somewhat larger numbers during the rise of the pulses. Brachionus mollis Hempel.—Average number of females, 137; of eggs, 10. More abundant in previous years, the average in 1897 being 1,092 and 277, and in 1896, 428 and 56. This likewise is a summer planktont. The earliest record of its appearance in the plankton is June 17, 1896, at 76°; and the latest, 181 October 17, 1894, at 58°. With but two exceptions the species was taken only-above 70°, and the period of most continuous occurrence and largest numbers is near the summer maximum of 80°. The optimum is thus near the summer maximum. This species was never taken in the plankton in large numbers, the greatest being on sept. 14, 1897 (20,000), at 84°. On account of the small numbers and somewhat irregular occurrences the phenomenon of recurrent pulses is here less apparent than it isin more abundant species. The appended table records the best-defined ones. These pulses share in the general ploiman pulses in only about 50 per cent. of the cases, and the most of them coincide with or follow shortly after the pulses of chlorophyll-bearing organisms. PuLsEes oF BRACHIONUS MOLLIS | Year Date Temp. | No. | Date Temp. No. 1895....| July 6 81° 742 | Sept. 5 75° 054 W396....| July 18 79° et a2OOR Anaee 621 jee 8,400 1807-...| July 30- 85° =| 11,600 | Sept. 7 80° 20,000 i398... .| Aug. 23 sine 800 Sept. 27 | ioe 4,800 so far as J am aware this species has not been found in other waters than the Illinois River and its adjacent backwaters. Hempel (’99) reports it as most abundant in the marshy environment of Flag Lake. Brachionus pala Ehrbg.—Average number, including all varie- ties: females, 19,969; eggs, 25,974. The following table gives the average number,in the several years, of the varieties here included, and it will serve to show their relative frequency. This is the most abundant species of the genus in our waters, the grand total of all occurrences exceeding 9,000,000. As a whole the species was much more abundant in the stable year 1897 (180,998), and less abundant, all things considered, in the disturbed conditions 5 of 1896 (36,665). Asa whole the type form pala is less abundant % than amphiceros. It forms but 28 per cent. of the total, as compared with 68 per cent. included in the latter variety. Dorcas forms less a ee! 198 ‘P82 SL 6z9°% | 80r 2 | Tel. | PRS cS | S7e ror) 907 12 919 67 991 "+ "TepOL 3 | f 886° ST £96' 61 Z e¢ VL OLT COU Si TLO Ay COs Oc Meo) c cs 868T 60789 866° OST OF 901 0 ean SZ9‘SE | 967‘ 9ST] PHS‘ Ze | SLE HS o¢ LOST 867 $S$9'9¢ 1Z AO GE NOLO Ge MN OAL TO Sia O87 Ss eSh WH ooo. ce er 9681 ; \ 876 ‘TE -8ZE SP or | 908 G20! 086 Ie 690 Tle 698 Sialuccs cosicy Ons Te S6st ors clot 0 0 0 v GE 6st SL OSL‘ T Ol Fost ssaq } ssaq 5 S33 q 35 s33q 3} . $330 S[ENpIAtput SUOTJOIT[OO re TeIOL TROL JO “ON A snsourds SDI40p sosanvyd wp ppd ‘NOILOGTION) Add aad WAN GAOVAGAY “SHILAIaVA UNV VWIVd SONOIHOVUG 182 183 than 2 per cent; and the form spznosus, less than 1 per cent. The proportions formed by the several varieties fluctuate from year to year and from season to season,—indeed, from collection to collec- mon (lable I.).. Thus in the first three years pala exceeded am- phiceros, while in the last two these conditions were reversed ; and in 1896 the form spimosus contributes 6.5 per cent. of the ' individuals. The predominance of the pala-ampliceros group 1s, however, preserved throughout all of the years. The species as a whole is found throughout the entire seasonal range of temperatures but with very great fluctuations in numbers. Speaking generally, there are vernal and autumnal pulses separated by a midwinter minimum which 1s well sustained, developments in excess of 5,000 per m® being very rare in this season. There 1s also a midsummer minimum more or less diversified by pulses of some magnitude. This sequence was not fully realized in any single year of our records, but this may be due in part to insufficient collections at times of the major pulses. Thus in 1894 only a small autumnal pulse (13,650) was detected. In 1895, there was a small vernal pulse (67,338), and a belated autumnal pulse (320,915) lasting a full month in November—December. In 1896, there was a very abrupt vernal pulse rising from 53,618 on April 17 to 1,012,350 on April 24, while in the fortnightly fall collections the only pulse detected was one of 14,000. In 1897, the monthly collections of the spring seem to have missed all considerable developments, the largest recorded being only 16,000. On August 31 and October 12 of that year, however, there were pulses of 1,398,000 and 1,605,600. In 1898 there was a well-developed vernal pulse of 451,200 and a small autumnal one of 83,200. The species is not, however, dicyclic, for both the winter and summer interims are marked by occasional recurrent pulses of smaller proportions. The table on the next page shows the loca- tions and temperatures of the culminations of these pulses. From this table it is evident that a wide range of optimum tem- peratures is possible. Nevertheless, 23 of the 31 pulses occur above 50°, and 21 of them above 60°. In 1898 only 3 per cent. of the individuals are found below 57°, and with the exception of 1895 approximately these conditions will be found in the other years. Brachionus pala is thus a perennial planktont, but as a rule it reaches its largest developments only above 60° in our channel waters. PuLsES OF BRACHIONUS PALA 184 AND VARIETIES. Year Date Temp. No. Date Temp. No. Date Temp. No. 18948 —=——— |) a eee 1895 — — fa! | a 1896 Yanzel 335 8,268 Jan. 25 sei? 5,928 —== == ——— 1897 —— a = 1898 == —_ ——— Mar. 22 Sule e720) Year Date Temp. No. Date Temp. No. Date Temp. No. 1894 =—= = = —S— — 2 = 1895 Apr. 29 64° 67,338 —— = == —— 1896 Apr. 24 722 11,012,350 May 25 fs 4,400 — —— ——<—<——— 1897 SS —— === May 25 66° 16,000 [= = 1898 SS => = May 3 60° 451,200 | June 14 83° 1,000 Year Date Temp. No. Date Temp. No. Date Temp. No. 1894 July 30 g2° 0 ——— Sept. 4 78° | 13,650 1895 July 6 S12 3,710 Aug. 21 81° 47,480 | Sept. 20 79° PDA IPIS) 1896 July 23 782 12,600 Aug. 3 80° 39,200 | Sept. 30 58° 14,000 AUS) 81° 12,800 1897 July 30 84° 11,200 Aug. 31 80° |/1,398,000 = == 1898 July 19 84° 6,400 Aug. 16 Udi 38,400 | Sept. 27 73° 83,200 Year Date Temp. No. Date Temp No. Date Temp. No. 1894 SS | | Oo | = SSS ee 1895 Nov. 27 Boe 320,915 == ——— 1896 —- — — —_ — Dec. 29 359 14,120 1897 @ct. 12 65° |1,605,600 Nov. 23 43° 1,160 =—= == 1898 Oct, 25 49° 8,500 Nov. 15 41° 1,100 Dec. 15 329 3,100 185 The pulses recorded in the table will be found to coincide (Table I.) with those of other species of the genus, and in the main with those of the total Plowma, thus indicating that this species responds, along with other rotifers, to some common factor of their environ- ment. The relation of these pulses to those of the chlorophyll- bearing organisms (Pl. I. and II.) is also striking. Of the 30 pulses recorded in the table, 6 fall outside of the period included in Plates T.and II. Of the remaining 24 there are 17 whose culminations in the main coincide with those of the organisms upon which they feed, and 5 of the 6 remaining follow shortly thereafter, usually at the next collection, at-an interval of a week or thereabouts. In one case only is there a delay of a fortnight after all of the plant pulses. The large pulses of August—October, 1897, were judged by the Chlorophycee only, as these overtop the other plants so greatly. The pulse of August 31 occurs a week before the culmination of the Chlorophycee is reached, but in the presence of abundant food. The dependence of these pulses of Brachionus pala upon the food supply is plainly suggested by their time relations with the pulses in the plant life of the plankton. Further reason for concluding that the species is polycyclic is found in the evidences of sexual reproduction, which will be noted in connection with the discussion of the varieties. In this connec- tion it will suffice to say that there is some evidence that the pulses are preceded by rapid parthenogenetic reproduction, and accom- panied or followed by the appearance of male:eggs, males, and winter eggs. The eggs of Brachionus pala are detached from the parent in such a large proportion of the cases in preserved material that the tracing of the reproductive cycle by means of attached eggs is ren- dered difficult if not impossible. Furthermore, eggs resembling the winter eggs of this species,and provisionally referred to it in our records, are to be found in the plankton at nearly all seasons of the year, and it is obviously impossible to determine the time at which they were produced. It seems probable that all of the varieties pass through recurrent cycles, and that none of them 1s a temporary phase of the cycle. Outbreaks of parasitic diseases in this species are very common. They almost always attend the larger pulses, but isolated individuals infested by some of these pests are not infrequent, especially during 186 the summer months. Thus in the vernal pulse of pala (type only) reaching 716,982 on April 24, 1896, 19,056 individuals were para- sitized by Bimerium hyalinum Przesm.,’or by something very similar to it, and 30,966 were infested by a fungus-like growth. This is about 7 per cent. of the total individuals. Similar though less pronounced outbreaks have attended other vernal and autumnal pulses. Species of Colacituwm are sometimes found attached to the loricee of this species. Brachionus pala is exceedingly variable, especially in the matter of the development of the posterior spines. Forms without the spines (pala type) intergrade, by only slight gradations, into those with fully developed spines (var. amphiceros). The angle which these spines make with the lorica is also a matter of great variation, in preserved material at least. Individuals with the spines at right angles to the antero-posterior axis are occasionally seen. The species also varies in the matter of the dorsal-ventral curvature of the antero-median spines (var. dorcas). Individuals with such curved antlers are sometimes provided with posterior spines (var. dorcas form spinosus). I have followed Weber (98) in placing B. amphiceros Ehrbg., B. dorcas Gosse, and its form spinosus Wierz. as varieties of B. pala. They do not, however, all stand upon an equal footing. B.amphiceros grades imperceptibly into B. pala, and has the same seasonal distribution. 6. dorcas and its form spinosus intergrade with each other as do pala and ampluceros, and they also exhibit some intergradations with 6. pala; but they are winter varieties, or at least belong to the colder season, as will appear later. Their differentiation in this respect is thus more striking than that of B. amphiceros, and makes it probable that we have in dorcas a seasonal variety of B. pala. Zacharias (’98) has reduced B. pala to a variety of B. amphiceros because in his opinion the latter is the more widely distributed form in certain pond waters which he examined. This is a criterion which presupposes a wide knowledge of distribution and numbers, and, furthermore, a basis which can not fail to add to the confusion already existing in this genus, since it is hardly to be hoped that it will lead to the same conclusion in the hands of different investigators in different regions, or even in different seasons and years in the same region. As an illustration of the difficulties which might arise I may cite the yearly averages of amphiceros and pala in the table on page 182. In three 187 years the latter is more abundant, and in two, the former. The relative abundance of these forms in the river at a given point of collection 1s an epitome of their distribution in a wide area of channel and backwaters. An application of the principle advanced by Zacharias would in: this instance lead to constant change. The retention of pala (Ehrbg., 1830) as the type and amphiceros (Ehrbg., 1838) as the variety is in keeping with priority in nomenclature and with the principle of regarding the more highly differentiated or divergent form as the variety. Variety amphiceros occupies thus the same relation to the type that bidens does to its type angularis. Both are illustrations of the tendency common to all species of Brachionus to develop posteriorly directed spines. I shall proceed to discuss the salient points in the seasonal dis- tribution and statistics of the several varieties :— Brachionus pala Ehrbg., type.—Average number of individuals, 2,693; of eggs, 20,809, including all free eggs referable to the species in the broader sense. In the present connection I shall call attention only to the fact that the type form, without the posterior spines, is less abundant during the midsummer interval than the spinous variety amphiceros. This appears in Table I., and is to be found in the records of years prior.to 1898. A fuller comparison of the records of the two forms will be made in the discussion of amphiceros. I shall not discuss the recurrent pulses of this form or of amphiceros, since as they dominate those of the species as a whole it would lead to considerable repetition. The pulses of pala in the main (Table I.) coincide in location with those of the species as a whole, and the direction of movement of the seasonal curve of distribution is quite similar, save in the fact that the amplitude of the pulses is less, and that the differences in seasonal distribution between pala and amphiceros modify the curve of each. The decisive evidence of sexual reproduction in the species in the form of attached male and winter eggs is found repeatedly at times of the major pulses. In some instances they appear during the rise ot the pulse. The autumnal pulse of 1895 will serve as an illustra- tion of the character of these statistical data. (See following page.) _ This pulse is sustained much longer than usual, but it serves to show the prevalence of parthenogenetic eggs during the rise of the pulse, and the evidence of sexual reproduction during its progress. In some other instances the number of free winter eggs ajter the 188 BRACHIONUS PALA, TYPE Form. SEXUAL CYCLE. eee Winter eggs | Summer eggs See Total eo Pe caeed Car- : eggs* ee Free* ee Free* | Carried ets Oe. poussins Dy (ones a2 06)| === 96 48, Novae 765) | 765.|85.| 1700 | 30608) onaasal, me 1885 Nov. 14..... ‘4,140, 8,280 | 7,245 | — | 63,135 | 71,415 |150,075 | 134, 5350 Nowe20ee ees | == | 4/,680;| —" "| — | 434680.) 68,880 |114. 240) ) 1acmoam Nov, 27.:... == 742 | 2,226 | — | 46,746 | 89,040 |138,754 Bae Dace | = |» 4,380. |= = 9.68, 310" | 662400135, 0300 momtenaaim Wee. tard: — Aga |) ===) 9/46. 4194 | 17 34400247 On mesomnad Dech tes. : | | | | 17,808 | 1,113 | 18,921 | 14,469 Dee. 25.....| | | i amet al 742 | 371 * Includes free eggs of other varieties also. culmination of a pulse is very large. For example, the sudden ver- nal pulse of 716,982 on April 24 is accompanied by 28,584 free winter eggs. The pulse declines to 22,224 on April 29, and the free winter eggs rise to 95,841, and the empty lorice to 26,114. Females carry 1-5 summer eggs, and 1-8, or even more, male eggs. There is great variation in the size of ie summer eggs, these and the male eggs appearing almost to intergrade. Brachionus pala, including Bb. amphiceros, is a common constitu- ent of the plankton of shallow warm waters. It has not been reported from the larger and cooler lake waters by Apstein (’96), Burckhardt (’00 and ’00a), or Jennings (’94, ’96, and ’00). Zacha- rias (98) and Marsson (’00) find it in the summer plankton of smaller lakes and ponds in Germany. Seligo (’00) records it from April to October, with a maximum in August, in Prussian lakes; and Lauterborn (’98a) finds it to be perennial and polycyclic in the Rhine. Schorler (’00) reports both pala and amphiceros from the Elbe, the former being abundant in May and sporadic during the summer, while the latter was abundant in April, June, and Septem- ber, and rare at other times during the warmer months. Zimmer lili a tr 8 ER BON OS OED 4 | 189 ((99) finds amphiceros in the Oder, where it appears in April and increases until the end of August or the first of September, when it is the most abundant animal in the plankton. In no one of these instances was the examination so long continued or made at such short intervals as in the case of the exploration of the Lllinois. The diversity exhibited in these different waters may be paralleled by the fluctuations from year to year in the Illinois, and from all the data it may be inferred that the organism is probably perennial and polycyclic, the number of pulses depending upon local conditions, primarily of the food supply. Brachionus pala var. amphiceros Ehrbg.—Average number of females, 17,071; of eggs, 5,103. The numbers were much larger (158,299 and 35,392) in the stable conditions of 1897, and still smaller (5,430 and 715) in the disturbed conditions of 1896. The seasonal distribution of this variety with respect to that of the type constitutes the chief point of interest in the records. It is present throughout the whole range of temperatures, shares in the vernal and autumnal pulses noted for the species as a whole, but constitutes a much greater proportion of the amphiceros-pala group during the warmer months than it does in the colder ones. Thus, as shown in the accompanying table, the proportion which amphi- SEASONAL DISTRIBUTION OF BRACHIONUS PALA AND B. PALA VAR. AMPHICEROS. Jim|e at to.Oct. 1 | Oct. 1 to June 1 | | | | Year | pala amphiceros | pala amphiceros No. |%¢ No. |®8| No |S] No. |Se Oo 1S) | oO oO Pee ees) ena? | 4 | 4582324 | 96\| (863,247. |.71 | 336,618.) 29 | | | M306... 2... | 14,637 | 14|° 89,400 | 86| 958,265 | 87 |144,087| 13 | | haat | ae | 14,600 | 4| 3,776,400 | 96] 719,650 | 44 |912,580| 66 | | | | | MOBS... T |} 10,440] 4| 229,720 | 96| 129,623 | 17 |657,960| 83 OL Average.....| 11,679 |6.5 | 1,062,711 |93.5| 667,696 | 55 |512,811| 4 190 ceros forms of this group in the period from June 1 to October 1 is from 86 to 96 per cent., averaging 93.5 per cent..in the several years. On the other hand, in the colder months—Jan. 1 to June 1 and Oct. 1 to June 1—the per cent. is only from 13 to 83, averaging 45. The temperatures on June 1 (Pt. 1., Pl. [X.—XU1, average abou and on Oct. 1 about 67°. The spinous form (amphiceros) thus in- cludes about 45 per cent. of the individuals at low temperatures, and 93.5 per cent. at high temperatures; and the smoother form (pala type), 35 per cent.. and! 6:5 per cent-, respectively. This predominance of the spinous variety at high temperatures is apparently a striking illustration from statistical evidence of the hypothesis of Wesenberg-Liund (’00) that such elongations of the body of planktonts are adaptations to the lessened buoyancy of the warmer water. This relation of the spinous form to higher tem- peratures is evident in every year, 1895-1898, and the proportion of spinous forms, 86-96 per cent., exhibits all the constancy that might at the best be expected in plankton data. The relation is generally apparent (Table I.) in the individual entries as well as in the sumis total, and, considering the numbers concerned and the long period of observation, should have more weight than some of the exceptions to the hypothesis, which have been or will be noted, in which the data are less extensive. For example, Brachionus pala var. dorcas does not in its seasonal distribution support the hypothesis, but owing to its small numbers—especially of the form Spinosus—less weight should attach to its evidence. In 1897 the first autumnal pulse of the pala group consisted almost entirely of var. amphiceros. This pulse started August 10 at 3,600, culminated August 31 at 1,398,000, and declined to 800 September 29. Of the 3,500,200 individuals included in this pulse, all but 11,400 belonged to amphiceros. The temperatures recorded during this period ranged from 83° to 71°. A second pulse started October 5 at 1,600, culminated October 12 at 1,605,600, and declined to 0 on October 26. Of the total individuals (1,609,000) included in this pulse, 894,800 belonged to amphiceros and 714,200 to pala. The range in recorded temperatures in this period was from 71° to 59.5°. This may serve as an additional illustration of the relation of tem- perature to the spinous variety of Brachionus pala. This variety is itself polycyclic, as is evidenced by the recurrence of male and winter eggs carried by the female at times of the pulses. ail 191 Owing to the ease with which such eggs are detached, the records are quite imperfect indices of the actual numbers. In 1898 male eggs (carried) to the number of 70,400 per m.? attended the culmina- tion of the vernal pulse (419,200) on May 3. Winter eggs (carried) were recorded twice on the decline of the pulse of August 16; once on the decline of that of October 25 ; andonce on that of December 15. Brachionus pala var. dorcas Gosse.—The seasonal distribution of this variety is so sharply defined that it merits especial attention. The following table gives the dates and temperatures of last and first records in each year. SEASONAL LIMITS OF BRACHIONUS PALA VAR. DORCAS. Last records First records | Largest pulses Year i : Date Temp. Date | Temp. Date Temp. | No. Apr. 29 64° | Oct. 15 | 57° | Apr. 29 64° | 9,000 Nov. 14 46° May 1 202" >| Now 7 |) 44° | Apr. 24 72° | 183,000 Apr. 27 60° Oct. 12 65° INDI 2h C02. | 2,400 Jan 11°98 | “32° | Apr. 26 57° | Dec. 6 34° | Apr. 26 S724 | * 4.000 May 17 64° | The species practically disappears at the end of April, when temperatures rise above 70°, and it does not return to the plankton until they fall, in October and November. Its period of continuous occurrence does not begin in years of greatest numbers until tem- peratures reach 45°, and it remains throughout the period of mini- mum temperatures. As the collection-averages indicate, this species is relatively rare, and its numbers, even in its largest pulses, are usually smaller than those of the other varieties which it accom- panies. Although this species is a winter planktont it reaches its greatest development during the spring pulse, indicating an opti- mum near 65°, though it does not recur in numbers when this temperature returns in autumn. There is a single autumnal pulse in 1895 of 8,625, on November 14, at 44°, accompanying pulses in the other varieties. There was also one midsummer record. 192 The curvature of the median anterior horns which defines this variety results in a considerable elongation of these processes. With regard to the idea of Wesenberg-Lund (’00) that this tendency on the part of plankton organisms to elongate in “ Balanceapparat”’ is an adaptation to the lessened buoyancy of the warmer water of summer, it must be said that 1t seems difficult to apply this hypothesis in the case of B. pala var. dorcas, which is probably a seasonal variety confined to winter months. Ihave no data, however, on the relative development of these processes in B. pala at different temperatures, beyond the seasonal limitation of this variety to lower temperatures when it should be least expected according to the hypothesis. Brachionus pala var. dorcas has not been found widely distributed in the fresh-water plankton, or at least not reported separately from 6B. pala, which is widely distributed. Skorikow (’96) reports it from Charkow, Russia; and Kertész (94), in January from Budapest. Brachwonus pala var. dorcas forma spinosus Wierz.—Average number of females, 33; of eggs, 2. This form was always sporadic in its appearance in our plankton. Of 12 occurrences, 3 were in April, 2 each in November, December, and July, and one each in January, May, and August. The whole seasonal range of tempera- tures is thus included. It may be of significance for Wesenberg- Lund’s hypothesis that the spinous form of dorcas makes over 50 per cent. of its appearances between April 1 and September 30, whereas dorcas itself is much less abundant relatively within these limits. The largest occurrence of spimosus—100,044 on April 24, 1896—was marked by the fact that 97.5 per cent. of the individuals were infested with fungi. The nearest approaches to pulses in this form are the November-December appearances in 1895 and 1896. Females with winter eggs were recorded December 29 in the latter year. Brachionus quadratus Rousselet.—Individuals corresponding to Rousselet’s description have been found occasionally in the plankton from the last of May till the middle of August at temperatures of 70° and above. The species is somewhat closely related to the bakeri series, and may ultimately prove to belong to it. Rousselet (°97) is of the opinion that it is distinct by reason of the truncate posterior end, the absence of foot sheath, the reticulations of the shell, and the semi-jointed foot. It occurred only in small numbers, 19S and forms intermediate between it and bakert were not recorded. This is, I believe, the first record of its occurrence in American waters. Brachionus urceolaris Ehrbg.—Average number of individuals including all varieties, 468; of eggs, 56. The species was relatively quite abundant in 1897 (5,290 and 1,976) in the stable conditions then prevailing, but less so in the recurrent floods of 1896 (1,020 and 494). It is not a common species, being outranked by B. angularis, baker1, budapestinensts, and pala. The species as a whole is found throughout the entire year, though never in large - numbers since 1895. The following table, which gives the principal pulses in the several years, shows the wide range of the species and its varieties in seasonal distribution. PuLsEs OF BRACHIONUS URCEOLARIS. Year Date Temp. No. Date Temp. No. Me Pos, PEA Oa are wiaea cet | Mars 24 41° PPX) Apr. 17 66° 8,398 obec OERER ICRC ERTL Peg tes uel ese Apr. 27 60° 6,400 2 Ot IOC O RTE RENEE etre ates, |p Mare 22 Sits 2,000 Apr. 26 Sis 6,400 Temp. No. | Date Temp. No. | Date | Temp. | No. =| ip eee —= —— | Aug. 15 84° 181,764 June 19 80° 324,254 sz ——— ==} Tittise BE) Gfoe 10.0008) .—<—<$—= pes? Temp. No. Date Temp. | No. | Date |Temp. | No. Sept. 5 74° 28h |) —————— =— — | Dec. 4 Ay 345 = ee = | Dec., 3 ae 794 Sept. 21 ihe 1205200" | Oct. 5 ike 800 | Sept. 6 | 79° 5,600 | ———_ | ——| ———| Dec. 13 | 33° 500 194 There is some tendency, especially in later years, toward the colder months. Eight of the fifteen pulses occur below 70°, and twelve between September 1 and May 1. On account of the small numbers the pulses are poorly defined in our records (Table I.), but there are indications that-they coincide in location, in a general way, with those of other brachiomde and the Ploima as a whole. They also in many instances coincide with or follow shortly after the pulses of chlorophyll-bearing organisms, as has been noted in other Brachtonde. This species, B. urceolarts, is a cosmopolite,and of general occur- rence in the fresh-water plankton of smaller and warmer bodies of water. It is reported by Weber (’98) from Swiss marshes, by Zacharias (’98) and Marsson (’00) from many smaller German waters, and, by Seligo (00), throughout the year, from lakes near Danzig, where it attains maxima in April, July, and September. Since this author includes B. angularis (B. urceolaris forma angu- latus Seligo) with his records of urceolaris, it is probable that the species in the usual sense may have much more restricted numbers and range in his region. Kertész (’94) finds it about Budapest. It is reported as sporadic in the vernal plankton of the Elbe by schorler C00), and is listed from the Oder by Zimmer 3 99)P Skorikow (97) reports it once in summer plankton of the Udy near Charkow. The species is exceedingly variable in the development of the anterior spines, and in the proportions of the body. It varies toward the bakeri group, and individuals are sometimes found which seem to connect the two groups. I follow Skorikow (96) in placing B. rubens as a variety of B. urceolaris, including in it those forms whose anterior spines are least developed. The more slender summer forms I have listed as var. bursarius Barrois and v. Daday. From my observations on B. variabilis Hempel, I am inclined to regard it as a possible variety in the urceolaris group. In form, texture, proportions, and anterior spines it is certainly similar to this group. The presence of the posterior spines would not suffice to separate it, since these may or may not be present, and the existence of a variety of urceolaris with such spines would only present a phenomenon parallel to that observed in pala, angularis, and bakert. The quadrate foot-plate present in variabilis, which, according to Hempel (’96), is not found in other species of the | et vs ae 195 genus, serves to distinguish this form, and in the absence of proof of its occurrence in forms of urceolaris as here defined I prefer to leave variabilis as a separate species. In any event it is closely related to the urceolaris group,and may ultimately be found to belong within its seasonal range of variation. Seligo (’00) has suggested that B. angularis is also a variety of urceolaris, but I do not so regard it. The averages of the different forms in the several years are given in the table on the next page, which also includes B. variabilis. The discussion of the different varieties follows :-— Brachionus urceolaris Ehrbg., type.—Average number of 1in- dividuals, 18. The type form was not abundant in any year, and its appearances were sporadic. It was recorded in February, June, pnd July.- It imeludes less than one per cent. of the individuals referred to this species. Brachionus urceolaris var. rubens Ehrbg.—Average number of individuals, 244; of eggs, 41. This variety was more abundant during the stable conditions of 1897 (5,290 and 1,976) and the low- water years of 1894 and 1895. It includes over 99 per cent. of all the individuals referred to this species. It is apparently the winter form of the species. This appears clearly in its seasonal distribution in the later years, but in 1894 and 1895 it was found in summer months and in large num- bers. It is thus capable of development in the whole range of temperatures. The pulses recorded in the table on page 193 are in the main composed of this variety. It is quite abundant during the summer of 1894, attaining a pulse of 181,764 on August 15 at 84°, disappear- ing in September, and not reappearing until the April collection. It attains a pulse of 324,254 on June 19 at 80°, declines in July, then occurs sporadically until the following February. It then continues till June 6, with a pulse of 8,398 on April 17 at 66°. An isolated occurrence of 10,000 in July is the only record in the summer of 1896. It is in the November—December plankton of 1896 and the March—May plankton of 1897, and attains a pulse of only 6,400 on April 27, at 60°. It does not reappear until the 14th of the following September, in whose stable conditions a pulse of 121,200 on the 21st, at 71°, isfound. It disappears October 5, and is irregularly present from January to April, with larger numbers in the latter part of the period. It is not found in 1898 (Table I.) from May 1 to Decem- (14) ch CSP NVOS le 90Gs OF | 199: OS ST Cog “PLO OlleLoe: OS O7T $9 SS Gar, au PSO EANON p, An 0 F 9S 89F ST 902 | IF TPT 07 ST zs Sas ce all De a 86ST i 99 OFT 946‘1T | 062‘S 0 0 OV | \206G. 5 0 0 o¢ Paine Sip Noe L681 : tI OLT FOF 07ZO'T 0 Le L8V 986 l l St ae) eae ek a i 9681 4 l St ee UG 0 0 S97 ray 1G) O@I aS | Sioa (Sie iaar cata Sas a: S68T GOSS SEOs Pe ee OCe Onn GH Cs (0) 0 SOS Ob hae -ce 0 S Ole sen( Vo ewe teeta F681 sso & ss3q & ssaq 5 sso ro) ss3q & SUOT}IIT[OO OD SU]UQDIUADA SVADIOIIAN SNUADSANG SUdqnd SR pIRSOUT oN a SNUOLY IDA T SEAS st SAN a 1 Te}0L, “NOILOATIO“Z) Add ATEWNN AOVVAAGAVY “SITIGVIAVA “GF AGNV “SHILGIUV A GNV SIYVTOHONN SANOIHOVaA 4 197 ber 1, but is continuously present in the winter of 1898-99 from December 6 till March 28, when collections ceased. Male eggs were recorded but.once—April, 29, 1895—and there is no other evidence of the cycles of reproduction beyond the pulses in numbers. They suggest a polycyclic habit with major pulses in spring and fall. Itis apparent that conditions affect these cycles greatly, as is seen, for example, in the contrast between the earlier years, with low water in the spring, and the later ones, when high water was longer continued. This variety, rubens, has not been widely reported in the plank- ton. Skorikow (’96) finds it in June in the River Udy, and Kertész (’94) reports it from Budapest, while Stenroos (’98) finds it in the littoral fauna of Lake Nurmyarvi in Finland, and also in the plank- ton in July and August. Brachionus urceolarts var. bursarius Barrois and v. Daday.— Average number of individuals, 206; of eggs, 33. This is a sum- mer variety, and forms but a small part—less than one per cent.— _of the total number of individuals referred to the species. Brachionus variabilis Hempel.—This species was found but once in 1898, but was more abundant 1n former years (see table on oppo- site page). The largest development which it attained in the Illinois was a pulse of 168,222 on August 15, 1894, at 84°. The largest number in subsequent years was 5,200 per m.* on August 8, 1896. It may be significant of the connection of this form with the urceo- laris-rubens group that the great pulse of 1894 was coincident with an unusual development of rubens on that date. This species is a Summer form, the earliest record being May 24, 1898, at 74°, and the latest September 25, 1895, at 73°. Its opti- mum temperatures lie near the summer maximum. If this form should prove to be merely a spinous variety of B. urceolaris it wall afford another illustration of spinous varieties of Brachionus appear- ing at high temperatures, in accordance with the hypothesis of Wesenberg-Lund (’00). In Table I. there is given for 1898 the seasonal distribution of the free winter eggs of Brachionus. It will be seen that they occur throughout practically the whole year, with some increase after the times of the April-May and September pulses. Cathypna leontina Turner.—Average number, 47, in 1896, a year of disturbed hydrograph; less abundant in previous years, and not 198 recorded in subsequent ones. Earliest record, June 17, at 76°; and latest, October 2, at 63°. Always present in small numbers and evidently adventitious. Cathypna luna (Ehrbg.) Gosse.—Average number, 47. Found in every month but November, though always in small numbers and irregularly. All but six of the thirty-three records fall between April 1 and October 3 and above 50°. Over half of all the individu- als were found in 1896. This fact, together with the nature of the seasonal distribution, indicates plainly its adventitious character. Cathypna rusticula Gosse.—Found once, March 22, 1897, at 44°. Not previously reported from American waters. Celopus porcellus Gosse.—Average number, 106. From March to September, at 37° to 80°, and apparently adventitious. Colurus bicuspidatus Ehrbg.—Average number, 274. This species is apparently a winter planktont. In 1897 1t appeared first November 9, at 50°, and was found somewhat irregularly through the winter until May 17, at 64°. There is a pulse March 15, at 46°, of 6,400. Ovigerous females were found during the rise of the pulse, and males on April 12, on its decline. A few scattered records were made in the following winter, beginning November 8, at 46°. It occurs in the plankton during flood season and may be adventitious. Colurus obtusus Gosse.—Average number, 38. In small numbers and irregularly in March and April at temperatures below 50°, and in September at 73°. Hempel (’99) lists also C. deflexus Ehrbg. Diglena circinator Gosse.—Average number, 121, in 1896, a year when many adventitious rotifers were brought into the plankton by disturbed hydrographic conditions. All the records lie between April 29, at 70°, and July 28, at 81°. Anovigerous female was found in July. The species is adventitious in the plankton. Diglena forcipata Ehrbg. was recorded once—October 12, 1897, at 65°. Diglena giraffa Gosse was observed but once in the river plank- ton. Not before recorded from American waters. Diglena grandis Ehrbg. was recorded in July and September at (Ocsamc h9e. Diglena uncinata Milne was found August 12, 1898, at 82°. Hempel (’99) reports D. biraphis Gosse and D. catellina Ehrbg. in waters immediately tributary to the river. All members of the 199 genus belong to the littoral fauna among vegetation, and are adven- titious in the plankton of open water. Euchlams pyriformis Gosse.—Recorded April 12, 1898, at 52°. Hempel (’99) reports 1t from June to October in collections in the river in 1894 and 1895. Euchlants triquetra Ehrbg.—Average number, 19. Found irregu- larly from July to November at 84° to 41°. Hempel (’99) reports it alsoin June. It is probably adventitious. Hempel (’99) also reports FE. dilatata Ehrbg. in the river from July to September, and EF. deflexa Gosse in tributary waters. Gastropus stylifer Imhof.—A rotifer doubtfully referred to this species was found sporadically in the plankton of the river. It was recorded in June, 1894, and July, 1896, at temperatures above 75°. It was almost continuously present in 1896 from February 20 to April 10, and again on November 17 and December 3. It did not reappear until January 31, 1899, from which time it continued present until the close of operations in March. Most of these oc- currences are at minimum temperatures and all of them below 45°. I have followed Weber (’98) and Jennings (’00) in using Imhof’s name Gastropus stylifer instead of Hudsonella picta Zach. or Notops pygmeus Calman, by which names the species has been frequently designated. The evidence from our records indicates that it is a somewhat sporadic winter planktont in our waters. Lauterborn (’93) finds it to be a perennial planktont in the Rhine, with its largest numbers in summer. Hydatina senta Ehrbg. was found September 20 at 73°. Hempel (°99) also reports it in towings from the river in March and July, 1895. This species is very common in European waters, but has as yet been found in America only in the Illinois River and, by Kel- licott (’88), at Corunna, Mich. Mastigocerca bicornis Ehrbg.—Average number, 42. Found irregularly and in small numbers from June 28 to September 13 above 63°. Hempel (’99) reports it from Quiver Lake among vegetation, and it is evidently adventitious in the river plankton. Mastigocerca bicristata Gosse was found but once, late in Septem- ber, 1895, at 73°, but it is more abundant in the backwaters. Mastigocerca carinata Ehrbg.—Average number, 1,674. This species was present in the plankton from the middle of June till the 200 first of October, and at irregular intervals and in small numbers in fall and winter months. The distribution in years prior to 1898 falls within the limits shown in Table I. In this year the bulk of the occurrences le between June 21 and August 4, and above 77° and 72°. The optimum lies near the summer maximum, though occurrences at minimum temperatures in March and December reveal acclimatization to a wide range of temperatures. In this year there are several somewhat irregular pulses, the best-defined of which follow the pulses of chlorophyll-bearing organisms (cf. Table I. and Pl. II.) at an interval of one or two weeks. The species was not recorded so frequently in previous years, in some of which also pulses are indicated. These pulses are not consequent upon floods, and the species is apparently not adventitious in the plankton but a normal constituent. Apstein (’96) reports M. capucina as abundant in Dobersdorfer Lake from June to October—a seasonal distribution similar to that found in the Illinois River for M. cartnata. Mastigocerca elongata Gosse was found once—March 28, 1899, at 38°. Hempel (’99) reports it in June in Quiver Lake. Mastigocerca mucosa Stokes was taken in August to October, 1898, at 82°-62°, in small numbers. It is reported by. Jennings (700) as “one of the most abundant of the Rotifera among the vegetation of the shallow parts of Lake Erie,’ but it was not reported by Hempel (’99) in similar environment about Havana. Mastigocerca stylata Gosse was found in the plankton in small numbers in June and July at temperatures approaching 80°. Hempel (’99) reports it also in August. In addition to the species of this genus above listed, Hempel ('99) records M. lata Jennings. There are also in our records a considera- ble number of individuals referred to this genus but not specifically identified. Many of these belong to one, or possibly several, very small species. They are most abundant during the summer months, reaching a pulse of 16,800 on June 28. They occur in large numbers in the filter collections (average for 1898, 798; filter-paper, 145,384), and, it seems, must escape with ease through the silk net on account. of their small size and their active movements. A number of species in this genus have been described of late from the fresh-water plankton, but in the present state of the litera- ture of the subject I am not certain to what species these forms er i ial Nr 201 should be referred. The genus is sadly in need-of critical revision. It includes-a number of semi-limnetic species, whose importance in the plankton will probably be revealed by more perfect methods of collection. Metopidia lepadella Ehrbg. was found only in March and June at temperatures above 46°. It is apparently adventitious. Metopidia oblonga Ehrbg. was found once—July 29, 1895, at 75°. Metopidia salpina Ehrbg. was recorded June 28, 1898, at 78°. Metopidia solidus Gosse.—Average number, 67. This is the most abundant representative of the genus in our plankton. It was recorded from March 15 to November -14, at temperatures above 45°. Most of the occurrences are in the summer months (Table I.), at maximum temperatures. The numbers are small, the occurrences irregular, and the species evidently adventitious. M. rhomboides Gosse is recorded by Hempel (99) from the river plankton, as also M. acuminata Ehrbg., triptera Ehrbg., and bractea Ehrbg. from the backwaters. Monostyla bulla Gosse.—Average number, 50. Present in small numbers and irregularly from April till the middle of October at temperatures above 50°. It is evidently adventitious. Jennings _(700) finds this one of the most abundant rotifers among the aquatic vegetation in Lake Erie. It is in our waters the most abundant of the genus in the plankton, especially in the vegetation-rich back- waters. Monostyla lunaris Ehrbg.—Average number, 37. Found in the extremes of the temperature range, but over 50 per cent. of the occurrences are in August—October. Its numbers are always small and its occurrences irregular. It is plainly adventitious. Monostyla quadridentata Ehrbg.—Average number, 10. This species was found in the plankton irregularly in July-September, at maximum temperatures. It is abundant (Hempel, ’99) in the backwaters, where vegetation is abundant, and is apparently adven- titious in the plankton. In addition to the species here recorded Hempel (’99) lists M. cornuta Ehrbg. and M. mollis Ehrbg. from collections in the river,and M.closterocerca Schmarda from the back- waters. This is an exceedingly variable group, and will repay a thorough revision in the light of a study of the variation of its species. A considerable reduction in the number of these so-called species will doubtless result from such a study. ZO2 Noteus quadricornis Ehrbg.—Average number, 19. This is a rare species in the plankton, being found in 1895 and 1896 in July at maximum temperatures, and in 1898, on April 12, at 52°, and on November 8, at 46°. Notholca longispina Kell.—This species, which has been found in the summer plankton of many European and American waters, especially our Great Lakes, was noted but once in the Hlnois—in January, 1895 (Hempel, 99). It seems to prefer cooler and purer waters. Notholca striata Ehrbg.—Average number, 437, including varie- ties. This is a winter planktont in our waters, appearing in 1897 on November 30, at 34°, reaching a maximum of 10,840 March 22 (Table I.), at 51°, and disappearing April 19, at 52°. It reappears the following autumn on November 1, at 45°, and attains a maxti- mum of 4,000 March 21, at 37°. In previous years the occurrences all lie within the limits of November 1 and April 24 with the excep- tion of two records in 1895—September 5 and October 15, at 74° and 56°. The spring maximum in 1896 (7,778) was on April 10, at 52°, and in 1897 (4,260) on March 22, at 43° “In each year bute single pulse, that of March—April, is indicated. Minor fluctuations during the winter (Table I.) are in some cases attributable to flood agencies. The temperature limits of this species are quite definitely estab- lished. The species reappears in autumn when 45° is reached, and declines rapidly in the spring after 50° is passed and is but rarely found above 60°. It attains its greatest numbers late in winter or early in spring in the face of flood conditions, though the numbers attained in the channel waters are never very large. Empty loricee have been found in the plankton after the decline of the species in April, and females with a single egg were noted in small numbers in 1895 during the rise of the pulse. I follow the suggestion of Weber (’98) that N. striata should include as varieties the following: WN. labts Gosse, N. jugosa Gosse, and N. acuminata Gosse. Examination of many individuals in the plankton proves beyond a doubt the great variability of the organ- ism whose seasonal occurrence we have traced. It varies in the length of the posterior spine, in the proportions of the lorica, and in the development of the striz and the anterior spines. Of a total of 81,227 of Notholca striata in this wider sense, 68,887 were referred 203 to var. acuminata, 3,852 to var. jugosa, 7,029 to N. striata in the narrower sense, and 1,469 to other varieties, including var.labzs and var. scapha. The seasonal distribution of N. striata (sensu strictu) and var. jugosa lies within the limits of that of var. acuminata, but occurrences are too few to trace their seasonal fluctuations. This species is reported by Lauterborn (’94) in the winter plankton of the Rhine. He also notes the connecting links between N. acuminata, N. striata, and N. labis, and regards them as belonging to the same “ Formenkreis.’’ Apstein (’96) reports N. acuminata, N.labis,and N. striata in lakes of northern Germany and indicates a seasonal distribution which coincides closely with that found for these forms in the waters of the Illinois. He also reports a March— April maximum and only isolated occurrences in midsummer. Forbes (’83) finds the species in the stomachs of young Coregonus feeding upon the March plankton of Lake Michigan. Seligo (’00) also finds it in the winter plankton of Prussian waters. Notommata cyrtopus Gosse was found in the plankton in April and September at temperatures above 50°. Hempel (’99) reports N. aurita Ehrbg. from the river, and N. tripus Ehrbg.and N.lacinu- lata Ehrbg. (=Dzaschiza lacinulata Ehrbg.) from the backwaters. Plesoma lenticulare Herrick was found in the plankton of the river from September to December, 1896, throughout the whole range of temperatures from 75° to the winter minimum. Hempel (799) reports it from May to December, but principally in vegetation Polyarthra platyptera Ehrbg.—Average number of individuals, 86,674; of eggs, 52,560. In 1897, 94,653 and 58,235; in 1896, 29,653 emcell 138: in 1895, 28,947 and 20,074; in 1894, 743 and 217. The effect of the stable conditions of 1897 and of the recurrent floods of 1896 is seen in the larger averages in the former year and in the smaller ones in the latter. This is one of the most abundant rotifers in our plankton, includ- ing, as it does, one seventh of the total Rotifera, and exceeding in numbers all other species of the group excepting only Syncheta stylata. It isa perennial form, and was recorded in every plankton collection but two, and it may have been present then. The seasonal distribution of this abundant species is very char- acteristic of the form which most, though not all, plankton organ- isms exhibit. Two prominent features are (1) a limitation of large numbers to the warmer months and (2) a rhythmic occurrence of 204 t recurrent pulses at approximately monthly intervals. In Plate V. I have plotted the seasonal distribution of this species for the years 1894-99. The plate will serve as one of the best illustrations of the nature of the data contained in my statistical records that could be chosen from them. It illustrates graphically the character of the seasonal distribution of this species and the nature of what I have called recurrent pulses. In the table which follows, as elsewhere in similar tables, these pulses are listed by the number of individuals attained at their maxima, and are located according to the dates of these maxima. PuLSES OF POLYARTHRA PLATYPTERA. Year Date Temp. No. Date Temp. No. Date Temp. No. 1895 == rT i 1896 Jan. 6 32° 5,406 | Feb. 25 34° 7,852 | Mar. 24 41° 57,267 eS) ooe 2,736 1897 ——= = —=> ——$—— ||| == Se 1898 Jan. 25 32° 11,997 Feb. 22 32° 6,318 == 1899 Jan. 17 33° | 20,800 | Feb. 14 Sis? 145,600 | Mar. 7 335 71,200 | } | Year Date Temp. No. Date Temp. No. Date Temp. No. } | 1895 | Apr. 29 GAO Nhs SOAR 0e | ee 1896 Apr. 24 (oo 233,436 | May 8 76° 54,365 | June 1 69° 18,000 eal 73° 35,200 1897 | Apr. 27 60° 472,000) | ——— == = 1898 Apr. 26 Sifis 696,000 | May 17 64° 195,200 | June 14 82° | 432,800 | Year Date Temp. No. Date Temp. No. Date Temp. No. es 1894 July 30 82° 1,908 — ————— = 1895 july 6 | 81° 231,504 | Aug. 1 ¥9OS - 6,350 | Sept. 12 79° 19,272 seat PK 82° TASS 1896 July 10 | 980° 90,000 | Aug. 8 86° 39,200 | -—— —_ | —— | Pedi 2s 82° 71,000 1897 July 21 | 81° | 172,000 | Aug. 24 78° 230,400 | Sept. 14 |~ 83° 50,000 {9384 Aug. 2 78° | 288,000 | Sept.27 | 73° | 238,400 | es 82° 96,000 205 PuLseEs OF POLYARTHRA PLATYPTERA—continued. : | Date’. | Temp. No. Date Temp. No. Date | Temp. No. es ea | Oct. 17 58° 1,140 — —— Oct. 23 SAS 408 Nov. 27 SSE 74,942 | Dec. 18 39° 21,147 —— —- | Dec. 29 | 35°] 37,560 Octars 7 fa et 816,000 | Nov. 15 47° 22,400 | Dec. 14 40° 7,300 Och 11 65° 47,500 | Nov. 22 40° 6,000 | Dec. 20 33° 63,400 mom 2) 49° 37,500 An examination of this table and the graphic presentation (PI. V.) of the seasonal distribution will show at once the uniformly small numbers attained at low temperatures. Between October 15 and April 15, that is below 60°, no pulse exceeding 100,000 is reached save one of 122,400, February 21, 1899, at 33°. Of all the records in this period only seven exceed 50,000. On the other hand, during the warmer months, above 60°, the pulses have a much greater amplitude. Four of them exceed 400,000, and there are twenty-two records above 100,000. The summer pulses are often separated by minima which approach midwinter levels, but in spite of this the general level of summer occurrences is much higher than that of the colder season. In 1898 the average from April 15 to _ October 15 was 30,861 per m.*, and for the other months of the year, ~ 15,813, or about half the number in the warmer season. From these facts of distribution it is apparent that though perennial the species finds its optimum conditions at temperatures above 60°. The statement of Hempel (’99) that it thrives best in cold water is not. borne out by the statistical examination in any of the years. _ The recurrent pulses of this species vary greatly in amplitude. The largest pulse recorded was that of 816,000, October 5, 1897, at 71°. It appeared in a period of prolonged low water and at the close of one of high temperatures continued beyond the usual September limit (Pt. I., Pl. XI.),in a very unusual development of Carter1a and the smaller alge of the water-bloom (Pl. II.). Similar autumnal pulses do not appear in other years, the autumnal develop- ment as a rule not exceeding to any noticeable degree that of mid- summer. There has been in every fully tested spring a large vernal pulse, usually at the time of the spring volumetric maximum, or thereabouts. In 1896 and 1898 it was the largest pulse of the year. 206 This was not true in other years, but collections in those years were too infrequent to trace the seasonal distribution of the species with accuracy at that season. It is volumetrically of some importance in determining the quantitative fluctuations in the total plankton. Computations based on its average size indicate that approximately 600,000, including eggs, would be required to form 1 cm.* of plank- ton. On this basis, and allowing 10) pertcent. for interstices au constituted at the time of its vernal maximum in 1898 about 10 per cent. of the total volume of the plankton (silk-net catch). The table on pages 204 and 205 lists 43 pulses, of which 6 le out- side of the period included in Plates I.and II. Of the 38 remaining pulses 16 coincide in location with the whole or a part (in case of divided culminations) of the pulses of the chlorophyll-bearing organ- isms; 12 follow at the next collection, usually at intervals of one week; and 6, after a fortnight. The remaining 4 do not bear this rela- tion, occurring in autumn or midwinter, when all pulses were feeble and ill-defined. A comparison of Plates I. and II. with V. will show that not all of the chlorophyll-bearing pulses are attended by pulses of Polyarthra; nor is there any constant relation, excepting the vernal pulse, between the size of the pulses of the two groups of planktonts in question. Nevertheless, the dependence of the recurrent periods of rapid multiplication of Polyarthra upon the rhythmic occurrences of the chlorophyll-bearing organisms upon which they largely depend for their food is strongly suggested by the data here offered. Food relations thus dominate the repro- ductive cycles. The pulses of Polyarthra form a considerable portion of many of the pulses of the total Ploima, and it is but natural that we should find a coincidence in their locations. This may be followed for 1898 in Table I. Ina number of instances the culminations of the pulses are not exactly coincident, but separated by the interval between two collections. The association of the two pulses is, however, apparent in every case, and a similar relation may be traced in prior years. These recurrent pulses afford evidence for the polycyclic habit of this species. Additional proof of this phenomenon is found in the evidences of sexual reproduction—either male or winter eggs attached to the female—which have attended many of the pulses. The eggs of this species, both summer and winter forms, are very 207 readily detached in the manipulation of the plankton, so much so that in 1898 less than 6 per cent. remained attached. More or less uncertainty attends the determination of the parentage of detached winter and male eggs, so that decisive proof of sexual reproduction is best obtained from the attached eggs. In Table I. will be found the records of free and attached male and winter eggs recorded in 1898. Evidence will be found in this of sexual reproduction at- tending the pulses of March, April, May, September, and December. The presence of winter eggs at intervals throughout the greater part of the year may be due either to their continual production or, as seems more probable, to their continuance in the plankton for some time after their formation. The presence of attached winter eggs, or of larger numbers of free winter eggs, seems to mark the culmina- tion and decline of the pulse. Male eggs, on the other hand, are more generally present during both the rise and decline of the pulses. Somewhat similar evidence of sexual cycles attends many of the larger pulses in years prior to 1898. This species affords a striking example of a perennial eulimnetic planktont. It is found in midwinter under the ice in water at the freezing point, and even under these conditions it multiplies, pro- ducing pulses whose amplitude surpasses that of many rotifers of the plankton, and runs a reproductive cycle similar to, though of less amplitude than, those at other seasons of the year. It shares with other organisms the vernal outburst, and repeats the process in summer months under maximum conditions of heat and in waters whose chemical condition is very different from that in which the hiemal and vernal pulses appeared. Successive generations of this species are thus adapted to widely different conditions. Through all the changes incident to ice, stagnation, flood, sewage pollution, changing temperature, the wax and wane and change of food, the constant and unceasing warfare of enemies which prey upon it and of parasites which plague it, and, above all and continuously, the removal of countless individuals from the place of their origin by the ceaseless current of the stream, this species lives on, holds its own in the plankton, and repeats year after year the same sequence of rhythmic pulses of occurrence in the river water. The secret of the process doubtless lies in its capacity to produce repeatedly these crops of winter eggs which serve to seed the environment and start 208 anew the cycle of growth and reproduction whenever the favorable conditions prevail. There is in this species no hard lorica whose variable processes might serve to demonstrate to every observer its capacity for varia- tion. This is doubtless one of the reasons why we do not find a host of new species and varieties of Polyarthra as in the case of Brachionus. It is subject to considerable variation in size,and the swimming lamellz vary in length, width, and serrations. Hempel (’99) records Wierzejski’s var. euryptera in our plankton, and I have often observed it, but no record was kept of it since the characters which define it are not readily seen in plankton enumeration. Weber (’98) has mentioned, without designating by name, a long-spined variety which I find very common among the individuals which occur in the Illinois. This planktont is subject to attacks of internal parasites (S poro- zoa?) which infest it at the times of its maximum pulses, though never to the extent observed in the case of Bimerium in Brachionus. It is very frequently loaded down by Colacium, and some of the smaller peritrichous Ciliata are often found upon it. The absence of a hard lorica has served to obscure somewhat its food relations to whatever animals prey upon it. Polyarthra platyptera is a cosmopolite, and is apparently found generally in the fresh-water plankton. Jennings (’00) reports it as abundant in the waters of the Great Lakes, and it has been found generally in American waters. Zacharias (’98) and Marsson (’00) find it in pond and stream waters of Germany; Stenroos (’98) reports it asa predominant rotifer in the planktonand littoral regions of Finland waters; and Borge (’00) finds it in Swedish plankton. It has also been found to be an important constituent in the plankton of European streams. Skorikow (’96) finds that it is the most abundant rotifer in the summer plankton of the River Udy, consti-. tuting almost a third of the total rotifers. There are indications in his records of recurrent pulses, and the largest numbers are found in September. Zimmer (’99) finds it perennial in the Oder, but never abundant. Schorler (’00) finds it in the Elbe from April to September, with maximum in August. Lauterborn (’98a) lists this species among the perennial rotifers,and states that it is dicyclic in the Rhine and its adjacent waters, which he has examined quite thoroughly. The vernal sexual period begins with the appearance Prints ai, 209 of the male eggs in March, and winter eggs follow in April and May. The second sexual period extends from the end of July to the end of October, with a maximum in September—October. This bears some resemblance to the distribution in the Illinois, with the exception that the recurrent cycles which make the species poly- cyclic were not noted, and that male or winter eggs were not present in the colder months. It may be that the application of the quantitative statistical method with brief intervals of collection in the Rhine would reveal a still closer correspondence in the seasonal routine of Polyarthra in the two streams. Wesenburg-Lund (’98) finds that temperature has nothing to do with the appearance of the sexual cycle of this species in Danish waters. Males were found in December, as also (eggs only) in the Illinois. He also found differences in different bodies of water as to the times of the sexual cycles. Apstein (’96) has found this species perennial and one of the most abundant rotifers in plankton of the lakes near Plén, Germany, with maximum period from April to August, and in November in one lake, and in July-August in another. The sexual cycle was noted in May—June only. Seligo (’00) finds the species perennial in lakes near Danzig, with large numbers in April and July. His collections were too widely separated to trace fully the seasonal fluctuations. Burckhardt (’00a) finds Polyarthra in small numbers in winter months in the plankton of Swiss lakes, and in larger numbers in the summer, but does not trace their seasonal fluctuations. Pterodina patina Ehrbg.—Average number of females, 37. With two exceptions all the records of this species lie between the last of May and the first of October. There are but four records below 70°. This indicates optimum conditions for the species during the period of maximum heat, and further evidence of this lies in the occurrence of the larger numbers during this period. Appearances in January—March suggest a perennial habit; and small and irregular numbers, that the species is largely adventitions. Hempel (’99) also records P. valvata Hudson from Quiver Lake. Rattulus tigris O. F. Mull.—Average number of females, 207. I have not found this species in any year later than October, though, as shown in Table I.,it appears in January at minimum temperatures, and continues in small numbers and somewhat irregularly until autumn. These conditions and the absence of pulses suggest that 210 the species is adventitious in the plankton. The greater part of the occurrences were recorded above 50° and the larger numbers above 60°, indicating an optimum during summer months. The record in Table I. refers to the species figured by Jennings (’00) under this name. Rattulus sulcatus Jennings was found seven times in the plankton in July and August during maximum temperatures. It 1s probably adventitious in the plankton. Salpina brevispina Ehrbg. was found September 5, 1895, at 74°, and Aprile29, 1890, at 70° Salpina eustala Gosse was found July 13, 1894, at 82°. Salpina macracantha Gosse was found September 5, 1895, at 74°. Salpina ventralis Ehrbg. was found July 29, 1895, at 75°. In common with other species of the genus it is adventitious in the plankton. Schizocerca diversicornts v. Daday.—Average number of females, 46. The earliest record of this species was June 1, 1896, at 70°; and the latest, September 20, 1895, at 78°. Most° ot the recordszamal the larger numbers are in July-September during the period of maximum heat, in which its optimum conditions must be found. Egg-bearing females were also found in these months. This species is closely related to the Anurea aculeata group, and like it is exceed- ingly variable, especially in degree of development of the various spines. Variety homoceros Wierz. was found in May, June, and August, 1896. Five sixths of all the individuals recorded were found in 1896, and the fact that this was a year of unusually dis- turbed hydrograph (Pt. I., Pl. X.) suggests that this form may be to some extent adventitious in our plankton, but no direct relation to the access of flood waters can be traced. Lauterborn (’98a) lists this speciesamong the summer planktonts of the Rhine, and Seligo (’00) finds it in large numbers, with a maxi- mum in July, in lakes near Danzig. Zacharias (’98) reports it in German pond plankton, Zimmer (’99) finds it in the Oder, and Schorler (’00) in the summer plankton of the Elbe. Syncheta pectinata Ehrbg.—Average number of individuals, 3,950; of eggs, 13,823. It was much more abundant in previous years, averaging in 1897 23,227 and 28,230; in 1896, 7,064 and 7,927; in 1895, 13,071 and 4,730" 1m £894 7-520. and 65 O=maibae effect of the disturbed hydrograph of 1896 is seen in the smaller 214 numbers of that year, while the larger numbers in 1897 may be attributed to the more stable conditions. The small numbers in 1898 do not seem to be correlated with any feature of the environ- ment. This species has been found in every month of the year, and is thus perennial in our plankton. As will be seen, however, in Table I., the most of the occurrences and a much greater proportion of the individuals are found between May and October, and thus above 60°. The same limitations are found in the other years, with the exception that in 1896 there was a more continuous and larger de- velopment from the last of February. In the table which follows it may be noted that all of the pulses but four are at temperatures above 70°, and of these four none exceeds 25,000, and two do not ~ exceed 2,500. The optimum conditions for the species in our -. waters are therefore above 70°. The average temperature at the time of the larger pulses is near 80°. The vernal pulses are poorly defined, as are likewise the autumnal ones. It is a midsummer species in our waters, with its maximum in August. PULSES OF SYNCHETA PECTINATA. Year Date Temp. No. Date Temp. ‘No. Date Temp. No. 1896 Mar. 3 3D 6,360 Apr. 10 46° DUBE CNR) | —= | oS 1898 ——_ = |S eS ZO iif 1,600 | June 21 io | 1125000 Year Date Temp. No. Date Temp. No. Date Temp. No. 1894 July 13 83° 74,606 1895 July 23 go° 1,749 | Aug.12 | 85° 175,230 | Sept. 12 | 79° |. 27,740 1896 July 10 go° | 22,200 | Aug.26 | 75° oe40g —— : MPS 82° | 38,000 1897 — Aug. 10 | 81° 5382008 at2a e780 9\|-964 000 1898 July 19 | 84° | 20,800 | Aug. 2 78° 12,000 | Sept. 27 | 73° | 30,400 eG ee WO 3,200 1898 Deen a) 2,500 ZZ Of the 18 pulses listed in the preceding table 17,fall within the limits of periods included in Plates I. and II. Of these 17 there are 7 which coincide with, and 9 which follow shortly after, the culmina- tion of the pulses of the chlorophyll-bearing organisms, while 1, a small one in March, 1896, shows no such correlation. Food is thus a primary factor in the production of these recurrent pulses. As will be seen in Table I.,these pulses uniformly coincide with those of the total Ploima, and a similar relation may be followed in prior years. The eggs of this species are not usually carried by the female for any length of time, and are rarely found attached in preserved material. For this reason the sexual cycles are not easily followed with accuracy in the statistical data. It may be seen in Table I. that the free winter eggs belonging to both species of Svucheta are most numerous in the period of the larger pulses, and that their occur- rences show some tendency to coincide with these pulses. Proof that these pulses terminate in sexual reproduction is thus lacking, though it seems probable from some of the evidence. Syncheta pectinata has not been widely reported from American waters. Jennings (94) finds it in Michigan and Kellicott (’97) in Lake Erie, but it has not been elsewhere reported in American plankton. It appears, however, in many European records. Skori- kow (’96) finds it in the summer plankton of the River Udy, in Russia ; Zimmer ('99) finds it in common with S. tremula in the Oder throughout the year. He makes the statements-that it is never rare, 1s somewhat more abundant in the spring, and is, at other times, present “in relativ gleichmassiger Haufigkeit.’”’ In the light of our results it seems probable that the data at Zimmer’s disposal were insufficient to justify his conclusions as to the uniform- ity of its seasonal distribution. Schorler (’00) finds it in the Elbe in April, May, and October, with a maximum in May. Lauterborn (’98a) finds it perennial in the plankton of the Rhine, and lists it among the dicyclic species with two periods of sexual reproduction, one in April and one from the end of July to October. Judging from the character of the statistical data which have been presented for this and other species in the Illinois it seems probable that the later period noted by Lauterborn may include several cycles, and that the species is usually a polycyclic one. Seligo (’00) reports it perennial in waters near Danzig, with largest numbers in April and ¥ 213, September. Apstein (’96) finds that this species (including S. tremula and S. grandis) is one of the most abundant in lakes near Pl6n, with variable maxima in different bodies of water. He finds it perennial in one case, and reports vernal maxima. Winter eggs were found in March and April. Syncheta stylata Wierz.—Average number of individuals, 120,391 ; oi eggs, 17,/97. In 1897, 42,577 and 9,127; in 1896, 24,099 and pe>- im 1895, 155,880 and 2,418; in 1894, 8,582 and 132. This species affords an exception to the general rule hitherto observed among the rotifers of our plankton in that it is more abundant in 1898 than in the previous year. As will be seen in the following table both the vernal and autumnal pulses are unusually large in 1898, while in the previous year the vernal pulse is only moderate and the autumnal pulse is scarcely to be detected. For some reason the prolonged low water and sewage contamination of the autumn of 1897 was not favorable to the usual growth of this species. It may be that it was crowded out by the unusual develop- ment of Polyarthra at that season (Pl. V.). PULSES OF SYNCHATA STYLATA. Year Date Temp. No. Date Temp. No. Date | Temp. | No. on = Ee ee ees a ee 1896 | Jan. 6 ae | 9 Heese | eee) ee | ee Se es Soe 3,648 a) — || | |) ——— | —— 1898 Jane 25 SyHe LOS || =< — ——— || “Mars Bee 6,400 eee 2: byl 58,000 1899 Jan. 14 34° 12,000 | Feb. 14 320 19,200 | Mar. 21 Sih 5,600 ; Year Date Temp. No. Date Temp.| No. Date Temp. | No. 1894 ——— —_ — 1895 Apr. 29 64° 219,123 1896 Apr. 29 70° 380,586 May 25 58 10,800 | June 17 76° 79,200 1897 May 25 66° 643,680 Wales) | == — | May 3 60° /1,139,000 | June 21 TN 1955200 set 70° | ’ 61,600 | | 214 PuLSES OF SYNCHETA STYLATA—continued. | | | | Year | Date Temp. | No. | Date Temp. | No. Date Temp. No. | | | | | | 1894 | -—— 1895 | | | Aug. 1 79° | 10,287 | Sept.27 | 73° (20208 1896 es ——— || Ane, 8 86° 8,400 | ° | | 1897 July 21 So e103 200 | Sept. 7 80° 28,000 1898 July 19 34° 64,800 | Aug. 2 79° | 170,400 | Sept. 27 73° 265,600 TT Oe Bae 24,800 Year Date | Temp. No. | Date Temp. No. Date | Temp. No. | | | | | | | 1 894 Octy e179) 58° 9 263;.9351)] | — — | | | | | | 1895 | | | Nov. 27 33° | 901,901 | Dec. 11 32° | 1,121,056 | | | | | 1896 | | Nov. 17 44°} 114,000 | 1897 | Oct. 5 | 71°] 12,000] Nov. 9 50° 26,400 | Dec. 14 36° 72,200 | fe 498) y 652)! ) ase s00 “30° | 34.59 87,200 1898 Oct. 25 49° | 824,500] Nov. 15 41° | 110,000 | Dec: 6 34° 42,500 S220 33° 59,200 This is the most abundant of all the rotifers in our plankton, exceeding by 30 per cent. Polyarthra, the next in abundance. It constituted one fifth of the total Plowma in 1898, and is accordingly a large factor quantitatively and ecologically in the economy of the plankton of the Illinois River. It is a perennial planktont, occurring in six sevenths of our collections and usually in considerable numbers. The distribution in 1898 (Table I.) is a fair index of the usual seasonal routine, with the exception that in all prior years the July-August minimum is more pronounced and better sustained. The development in January—February is never large, rarely exceeding 20,000. In March, numbers rise rapidly, usually with a minor pulse, the re- covery from which in April culminates in a vernal pulse, which in three of the six years was the largest of the year. Following this vernal pulse there is a series of smaller pulses throughout the sum- mer. The decline of the June flood, when this occurs, seems to offer favorable conditions (cf. foregoing table and Pt. IJ., Pl. [X.—XII.) for the development of a pulse which is but little smaller than the vernal one. It may be of some significance that this pulse and the to y 2195 vernal one both occur on the decline of the major floods of the year, and that the relative proportions of the two floods are to some degree paralleled by the amplitude of the pulses of Syncheta which attend their decline. The effect of the impounding backwaters as reservoirs for the greater development of the plankton is suggested by these data. Following the midsummer minimum is an autumnal pulse whose amplitude and location alike are subject tomuch variation. Aswill be seen in the table on pages 213 and 214, the maximum autumnal pulse is located twice in October, twice in November, and once in Decem- ber. This may be due to the fact that the collections are insufficient in some of the years, or to the probability that any one of several recurrent autumnal pulses may be the major pulse of that season. An examination of the seasonal distribution in 1898 (Table I.) and of the location and temperatures of the pulses recorded in the table on pages 213 and 214 will suffice to demonstrate the capacity of this species to develop at all temperatures within the seasonal range. The largest pulse (1,139,000 on May 3, 1898) is at 60°, and the next in size (1,121,056 on December 11, 1895) is at 32°. It will, however, be seen in the two tables that the pulses and the numbers in general during the periods of maximum heat and cold are not so large as in the intervals of more moderate temperatures. The impetus of the autumnal development may carry some of the pulses over in to minimum temperatures, but the level of development declines thereafter. There is thus something of a tendency for the average temperature of the larger occurrences to approach the average temperature of the year. The number of pulses listed in the table on pages 213 and 214 1s 38. Of these, 34 fall within the period included in Plates I.and IJ. of the pulses of chlorophyll-bearing organisms. Of the 34 there are 18 which coincide in location with these plant pulses, 12 which follow at a brief interval, and 4 which bear no such relation, three of the last being minor winter pulses. The dependence of the recurrent periods of rapid multiplication of Syncheta—the most abundant rotifer of the plankton—upon the rhythmic increase of the food supply is thus fairly demonstrated. The coincidence of the pulses of Syncheta with those of the total Ploima is readily seen in Table I.,and is equally apparent in prior years. 216 Eggs of this species are not carried by the parent for any length of time, so that reproductive cycles are not easily traced. The total number of the summer eggs of Syucheta will be found (Table I.) to fluctuate somewhat with the pulses of the species. The free winter eggs, belonging probably to both species of Syncheta, also show some tendency to predominate at and after the culmination (Table I.) of the pulses. A female carrying a male egg was recorded during the rise of the spring pulse in 1898, and attached winter eggs were noted at the vernal pulse in 1895 and 1897. The evidence points toward the culmination of these pulses in a sexual cycle. The soft and flexible nature of this rotifer and the absence of spinous outgrowths have made whatever variability the species possesses less evident than it isin such a genus as Brachtonus. ‘There is considerable variation in size—possibly due to age—even in the same collection. The determination of preserved material of this genus is fraught with insuperable difficulty. The separation of pectinata and stylata in our records is at the best only probable. It may be that other species of Syncheta have been included with the individuals referred to stylata. In any event the result of the division has led to symmetrical results comparable with those of other planktonts. Syncheta is often parasitized at the times of the larger pulses by some sporozoan (?). At the maximum of the vernal pulse in 1898 over 4 per cent. of the individuals were thus affected, the infestation continuing through the decline of the pulse. External parasites, Colacium and ‘Rhabdostyla, are rare. This species has not been found widely in the plankton, possibly because of the confusion of stylata, tremula, and pectinata in identifi- cation. From the large numbers reported in almost every instance where it has been found, the expectation of its wide-spread occur- rence is at least raised, waiving in this connection the possibility of specific confusion. Jennings (’94) found it to be very abundant in towings in Lake St. Clair, and (96) in Lake Michigan near Charle- voix. He finds it less abundant in the summer plankton of Lake Erie (’00). Stenroos (’98) reports it as one of the most abundant limnetic rotifers in Lake Nurmijarvi in Finland in the summer, and Skorikow (’97) finds that next to Polyarthra it is the most abundant rotifer in summer months in the River Udy near Charkow, Russia. His figures of occurrence show some traces of recurrent cycles in these months, with maximum numbers at the first of August. Lau- a 217 terborn (’98a) lists it among the summer rotifers of the plankton of the Rhine. The genus is in need of a thorough revision in the light of possible variation.* Taphrocampa annulosa Gosse.—Average number, 71. Found in September, at 73°. Evidently adventitious. Triarthra longiseta Ehrbg.—Average number of individuals, 3,147; of eggs, 293. This species was about twice as abundant in the stable conditions of 1897, and was present in less than half these numbers in the recurrent floods of 1896. It is a perennial species, having occurred in every month of the year. The continuous occurrences and the larger numbers le in all years between May and October and above 60°. In 1898, only about 3 per cent. of the total individuals were found below this temperature. With the exception of the vernal pulse of 1898 all of the larger numbers were found in the period of maximum heat. The optimum conditions for this species are thus found within that period and above 70°. The seasonal routine of the species is varied somewhat from year to year. There is usually a slight vernal pulse—larger than usual in 1898—and this is followed by recurrent pulses throughout the summer. The season closes without a predominant autumnal pulse, and after September the numbers fall and the occurrences become sporadic until the following April. The pulses of this species are listed in the following table, which gives their locations and temperatures. Of the 21 pulses recorded, 18 are within the periods of the plant pulses shown in Plates I. and II. Of these 18 there are 8 which coincide with these plant pulses, 9 which follow after a short interval, and 1 which shows no such relation. The dependence of the pulses of Triarthra upon food conditions is suggested. The pulses of Triarthra will be found on examination of Table I. to coincide in 1898 in the main with those of the total Plowma. The pulses are never very large, and the evidences of reproduc- tion are not well defined. Attached summer eggs attend the larger pulses, and free winter eggs of the species were found in October— November in 1898. In previous years free or attached eggs attended vernal or summer pulses at times. The evidence indicates a poly- cyclic habit. * See Rousselet, 02. PAN Ice) PULSES OF TRIARTHRA LONGISETA. | | Year | Date | Temp. | No. Date | Temp. | No. Date Temp. No. 1894 | 1895 | Apr. 29 | 64° | 2,332 2S) gee), eae 1896 Apr. 29 102 5,556 June 11 ase 4,000 st 80° 6,000 1897 —_— ——| —— | May 25 66° 8,800 1898 —_——— — —- May 10 62° 38,400 June 28 78° 800 oil 70° 1,000 Year | Date Temp. | No. Date Temp. | No. Date | Temp. No. | | eS | 1894 —~- a Aug. 15 g4° A, 337, /— 1895 July 18 80° 19,080 Aug. 21 82° 10,683 Sept. 12 792. 2,336 1896 | July 6 80° 2,800 Aug. 8 86° 7,200 == 1897 July 21 81° 49 ,600 Aug. 17 79° 9,600 Sept. 7 80° 70,000 1897 Oct. 5 ial 8,000 —=— =— ——e 1898 July 26 89° 28,000 | Aug. 30 83° 6,400 Sept. 27 Wee 14,400 This is an exceedingly variable species. It varies in the relative length of the three long setz, in their spinosity, and in the location of the posterior one. Many of the individuals in our waters resemble the form described by Plate (’85) as T. terminalis. The long-spined form described by Zacharias (’94) as var. liamnetica is also abundant. It is doubtful if either form is worthy even of varietal distinction. This species has been reported only from Lake Erie and the Illinois River in this country, and seems to be rare in the former. Weber (’98) finds it abundant in the plankton of Lake Leman; Burckhardt (’00 and ’00a) reports it as wide-spread and almost perennial in Swiss lakes, but with its maximum in December— February, and slight development during warmer months. Borge (700) finds it to be one of the common rotifers in the summer plank- ton in Sweden; Marsson (’00) reports its perennial seasonal range in several German waters, with greater numbers during the warmer season. Apstein (’96) gives it a perennial distribution in Lake Plén, with larger numbers in June-November, and maximum in June-July or August. According to Seligo (’00) the species is per- i 29 ennial in lakes near Danzig, rivaling Polyarthra in abundance, and exhibiting maxima in the warmer months from April to October. It is also a member of the potamoplankton of European streams. Skorikow (’97) finds it in summer months in the Udy, and Zimmer (99) reports it as present in small numbers and irregularly in the Oder from April to November. Schorler (’00) finds it in the Elbe -in May—October with maxima in May and September, and Lauter- born (’98a) includes it in ‘his list of perennial rotifers in the plankton of the Rhine. It has two sexual periods, the first in March—May and the second in July—October, and he suggests the probability of a polycyclic habit in some waters. Trochosphera solstitialis Thorpe was found June 27, July 2, and August 15,in 1896; in 1897,on May 25 and July 14-30. Free winter eggs were taken August 15, 1896. All occurrences were above 66°. These records were all from plankton taken in mid-channel of the main stream. Tyrochosphera was found in greatest abundance at the outlet of Flag Lake (Pt. I., Pl. II.) in July, reaching 9,664 per m.? at 72°. It was also found in August in the weedy backwaters of Dogfish Lake. Both of these backwaters connect with the river (Pt. I., Pl. II.) below the point at which our collections were made. It was either introduced from some similar backwater higher up the stream than our plankton station, or developed in the river itself. SCIRTOPODA. This order is represented in the plankton by a single species, whose discussion will suffice for the order. Pedalion mirum Huds. Average number, 4,524. This is a summer planktont of somewhat definite temperature limits. The following table combined with the data in Table I. will suffice to characterize its seasonal fluctuations. Its limitation to temperatures above 60°, indeed almost 70°, is apparent. There are in all but two records below 60°, and but four below 70°. It is a typical midsummer planktont, with several recurrent pulses during the period of maximum temperatures. The location of these pulses with reference to those of the chlorophyll-bearing organisms is significant. AS shown in Table I., they follow immediately, or coincide with, those of the synthetic organisms. For example, the apices of the pulses of Mastigophora, 220 First record First maximum Year Date Temp. Date Temp. No. TOE tne spot eee eee a | iis CO) 83° 2,592 PROG el oa EA tere SS || nie 6 80° 330,932 TSO Gf ee cone eee aaitenect 4 Sete yeaa May 25 102 July 28 80° 20,000 LiSO78 sine AEA We ee June 28 75° | July 21 34° 80,000 TS9Ss ce, Lee, Sa ee June 21 77° | July 26 89° 99 600 Second maximum Last record Year Date Temp. No Date Temp. 1S OAR Me ieee ras tick: oe == Sees Sepi yal 72° NES OSer eae Met ahs fie diosa ia eranenens | Aug. 21 Sie 3 Ode | Octa 22 6305 SD Ope were Ae soscctiot cheese Saete womse ee | Aug. 15 81° 77,600 | Sept. 16 qe Roe 7 PR m Retr ire Maer enrte Ar eats oR SeH ic | Aug. 17 ee 79,200 | Sept. 14 (ee RS OS Ne weh aie bio ee ee cve, cosy saa eevee Aug. 16 he No 2224000) Nowa sa 45° Bacillariacee, and Chlorophycee in the period in question in 1898 are (PI. II.) July 19, August 9, August 30, and September 27. The apices of the Pedalion pulses are July 26, August 16, and September 27, the last coinciding with the pulse of chlorophyll-bearing organ- isms. In 1897, the intercalation of the two pulses is apparent, and in 1896, two out of three pulses are intercalated and a third is coincident. As will be seen in Table I., these pulses of 1898 are approximately coincident in many cases with those of other roti- fers—Syncheta, Polyarthra, Triarthra, and Brachwonus. The sig- nificance of this intercalation lies probably in the food relations of the two groups of organisms. Females with a single egg attached to the body have been noted at the times of the maxima of the pulses, or immediately thereafter, Fit 4 ia i 22h. in five instances. On the pulse of July 26, 1898, a female with four male eggs was found. This species was not reported by Apstein (’96) from the lakes of Holstein, but was found by Lauterborn (’98a) in the Rhine and its backwaters. Here also it was a summer form, appearing about the middle of June, with a maximum in August or September and disappearing late in October, conditions of distribution much re- sembling those in the Illinois. It is regarded, along with other summer forms, as monocyclic. The appearance in our waters of male eggs July 26, at the height of the first pulse, leads to the in- ference that there may be several cycles; for example, three in 1898, with the recurrent pulses, in a single summer season. Weber (’98) gives it as a summer rotifer in Switzerland, and Skorikow (’97) finds it in July-September in the Udy River,in Russia; but it is not reported from the Oder by Zimmer (’99), nor from the Elbe by Schorler (’00). Kellicott (797) finds it in Lake Erie in small numbers in the summer. In addition to the species of rotifers noticed above, Hempel (’99) has reported the following in the Illinois River or its backwaters: Floscularia ornata Ehrbg., Limntas ceratophylla Schrank, Cephalosi- phon limnias Ehrbg., Cectstes intermedius Davis, O. mucicola Kell., Pedetes saltator Gosse, Furcularia forficula Ehrbg., F. longiseta Ehrbg., Eosphora aurita Ehrbg., Diglena grandis Ehrbg., D. catellina Ehrbg., D. biraphis Gosse, Celopus tenmor Gosse, Scaridium longt- caudum Ehrbg., Distyla gissensis Eckstein, D. olioensis Herrick, D. stokesi Pell, and D. hornemanni Ehrbg. GAS RIO MR CareAu: Chetonotus sp. occurred singly in the plankton August 29, 1896, July 30, 1897, and February 15, 1898, with a temperature range of $2.5° to 84°. ENTOMOSTRACA. Average number, 47,042. In 1897, a more stable year, 91,050; in 1896, a year of disturbed hydrograph, 50,158; in 1895, in more stable conditions, 148,348. The Entomostraca appear in every collec- tion at all seasons of the year. The decline to the winter mini- 222 mum occurs in November—December. Numbers are at a minimum generally less than 5,000 per m.*) in midwinter (January—February) ; rise in March to about 25,000 per m.?; and attain the maximum for the year in a vernal pulse of 200,000 to 1,500,000 in April-May. Following this, there is frequently a second pulse of large proportions in June, which in 1898 exceeds (Table I.) that of May. During the remainder of the year there is usually a series of recurrent pulses, of declining amplitude in 1896 and 1898, but rising to unusual heights (618,750 on September 9) in the stable conditions of 1897. In the main the pulses of Entomostraca coincide with or approximate to the location of those of the other organisms of the plankton, and often show correlations in amplitude. BRANCHIOPODA. Eubranchipus serratus Forbes. Young branchiopod larve questionably referred to this species appeared in the plankton in January—March, 1899, in small numbers at minimum tempera- tures. CLADOCERA. Average number, 6,068 per m.? In 1897 they were more abun- dant, averaging 17,863 per m.* in the more stable conditions of that year. In 1896,a year of recurrent floods, numbers fell to 7,719, while in 1895, a year of low water in spring, when many of the Cladocera attain their maximum, the greatest average, 31,937, was recorded. The phenomenal number of 443,716 per m.* appeared on June 19 in the stable low water (1.80 ft.) then prevailing. In 1894, another year of low levels, the annual average was also large (23,952), though probably enhanced by the fact that collections were not made in flood waters in this year. The Cladocera appear in all but 10 of the 182 collections enu- merated, the ten exceptions falling in November (1), January (2), February (6), and April (1), and usually in flood waters or, as in 1895,in stagnation conditions under the ice. Although the Cladocera occur in all months of the year, they nevertheless, as a group, exhibit decided temperature adaptations, as appears from the fact that all records 1n excess of 4,000 per m.' fall between May 1 and September 1 with but 6 exceptions,—4 in-the phenomenally Wa ts ee Vey . pi iy ey iby 223 early spring of 1896, and 2 in the delayed high temperature of October, 1897. The minimum records (less than 500 per m.*) are found during minimum temperatures. The numbers increase slightly (generally less than 2,000) as temperatures rise in March—April, rise abruptly, as they approach or pass 70°, to a vernal maximum in May—June, and decline during midsummer excepting when unusual pulses of Moina or Diaphanosoma raise the level of the pulse maxima above 25,000. This decline continues in channel plankton through the autumn until the low level of approximately 2,000 per m.’, at the most, is again attained in October, and falls irregularly to 500, or less, as minimum winter temperatures arrive in December. Ex- ceptions appear in 1897, when a well-defined autumnal pulse of large amplitude (193,500) is found on September 14, and is followed by others of declining amplitudes (137,600, October 5; 5,520, No- vember 15; 4,240, December 14) during stable autumnal conditions. All of the records above 4,000 per m.?, with one exception, are found at temperatures above 45°, and all in excess of 8,000, with 4 exceptions, after the vernal rise in temperature passes 70° in: April— May, and before the autumnal decline reaches this point in Septem- ber. The Cladocera are thus planktonts of the warmer channel- waters. The relation which hydrographic conditions bear to the seasonal occurrences of Cladocera is apparent in the yearly averages above quoted, and appears still more clearly in a comparison of the cladoceran population and movement in river levels in July— December, 1897 and 1898, as given below. July August Sept. +) Ost. Nov. Dec: Average No. | Cladocera 1897 |1898| 1897 |1898| 1897 |1898) 1897 1898/1897 1898|1897)1898 per m.3 | 127203050 139603756) 70675/1700 40350 1615/2532 620)1945) 236 | | Total movement | ‘in river levels, OO Nap AM DOA as) Ose iOoZ O26 BOE? 2) 56108 Sil 2a in ft. 224 Hydrographic changes affect the Cladocera by increasing the amount of silt and flocculent debris in suspension, which, by ad- herence to the swimming antennz and flotation processes of the animal, tend to impede its movements and sink it to the bottom, where it is removed from its normal feeding area and readily becomes the prey of the larger organisms of the bottom fauna. Barren flood waters also tend to displace and wash away 1n the increased current the Cladocera which have developed in the stream, and to afford both less food and less time for their further development. The occurrences of the total Cladocera fall into the type of recurrent pulses, though with slightly less distinctness than in the case of individual species of the group. Such pulses can be traced in all seasons in which records ‘were made at short intervals, and suggestions of their occurrence appear in the less frequent records of other seasons. Thus in July-December, 1897, (Pl. IV.), there are 6 well-defined pulses culminating at intervals of 3(1), 4(2), 5(1), and 6(1) weeks. In 1898 (Table I.) the pulses are less regular in the flood waters of the disturbed year. In 1896, when records were frequent, we can trace pulses in March, May, June, July, August, and September. The character of these pulses is well illustrated in the vernal pulse of 1898 (Table I. and Pl. IV.), culminating June 7 at 136,000. The species which share in this pulse are Alona affinis, A. costata, A. quadrangularts, Bosmina longtrostris*, Certodaphnia scitula*, Chydorus sphericus*, Daphnia hyalina*, D. cucullata*, Diaphanosoma brachyurum, Leptodora hyalina, Macrothrix laticornts, Motina micrura, Pleuroxus denticulatus, Scapholeberis mucronata, and Simocephalus serrulatus. Of these, only the five marked by the asterisk occur in numbers sufficient by our methods to delineate a pulse. The other species are accordingly of little consequence in modifying the form or location of the pulse. The June volumetric pulse (Part J., Pl. XII.) culminates June 14 at 6.99 cm.* per m.%, though the record for June 7 is also high (5.28). The cladoceran pulse culminates June 7 at 136,000. On this same day four of the dominant species also reach their culmination, viz.: Bosmina longirostris (62,800), Cericdaphnia scitula (55,800), Daphnia cucul- lata (3,400), and D. hyalina (11,600), the remaining 2,400 being contributed by other species. Chydorus sphericus, which appears this spring only in small numbers, attains its maximum (7,880) on May 24, two weeks earlier, though the record for May 31 is also high 225 (5,040), indicating a probable maximum between these dates. In other seasons, for example in 1896 and 1897, the maxima of this species coincide generally with those of other Cladocera, so that this divergence seems to be anomalous. An inspection of the table of records for 1898 gives a remarkably uniform and coincident rise and decline of the pulses of the several species which constitute this characteristic vernal pulse. No effort has been made by me to determine the total cladoceran fauna of the Illinois River. Only those species are here given which have appeared in our plankton enumeration. A number of others are known to occur in the littoral fauna, and a few scattering indi- viduals found in the plankton were not identified. Of the 25 forms here listed, only 10—named in the sequence of their relative numbers as shown in grand totals—may be regarded as typical planktonts, autolimnetic in channel plankton, viz.: Moina micrura, Bosmina longtirostris, Daphnia cucullata and vars. apicata and kahlbergiensis, D. hyalina, Certodaphnia scttula, Chydorus sphericus, Diaphanosoma brachyurum, and Leptodora hyalina. Of the ten, the last named and the varieties of D. cucullata appear to be of little quantitative importance in the channel plankton, though it may be that our methods of collection fail adequately to represent Leptodora. Of the remaining 15 species, Alona ajfinis, Ceriodaphnia reticulata and C. rotunda; Scapholeberis mucronata, and the two species of Szmocephalus are the only adventitious Cladocera of quantitative importance, and this only to a relatively small extent. DISCUSSION OF SPECIES OF CLADOCERA. Alona affinis Leydig.—Average number, 36. This species has a well-defined seasonal distribution. It appears in autumn in the last of October, as temperatures approach 40°, and remains until the end of June, when the summer maximum of 80° is re-established. The numbers are too small (Table I.) and irregular to define its seasonal fluctuations, though there are suggestions in the records of late autumnal and of vernal pulses. Egg-bearing females were recorded in January—February at minimum temperatures. No close dependence on hydrographic fluctuations is apparent to account for their occurrence in the plankton. Alona costata Sars.—Average number, 11. Only a few scattered occurrences of small numbers. Earliest autumnal record, Novem- ber 22, at 40°; latest vernal, May 24, at 73°. 226 Alona quadrangularis O. F. Mull—aAverage number, 5. A few scattered occurrences in March—May. Alona spp.—lIt is probable that some of the foregoing species of Alona are here included. There are 16 occurrences, scattered through all months but January, April, and November, with no large numbers and no marked seasonal distribution. Bosmina longirostris O. F. Mull.—Average number, 2,441, of which 1,527 are adult females without large embryos, 390 with them, and 524 immature. I include in this species B. cornuta Jurine, for I am unable to find any constant line of demarcation between these forms. The longirostris form is the dominant one in the channel plankton, the cornuta form being relatively rare. Bosmina is a perennial planktont in our channel plankton, but occurs in small numbers only in October—May, no record in this period with the exception of that of October 5, 1897 (20,400), at 71°, exceeding 5,000 per m.*, and most of them falling below 2,000. The records in November—March, with the exception of November— December, 1897, all fall below 1,000 per m.* In lke manner the percentage of collections containing Bosmina in December—April is lower than that in the summer, the percentages being 64, 16, 26, 47, and 55 per cent. respectively for these colder months, and averaging 82 per cent. for the rest of the year. The percentage of occurrences in October-November remains high (82 and 81 per cent.), though the numbers per m.° fall off greatly. The usual seasonal distribution is as follows: In January—March the occurrences are scattered and irregular and the numbers very small—less than 500 per m.* Toward the close of April the vernal increase makes its appearance, continues slowly through May, rarely attaining more than 5,000 per m.’, and at the end of this month or early in June reaches the maximum development of the year in a vernal pulse of 40,320 (1896) or 62,800 (1898) per m.’ From this summit there is an abrupt descent in a period of exhaustion to a level of less than 2,000 per m.* in the last fortnight of June. During the remainder of the year there appears a series of recurrent pulses of less magnitude, exceeding 10,000 per m.* in but three instances. These follow at intervals of four to six weeks. In July-September the amplitude of these pulses exceeds in all cases 5,000 per m.? In October (with the exception of 1897, when temperatures were un- jhe usually high), they decline in amplitude, and in November—Decem- ber often fail to appear in the small numbers recorded. In 1894, records are too scanty to be of significance. In 1895 there are three well-defined pulses, and traces of a fourth in August-Novem- ber. In 1896 there are five in May—September. In 1897 there are six in July-December, data during the remainder of the year being insufficient to define the pulses. In 1898 the vernal pulse in June and a feeble one in October are the only ones which appear. The pulses of Bosmina are best defined in the stable low water of the last six months of 1897. During that period they closely approxi- mate in location of maxima and minima the quantitative pulses and those of the chlorophyll-bearing organisms and of the rotifers. (Compare on this point the plates for 1897 in Part I.—Kofoid, ’03— and Pl. III.andIV.). The slopes of the pulses indicate that Bosmina is capable of very rapid multiplication; and their coincidence with other pulses just noted, taken in conjunction with the fact that males and ephippial eggs appear but rarely, suggests that these pulses of Bosmina are immediately dependent, in large part, upon fluctuations 1n the food supply for their origin and for the varying courses which they run. The relations of Bosmina to temperature appear in the facts that all pulses exceeding 5,000 per m.* in amplitude occur at tem- peratures above 70°, that the vernal rise does not proceed with any rapidity until this temperature is attained, and that the depressing effect of the autumnal decline below 70° is at once apparent in the reduced numbers per m.* No constant relation between the pulses of Bosmina and the midsummer heat pulses—such as appears in the records of Diaphanosoma—can be traced in the occurrences of Bosmina. An inspection of the accompanying table, in which the mean monthly Bosmina population per m.* of channel water in July—De- cember, 1897 and 1898, is given, and also the total + and — move- ment in river levels for these months in each year, will suggest an intimate connection between stability of hydrographic conditions and the increase of Bosmina. In 1897 the total movement for these months is from five sevenths to one tenth of that in 1898, and in every instance the Bosmina population is also greater by from 7.5 to nearly 400-fold in 1897, the more stable year. The means of the six months are 2.03 ft. and a population of 3,691 in 1897 to 5.3 ft. and (16) 228 BosMINA AND HYDROGRAPHIC FLUCTUATIONS.* | July | August September Year| | | Total Bosmina | Total | Bosmina Total Bosmina | movement, 3 | movement, es movement, Beate in feet Pee: in feet aaa ts in feet in —3.9 —2.6 | — .2 | 13975 6,213 4) 256 3973 6 |v aoe | +1.1 +0 | + .4 | = (9) —3.3 —2.6 T3983 |) 7. 140 etl 10 6 15 + .1 +4.4 +3.4 October | November December ae Total Bosmina Total Bosmina Total Bosmina movement, eae as movement, tne movement, i ne in feet Be ; in feet pea in feet Pp — .l — 7 — .6 1897 6 5,875 Da) 1,680 il 1585 + .5 +1.5 =O —1.1 — .6 —2.8 1898 | 3.9 780 See. 32 3.8 60 | +2.8 +2.6 +1.0 * + = rising levels; — = falling levels. 173 Bosmina in 1898. It is also true that months in which the disparity in stability is greatest are those in which the Bosmina ratios are greatest, and vice versa. It seems very probable that the increased current, the lessened time for breeding, and the greater burden of silt in flood conditions, especially rising waters, do not conduce to the rapid increase of Bosmina in channel plankton. The effect of the high temperatures of the late autumn of 1897 is apparent in the amplitude of the October, November, and De- cember pulses (20,400, 3,440, and 3,440, respectively), which exceed those of all other years at this season. Temperature thus plays— perhaps by virtue of its relation to the food supply—an important a i a —e 229 part in the seasonal delimitation of the amplitude of Bosmina pulses. The Bosmina population in the plankton consists largely of parthenogenetic females. Males and females with ephippial eggs, were recorded only in October-December, 1897, and then only in small numbers and isolated occurrences. Females with eggs or embryos and the free young were found at all seasons of the year and at all temperatures, but most abundantly at the time of the pulses. Parasitized or fungused individuals are also found occasionally at these seasons of greatest numbers, and the high mortality following a pulse is evidenced by the large number of dead occurring in the plankton. The proportions of females, females with eggs or em- bryos, young, and dead during the May—June pulse of 1898, may be traced in the following records. BosMINA PER M.’, May—JuNE, 1898. Date Females ees Young Total living | Dead AG 800 0 0 s00.—s| 0 Maye a3 sacs ne ss 1,600 400 800 2,800 O eee Oger 3 1,600 1,000 1,000 | 3,600 400 aL fatacsy Sonic 37 1,300 1,100 1,100 3500 | 100 ee Ae ee es, .3,280 1,400 1,240 5,920 920 Re TLC ater cliche 25,120 2 ,000 6, 800 33,920 | 1,280 [CMe Woecomac 38,800 9,200 14,800 62,800 9,200 seam Ar a) 3,24, 2,200 3,000 800 6,000 1,400 OO PAS ee 1,000 500 O | 1,500 | 100 eS him cess 300 } 200 200 | 700 | 100 Bosmzina longirostris has been frequently reported in the plankton of European lakes. Apstein (’96) finds it perennial in Plonersee with larger numbers in June-September and a maximum in July. No pulse-like recurrence is noted, parthenogenesis prevails, and males and ephippia are rare. His results, save in the matter of pulses, are thus in general accord with ours. Stingelin (’97) notes 230) great seasonal polymorphism in B. cornuta near Basel. Zacharias (97a and ’98b) records it in the plankton of German carp ponds. Stenroos (’97 and’98) finds it in waters of Finland and Karelia, where the cornuta type is littoral, and a limnetic form, distinguished by him as forma vernalts, is abundant in the plankton in May. Scour- field (’98) finds it common in the waters of Epping Forest, where it is perennial, males and ephippia appearing only in September— November. According to Scott (’99) it appears at various seasons in the lochs of Scotland in both the littoral and limnetic fauna. Burckhardt (’00a) gives an extensive revision of the genus bosmina, and includes in the B. longirostris group nine other so-called species, among which are B. cornuta Jur. The species is “ pelagic or hemi- pelagic” in various Swiss lakes, though apparently not in num- bers. The genus is there represented in the plankton princi- pally by the B. coregont group. Amberg (’00) lists it from Katzen- see, near Zurich, as a perennial planktont with large numbers in May, August, and February, but gives no statistical data. Fuhr- mann (’00) finds Bosmina perennial in Neuenburgersee, and B. - longirostris with a maximum in May. Marsson (’00) finds B. “longirostris-cornuta”’ in lakes about Berlin throughout the year, with larger numbers in some lakes during the warmer months and in others in November—December. In Barlewitzersee, near Danzig, Seligo (’00) reports B. cornuta as perennial, with maxima in June and in October-November, the latter being the greater. Larger numbers appear in summer than in winter. Cohn (’03), in waters near K6nigsberg, finds 6. longirostris only sparingly present, appearing in May—September with a maximum in July. In European streams, also, B. longirostris is widely distributed. Lauterborn (94) finds it abundant in the winter fauna of the Rhine. He also states that it is not acyclic in the backwaters, where he has found in three successive years both males and ephippia in May—June and again in November. There is thus a suggestion of a vernal and an autumnal pulse in these waters. Zimmer (’99) finds it through- out the whole year in the Oder. Schorler (’00) reports it from the Elbe at Dresden in May—October, with larger numbers in May—June and September, while Frié and Vavra .(’01) ‘find it in the same stream near Podiebrad. They state that B. cornuta is found in great numbers in 1 m.—surface in summer months, and B. longiros- tris sparingly in the littoral fauna. Steuer (’01) finds B.‘‘longtrostris- Zo cornuta’’ in the backwaters of the Danube at Vienna in April—-January. It exhibits a distinct seasonal polymorphism, with a large winter form and a smaller summer one. Data as to relative numbers during the year are not given. Skorikow (’02), in reviewing the investigations on the plankton of Russian waters, reports B. cornuta from the summer plankton of several streams, but expresses doubts as to whether “sie als autopotamische Planktonorganismen anzu- sehen sind oder nicht.’ Meissner (’03) finds B. cornitta generally in the Volga and its adjacent waters in the summer plankton, with largest numbers in August; and Zykoff (’03) reports it in small numbers from the same stream in May—July. It is not listed by Volk (’03) in the Elbe at Hamburg. B. longirostris occurs generally in American waters, though apparently, often in small numbers. Thus Forbes (’82 and ’90) reports it in the plankton of Lake Michigan and Lake Superior, and it appears generally in lists of Cladocera from many widely separated smaller bodies of water in this country. Burge (’95 and ’97) finds only a few Bosmina (species not stated) in Lake Mendota, but Marsh (’97) reports it (Species not given) as perennial in Green Lake, with a maximum in November. Huis records have also a suggestion of an earlier pulse, in June, in which month there is a sudden rise from a previous minimum. This partial survey of the literature of the records of Bosmina in the plankton shows its wide distribution, suggests the probability of great variation, necessitating caution in the description of new species in this genus, and indicates a wide diversity in its seasonal career even in waters with somewhat closely similar environmental conditions. . Ceriodaphma megops Sars was found singly but once—July 25, 1896, at 80°. Ceriodaphmia reticulata Jurine was found in the plankton occa- sionally, and always in small numbers, in April-September. All occurrences appear at temperatures above 66°, and the earliest is on April 17, and the latest is September 21. Females with summer eggs were found in June—September. Ceriodaphmia rotunda Straus was recorded in 1894-1895, but not thereafter. Its identification is somewhat questionable, and if correct, this is apparently the first record of this species in North American waters, unless it should appear that C. alabamensts Zaz Herrick or C. acanthinus Ross, which appear to resemble C. rotunda in some particulars, should be included here as forms or synonyms. The genus is sadly in need of revision. The forms referred to C. rotunda were found in August, 1894, and July-August, 1895, 16,536 per m.* appearing in the plankton on July 18 of the latter year. Ceriodaphnia scitula Herrick.—Average number, 1,539. This species is closely related to the European C. quadrangula O. F. Miull., if, indeed, it is not identical with it. It is not impossible that it is the form imperfectly described by Say (718) as Daphmia angulata. In the absence of a critical monograph of the genus I use the name applied in current American literature to this form. This is the most abundant species of the genus in our waters, outnumbering all others by over sixfold in the totals of our records. It is also one of the most important members of the Entomostraca in the channel plankton (total of all records, 156,119), being ex- ceeded in numbers only by Moina micrura (1,121,808), Bosmina longtrostris (381,598), Daphmia cucullata (237,444), and D. hyalina (231,746). It occurs in all months of the year except January and February, but in larger numbers and in more of the collections in May—Septem- ber. Thus less than 6 per cent. (reduced to 2 per cent. if one col- lection in the warm autumn of 1897 is omitted) of the individuals and only 20 of the 79 occurrences are found outside of the May— September period. Certodaphnia scitula is accordingly a summer planktont in channel waters. It is found in each year, though in varying numbers according to hydrographic and other conditions. Thus in 1898 the vernal pulse in June attains the unsurpassed amphtude of 55,800 per m.*, but declines in a fortnight and makes no recovery during the disturbed hydrographic conditions of the summer. In 1897, on the other hand, our records were too meager to delineate fully the vernal pulse, and in the stable conditions of the summer and autumn the species continued in numbers whose totals exceed those of 1898 by 81-fold. Similarly in 1896 the more gradual changes in levels which attended the floods of that year permitted a considerable development of Ceriodaphma throughout the summer. Stable hydrographic conditions thus conduce to increase in Certodaphnia. The relations which I have shown to exist between Bosmina and movement in river levels (see table on as —— 233 page 228) exist also in the case of Certodaphma and in much the same form. The relation of temperature to Ceriodaphnia is evident in its seasonal distribution. It does not advance rapidly in its vernal increase until after the water warms to 70°, and drops suddenly in numbers when the autumnal decline passes this point. Moreover, seasonal variations in temperature are accompanied by correspond- ing shiftings of the pulses of Certodaphmia. Thus in 1898 the water did not reach 70° until about May 20, reaching 73° on May 24, and the vernal pulse of Certodaphnia began at once its rise to the maxi- mum of June 7. In 1896 spring was early, 72° being recorded in surface waters on April 24, and we find a vernal pulse rising to a maximum on May 8. So also in 1897, when high temperatures continued into the autumn, the decline passing 71° on October 5, instead of in the first half of September as in other years, we find the pulses of Ceriodaphnia extending into October with unusual amplitude, reaching 5,200 per m.* October 5, while the highest record in this month, or later, in other years was 280 perm.? Tem- perature rather than season is thus the dominant factor in the seasonal curve of occurrence of Certodaphma. The form of this seasonal curve is typically that of a series of recurrent pulses of varying magnitude tending to reach the maxi- mum height in the vernal pulse of May—June, attaining often lower levels in July and rising again in August-September, and falling to Pemuinimum, or even to disappearance, in October. These later pulses do not appear in the disturbed hydrographic conditions of 1898 (Table I.), but are clearly delineated in the summer records of other years, especially in the stable conditions of 1897, where well-defined pulses appear in July, August, September, and October, at intervals of approximately four weeks, culminating July 14, August 10, September 14, and October 5. Their maxima attain respectively 5,600, 2,720, 6,000, and 5,200 per m.*, and the pulses are delimited in each case by minima of less than 500 per m.* They tend to coincide with those of other Entomostraca and to approach those of the Rottjera. The Certodaphnia population in channel waters is almost ex- clusively made up of parthenogenetic females. Males were not recorded at any time, though females with ephippial eggs appeared after the October pulse of 1897 and the vernal one of 1898. 234 Ceriodaphnia scitula appears but once in the records of European plankton, Scourfield (’98) finding it in the waters of Epping Forest in September. The closely related C. quadrangula as well as the other species have been frequently recorded by European investi- gators both in the littoral and the limnetic fauna, but they appear to be less generally found: there than the other dominant Cladocera of our waters. It does not appear in the plankton of our Great Lakes (Forbes ’82 and ’90, Birge ’95), or in that of Lake Mendota (Birge 795 and ’97), or Green Lake (Marsh ’97), but Herrick (’84) reports it as the most abundant species in Minnesota, and Fordyce (’00) finds it in Nebraska in shallow waters. A revision of the genus is needed before the seasonal distribution of the various species can be worked out ona basis that will make satisfactory discussions of the literature possible. Chydorus sphericus O. F. Mill.—Average number, 422, of which 26 are egg-bearing females, and 6 are immature, the remainder, 390, being females in which the ova were not prominent. The identification of species of Chydorus is attended by consider- able uncertainty. Comparison with named specimens from Europe supplied by Prof. G. O. Sars, leaves no doubt that C. sphericus is common in our waters, and it is apparently the dominant species. It is probable that several other species, as, for example, C. globosus Baird and C. celatus Schoedler, occur sparingly in our waters and have been included with C. sphericus in my enumerations. The difficulties which attend the attempt to assign every individual to one of the several species of Chydorus can be appreciated only by one who makes the effort. The problem of their specific validity should be solved by a statistical analysis of the range of varia- tion. The seasonal distribution of Chydorus sphericus in channel waters is in its general outlines very characteristic and well defined. The following table, which gives the average number of Chydorus per m.* for each month of our collections, shows clearly that it is a vernal planktont, and that there is a slight tendency toward an autumnal pulse in September, when vernal temperatures return. The number for November (222) would probably be considerably reduced if more than one collection had been taken in that month in 1896. Omitting this year, the average for November falls to ee eee Toe i a 230 78, and a secondary, hiemal rise becomes apparent in December. This December pulse of Chydorus is one of the elements in the upward movement of production in this month (see Part I.), and fuller data may serve to connect it fully with the September—October pulse, especially in more stable conditions. Both of these autumnal- hiemal movements have less than one tenth of the development that the vernal pulse exhibits. The number and percentage of occurrences also confirm the conclusions drawn from the numbers per m.* Percentages run higher in the spring, in March—May, and in September—October and in December, and lower in June-August, November, and January-February. Chydorus occurred in all March collections, and in only one third of the August collections. The analysis of the data in this table indicates the presence of Chydorus in the plankton practically throughout the whole year in the whole seasonal range in temperatures, with the larger develop- ments following shortly after the thermograph passes the yearly mean (57° average of monthly means of surface waters) in vernal rise and autumnal decline, the maximum development in April-May SEASONAL DISTRIBUTION OF CHYDORUS. AVERAGE NUMBER PER M.? | Year Jan. Feb. | March | April May June ROG 2 es ceen one a oe RE ECE —— ——— ————— 234 OST. See ee ae ere —— LAL a 2,044 | ———— O SOOM ee eada MeN: 304 167) |11.682 . | 10;070)|- 5: 704 448 MO enn Mega eres teat Sass ie —_ 20 540 320 | 32,800 900 HBOS teats Ses coerced errereere 160 0 256 300 3,364 356 TES OOS E Ree el rs eC, Wr 36 65 193 — | ——— — PAS ETEALS Co swwepataiate Recess 2 167 53 668 S235 0 132955 388 No. of occurrences...... 9 6 15 9 9 10 Percentage of occur- | TENCES SEM wise ves 75 40 100 82 90 UP 236 SEASONAL DISTRIBUTION OF CHYDORUS. AVERAGE NUMBER PER M.3—continued. Year July Aug. Sept. Oct. Nov. Dec: TOY oe WE a pee try see meer ve 95 0 461 100 16 56 SOS eee Ye Scene Slee wen bea by asters 91 103 164 38 203 448 LS OGM EN te de eee 64 104 78 160 800 277 US Oiaerene oe eee DAS 40 407 650 64 iil {SORE Ae es otaeemer eas 50 0 30 60 28 iL fD TRS OO pee te eros ae ey tee Parte —— ——. —— —_—_——_ | ———_ | —— INVETAC CH ey sioecitiers oro 103 49 228 202 222 214 No. of occurrences..... 11 7 13 12 10 14 Percentage of occur- TENCES HPT rete cee 61 33 81 71 63 82 occurring in average temperatures, for these months, of 60.5° and 68.3°, while the minor autumnal development appears in September— October at 74.2° and 57.6° respectively, and the December pulse, if indeed it be a separate and independent pulse, is at the low tempera- ture of 35.2°. The December movement may be simply the result of the more stable conditions which attend the appearance of the ice-sheet on the approach of winter. An analysis of the course of the seasonal distribution of Chydorus in channel waters, as given in Table I. and in statistics of other years, indicates the following seasonal regimen. In January—Feb- ruary, at minimum temperatures, the occurrences are irregular (75 and 40 per cent.) and the numbers small (average, 167 and 53 per m.*), while in March, with rising temperatures, occurrences are more numerous (100 per cent.) and numbers rise to 668 per m.* In April-May a high percentage of occurrences (82 and 90 per cent.) continues, and they mount rapidly to the maximum record of the year, which in our statistics varies from 4,088 in 1895 to 32,800 in 1897. This vernal pulse reaches its maximum in our records on April 29 in 1895, at 64°, and in 1896 on the same day, at 70°; on ge 0 VS » a — LL 23 May 25 in 1897, at 66.3°; and on May 24, in 1895, at 73°. From this maximum the pulse declines abruptly in a fortnight to a midsummer minimum during maximum temperatures, which continues until September. During this period the numbers are small, rarely rising above 400 per m.* (average, 388, 103, and 49), and the occurrences are also less numerous (72, 61, and 33 per cent.). With the decline of temperatures which begins in September the percentage of occur- rences mounts to 81, and the average per m.* to 228, and remains near this level during the remainder of the year. An analysis of the full statistical data, of which the records for 1898 are fairly typical, confirms the conclusions drawn from these averages. Chydorus in channel waters is monocyclic, with a well- defined vernal pulse in March—June which includes 95 per cent. of the total annual Chydorus population. There are suggestions of an autumnal pulse, but the data are not sufficient to delimit 1t. There is no satisfactory evidence that there are recurrent cycles or pulses at briefer intervals during the year. The dominating effect of temperature as a regulating factor in delimiting the seasonal distribution of Chydorus is very evident. This, inaddition to its appearance in the annual curve of occurrences, is also exhibited most clearly in a comparison of the vernal pulses in the two years of fullest representation in our records, 1896 and 1898. The following table gives the data of dates, temperatures of surface waters, and numbers of Chydorus. From these facts it appears that the late spring of 1898 delayed the vernal pulse of Chydorus, and that the early spring of 1896 accelerated it in that year so that their apices (April 29 and May 24) are four weeks removed from each other in seasonal location. In both years the rapid rise in the pulse appears ajter 60° 1s passed, the culmination occurs at about 70°, and the decline, im temperatures above 70°. Egg-bearing females were more abundant during the rise of the pulse, and less numerous during its decline. Evidence of great mortality during the decline of the pulses is to be found in great increase in the relative numbers of empty carapaces. Thus, during the decline of the vernal pulse in 1896 there were on the day of culmination, April 29, 2,780 dead to 18,904 living, on May 1, 3,570 to 14,875, and on May 8, 1,578 to 6,706. From 14 to 24 per cent. of the Chydorus population had thus recently perished. Parasitized 238 1896 1898 Date | Tempers.) Hoof | Dees eee Mae: Ufone 42°" || 256° |) Mary i5iseae 46° 440 Ta) Sen 40.7° | 610 | cole 9S Ale ood 480 Ome 48.1° | 6,405 eo oe. 495° 240 Apr dO). e ee 46.4° | 1,666 Are oSilee sameagace 200 CME tee :) 66.82 Ap OTS | We ee ee 200 CRY Siti 72° 1S MOMOL) Mee ano 56° SOC ERe 68° VS A004. «Dull eek! 2G ee Sy 800 May yileoagc ee 68.8° 14,875 May? 35. <3 60° Bea Bee 76° 6,706 apa (Ove 62° 600 Tee 72° 1,143 USE Aes 64° 35300 25% ea he 753° 80 SO An ise 7,880 | eos eae a 70° 5,040 UNE. Ovenn oe 719° 320 | June 7.... oe 600 a ig eae 73° 320 | caienarse 82:3 200 and fungused individuals were also noted in these periods of decline. Males were recorded in September, December, and February. Chydorus is not given as a constituent of the plankton of Nor- wegian lakes by Huitfeldt-Kaas (98) or of Swiss lakes by Fuhrmann (’00), Amberg (’00), or Burckhardt (’00 and ’00a). Its absence from these cooler waters stands in sharp contrast with its abundance in warm and shallow European lakes. It is reported as abundant in Chroococcacee-rich lakes of North Germany by Apstein (’96), where it is acyclic, with larger development in April—October, and maximum in August or in May-June. According to Weismann (’79) Chydorus in some waters is polycyclic. It is also reported by Zacharias (’97a and ’98b) from the pond fauna of Trachenberg and many other German localities, where it forms “ein notorisches ¢ - 239. Mitglied des Teichplanktons.’’ He also lists it (98b) from some German streams. Marsson (’00) found it in some waters near Berlin in April—-August, noting a great abundance in one instance in May. Seligo (00) gives a few statistical data indicating the occurrence of Chydorus in the plankton of Hintersee near Danzig in April-— December, with a maximum in August and a secondary one in October. It was, however, sparingly present in adiacent waters. Cohn (’03) finds a like irregularity in its occurrence in waters near K6nigsberg. Stenroos (’97) finds it to be one of the most abundant Entomos- traca in the waters of northern Russia and (’98) a littoral and bottom species near Helsingfors. Scourfield (’98) finds it to be one of the most abundant Cladocera in the waters of Epping Forest, occurring from March to December, with maxima of sexual reproduction in April and November. Scott (’99) reports it as abundant in the littoral fauna of Scottish waters, but rare in tow-net collections in open water. It also occurs in the potamoplankton of European streams, Zacharias (’98b) listing it from a few minor streams, but without seasonal, statistical, or temperature data. It was not separately listed by Skorikow (’97) in the summer plankton of the Udy at Charkow, or by Lauterborn (’94) in the winter plankton of the Rhine. Zimmer (’99) found it from February to July in the Oder, and Schorler (’00) finds it abundant in the plankton of the Elbe in April. Steuer (’01) finds it at all seasons in the backwaters of the Danube at Vienna, and in the plankton from March to November “oft in grossern Mengen,” but gives no statistics of its seasonal distribution. Fri¢ and Vavra (’01) find it in the channel and backwaters of the Elbe near Podiebrad, but more abundant in the littoral fauna, though no quantitative or statistical data of its occurrence are given. Zykofi (’03) reports it as present in the plankton of the Volga at all times in small numbers, and suggests a predominance in May—July. Meissner (’03) also reports it for the Volga, but states that it is predominantly a member of the littoral fauna though present in the plankton of the stream in restricted numbers. No statistical data are given by him. Volk (’03) reports it in the Elbe at Hamburg, but without any details. This species is reported generally from American waters. Forbes (90) reports it in the summer plankton of Lakes Superior and 240 Michigamme in small numbers, and (’93) in that of the Alpine waters of Wyoming and Montana, where it 1s, however, more abundant in smaller pools. Birge (’94) finds it generally distributed in collec- tions, including plankton, in Lake St. Clair and (’97) a member of the plankton of Lake Mendota, where its abundance is dependent on the supply of Anabena. Its maximum—only a single well-defined one occurring in each year—was found in July—October. Burge regards it as an accidental member of the limnetic fauna, maintained there as long as suitable food is present. Its mode of occurrence does not, however, differ from that of typical plankton organisms, which would doubtless likewise disappear from the plankton if their food should be lacking. It is noteworthy in this connection that it was only sparingly present in the channel of the Illinois in the midsummer—autumn plankton, when—as, for example, in 1897—-Anabena and its allies were abundant. It seems not improbable that temperature even more than food is an important factor in controlling its seasonal and local distribution. It is unquestionably a member of the plankton in our waters, though also abundant here, as elsewhere, in the lttoral fauna. In our locality in channel plankton it shows distinctly seasonal limitations which suggest the operation of tem- perature rather than food. Its occurrence in large numbers in Wisconsin lakes in midsummer and its absence in the Illinois at that time may also be correlated in part with the contrasted tem- perature conditions in the two localities. Its occurrence in our littoral fauna may also in part be due to the lower temperatures consequent upon spring-fed areas and the shade of aquatic vegeta- tion. Chydorus is one of those organisms capable of both the littoral and limnettc habit under suitable conditions of food and temperature. In our waters, at least,—and, as it seems from the data of distribu- tion, elsewhere,—temperature, rather than food directly, appears to be the factor controlling the occurrence of Chydorus in the plankton. Daphnia cucullata G. O. Sars.—Average number, 181. In 1897, very much greater,—5,483 per m.3 For the reasons given by Burckhardt (’00) I use Sars’s name cucullata rather than jardinez of Richard to designate those forms of the subgenus Hyalodaphnia in our plankton. In channel waters this species varies considerably, but not to the extent that it does eS 241 where its numbers are greater. The forms known as apicata Kurz and kahlbergiensts Schoed. appear in small numbers in some years. . This species appears in our collections in April-December only, with the exception of one occurrence in January and two in March. Its occurrences and numbers vary greatly in different years. In 1894—95 its numbers were small and occurrences scattering, it being ~ most abundant in November—December. In 1896 there was a large vernal development in April-June, and a series of diminishing pulses in July-September. In 1897 no vernal development appeared in our scattered collections, but in the stable conditions of late summer and autumn occurred the largest development recorded in any year, with a maximum record of 72,760 per m.?on October 5. In 1898 there was a small vernal development (3,400) in May—June and a still smaller one (600) in October. A well-defined seasonal routine is thus not demonstrable from our data, though the fact that both the percentage of occurrences and the numbers are highest in May—June and September—October suggests a tendency toward vernal and autumnal pulses separated by a period of less develop- ment in midsummer and of autumnal decline followed by a period of almost complete extinction in midwinter. The statistics of the D. cucullata population in all years in which weekly collections were made, exhibit very clearly the phenomenon of recurrent pulses of 3 to 5 weeks’ duration, with maxima of varying amplitude and minima of less than 400 per m.? in all cases but those which mark the September pulse of 1897. There are in 1896 pulses culminating April 24 (2,544 per m.’), May 8 (11,965), June 11 (12,000), July 18 (1,040), August 8 (800), and September 16 (507). In 1897, vernal records are incomplete. Pulses appear July 14 (800), August 17 (1,680), September 14 (57,000), October 5 (72,760), and November 15 (2,040). These pulses coincide exactly or approxi- mately with those of the other Entomostraca which exhibit the same phenomenon, and approximate also those of the Rotifera. A typical pulse, that of October, 1897, is shown in the following table. It is a noticeable fact that the proportion of immature forms is often greater at and after the period of maximum development than at other times, as appears in the table. The relations of temperature to the development of D. cucullata in channel waters appear in the fact that all occurrences in excess of 242 Date | Females eee Young At otal tS oouee SOs A omannoagauds 160 320 640 We ALAX0) 57 DO Eh cps yer aaah ees 7,520 4,000 12,800 24,320 52 Octane cea Ske 3700 10,800 58,400 72, 60 82 12 oraeaeieeey tee cas 1,600 7,600 9,200 83 OS ae cparins chro peters 560 840 4,440 5,840 76 600 per m.*are found after the temperatures pass 70°, with the’single exception of the decline of the October pulse and the rise of the November pulse to 2,040 per m.? at 47°, following the high tempera- tures in the late autumn and stable conditions of 1897. From the depression in numbers during the period of maximum heat 1n mid- summer and the occurrence of the major vernal and autumnal pulses before and after its reign it appears that the temperature optimum for D. cucullata in channel waters lies. below this level, that is, below 80°. D. cucullata is evidently very easily affected by the changes in hydrographic conditions. Thus, in July-December, 1897 and 1898, the total movement in river levels was 12.4 and 31.4 ft., respectively, while the total cucullata population for these months was 186,420 and 1,140—164-fold greater in the more stable year. D. cucullata thus exhibits the maximum sensitiveness among the Fntomostraca to these environmental factors. The D. cucullata population in the plankton consists almost entirely of parthenogenetic females and young. The immature stages form about 60 per cent. and the egg-bearing females 16 per cent. of the total individuals. Dead, parasitized,or fungused indi- viduals were found at times of the maxima or shortly thereafter, but never in very large numbers. Males were found once in December, 1896, and ephippial females also but once, on October 19, 1897, during the decline of the maximum pulse in our records. Daphnia cucullata var. apicata Kurz, in well-developed condi- tion, was found in relatively small numbers during the vernal pulses of 1895 and 1896 and the autumnal pulse of the former year. a “ew a a aH Ay eos OT rh GED Ort eta mH f 243 Incipient stages of this variety appeared also at other times. Burck- hardt (’00a) does not even concede varietal standing to aprcata, regarding it merely as a form of seasonal or local value. Its occur- rence in our plankton when reproduction and growth are most active suggests that it may have a growth value, and be in some way correlated with the factors involved in its cyclic production. Daphnia cucullata var. kahlbergiensis Schoed. appears but once in our records—in the plankton of June 11, 1896. The D. cucullata group is a cosmopolitan constituent of the fresh-water plankton, appearing frequently in the records of Euro- pean plankton. Apstein (96) finds it in lakes in northern Germany in April-October with maximum numbers in July. The seasonal limits thus resemble those in the Illinois, but the maximum falls at the time of our midsummer decline. Temperatures in these German lakes (16.3° C.) do not, however, reach the high levels attained in our waters in midsummer. Stenroos (’98) records it in several varieties in the plankton of Nurmijarvi See, the helmeted varieties being found in midsummer. Zacharias records it from the plankton of German ponds. Scourfield (’98) finds it in small numbers in Epping Forest interruptedly in April-November, a season coinciding with that in the Illinois. Burckhardt (’00) finds it represented by five different “forms” in Mauensee in the June plankton. Marsson (’00) finds representatives of Hyalodaphnia (species not given) in the April-June plankton near Berlin. Am- berg (’00) states that this species appears in April, increasing to a maximum in July-August, and disappears again at the end of November, a seasonal course similar in limits but not in maximum to that in the Illinois. His data are too scattered to trace the course of production with completeness. Seligo (’00), in waters near Danzig, finds the species present in June—January, with maxima in June-July and October. In the period of maximum summer , temperatures (16°-21° C.) the numbers decline as in this period in the Illinois. In Seligo’s infrequent (two to three weeks’ interval) data there are suggestions of minor recurrent pulses in other months. Cohn (’03) finds in Lowentin a Daphnia which he calls D. galeata with vars. kahlbergiensis and cederstrémit, and includes all three in his enumeration. His investigation covers the months of May— September, throughout which these forms appear, rising in a series of recurrent maxima on June 26, August 4,and September 2 and 29. (17) 244 Cohn seems not to have called attention to these clearly defined recurrent pulses. In European streams D. cucullata also forms an important part of the plankton. Lauterborn (’93) states that, with its varieties kahlbergiensis and cederstrémt, it appears abundantly in the plank- ton of the Rhine in summer, but is not found in itin winter. Zimmer (’99) states that D. kahlbergiensis was found constantly in the plank- ton of the Oder in July-September, and Schorler (’00) also finds it in the Elbe at Dresden in May—August, with larger numbers in June and August. Steuer (’01) reports it, in small numbers only, in August in the backwaters of the Danube at Vienna. Fri¢ and Vavra (’01) report D. kahlbergiensis as rare in the Elbe. Sowinski (’88) finds it in several varieties in plankton of the Dnieper and its tributaries, Rossinski (’92) finds it in the summer plankton of the Moskwa, and Zernow (’01) in the June-July plankton of the Schoschma and Wjatka. Meissner (’02 and ’03) finds it in several varieties in the May—August plankton of the Volga. D. cucullata in some of its various forms or varieties appears to be widely distributed in American waters. It was reported by Forbes (’82), as D. retrocurva, from the plankton of Lake Michigan, and also (’90) from Lake Superior and adjacent waters. Burge (91 and ’94) also finds it abundantly in Wisconsin waters and in Lake St. Clair. Herrick (’84) and Ross (’97) report it from Min- nesota and Jowa. Careful studies of its seasonal and vertical distribution in Wisconsin waters have been made by Marsh (’97) in Green Lake, and by Birge (95 and 97) in Lake Mendota. In Green Lake D. kahlbergiensis is reduced to a minimum or even extinction in December—April, rises in a late vernal maximum in June-July, falls again to a lower level in August-September, and then rises to a second and sometimes higher autumnal pulse in October. In its main outlines this conforms to the seasonal course of the cucullata form in our channel plankton. Our vernal max1- mum appears somewhat earlier, as a result probably of an earlier warming up of the water. According to Birge (’97) this species is more definitely periodic in its occurrence in Lake Mendota, being confined entirely to July-December. Here also the largest numbers are found in October, and the individuals gather in lower levels as temperatures decline. 245 Daphnia hyalina Leydig.—Average number, 417. In channel waters this species has appeared in but two years, in 1895 in April—July, attaining on June 19 a maximum of 166,208 per m.°, of which 150,626 were immature. The collections were too infrequent in these months to trace the course of this vernal pulse. D. hyalina did not reappear until the spring of 1898, on May 24, in a single vernal pulse culminating at 11,600 per m.* on June 7, and disappear- ing a fortnight later. Its occurrences with one exception were all at temperatures above 70°. There is no apparent cause for its absence in later months or in other years. Males and ephippial eggs were not found. Daphnia hyalina is an exceedingly variable species, and a large number of forms have been described which belong to the hyalina group. Burckhardt (’00), for example, recognizes 26 such forms as varieties of this cosmopolitan planktont. This variability and the difficulties attending the resulting synonymy cause any discus- sion of the species in other waters to be attended by much uncer- tainty. I shall therefore not attempt to distinguish in my dis- cussion between the various varieties included by Burckhardt in the hyalina group. In lakes of northern Germany, Apstein (96) finds that D. hyalina is essentially a winter planktont with a seasonal range of September— July, and with maximum numbers in November—January. The maximum thus appears there at the time of complete extinction in our waters. Stenroos (97) records it (as D. galeata) in the summer plankton of Karelia, Huitfeldt-Kaas (’98) finds it in Norwegian lakes in July and September in considerable numbers, and Scour- _field’s careful studies (’98) of its seasonal occurrence in waters of Epping Forest reveal an interrupted distribution in April-Novem- ber. Scott (’99) finds it in numbers in Scottish lochs in the plankton examined at long intervals in March—January. Fuhrmann (’00) reports it as perennial in Neuenbergersee, with a maximum in June followed by a midsummer minimum. Burckhardt (’00a) finds great diversity in different Swiss lakes and in different years in the relative numbers present. His intervals of collection were too great to detect any pulse-like movement in the production, and it may be that the diversity is due in part to the incompleteness of his records. He concludes that D. hyalina is at a minimum in March—May, increases in numbers slowly (with a preponderance of young indi- 246 viduals) in May—October to a maximum in November—January, which is followed by a rapid decline (with preponderance of adults) to the minimum. His results agree with those of Apstein (96) in the main rather than with ours in the Illinois. Seligo (’00) finds D. hyalina in Hintersee, though it is apparently absent from the adjacent Barlewitzersee. In the former lake it appears in May, rising to the year’s maximum early in June, continuing throughout the summer in diminished numbers, and disappearing 1n October. In his infrequent records there are suggestions of several recurrent minor pulses during the summer. Cohn (’03) reports D. galeata— regarded by Burckhardt (’00a) as a form of D. hyalina—tfrom the region of K6nigsberg, but refers it rather to the cucullata group. I shall therefore consider his results only in connection with D. cucullata. D. hyalina appears but rarely in the records of European potamo- plankton. Steuer (’01) reports it, in small numbers only, in May from the backwaters of the Danube at Vienna. Fri¢ and Vavra (01) state that D. mtcrocephala—regarded by Burckhardt (’00a) as a form of D. hyalina—is abundant in the plankton at a depth of O-1 m. in April-November in the Elbe and its backwaters at Podiebrad. It is also reported by Zykoff (’00 and ’03) in the late vernal (June-July) plankton of the Volga at Saratoff,and by Meissner (02 and ’03) in the same stream in May—June. The examination of the plankton of the Volga made by these authors is far less extensive than that made of the Illinois River plankton, but as far as it goes it indicates a similar distribution of D. hyalina in the two streams. Volk (’03) reports it from the Elbe at Hamburg without data. The species appears to be widely distributed in American waters, being reported, in some of its various varieties or synonyms, especially from lakes and ponds. Smith (’74) finds it in the plankton of Lake Superior, Forbes (’82) in that of Lake Michigan, and Birge (’94) in Lake St. Clair. It was also found in the Illinois by Forbes (’78) and in the backwaters of the Ohio River by Herrick (’84), who reports it also from Minnesota waters. Birge (’91) finds it in lakes about Madison, Wis., and Fordyce (’00) in deep pools in western Nebraska. The only investigation of its seasonal distribution in American waters is that of Birge (95 and’97) in Lake Mendota, where it forms about 3 per cent. of all the Crustacea. It is perennial +t > ea —— 247 in this lake but exhibits great differences in its seasonal course from year to year. The vernal development in May—June (the only one in our channel plankton) is relatively large in each year, but is sometimes exceeded by an autumnal one in October. A midsummer minimum sometimes appears between these pulses, and a winter minimum in December—April is always present. From the data here reviewed it seems probable that the very limited seasonal distribution and irregular annual recurrence of D. hyalina in our channel plankton is in a measure indicated in streams elsewhere, and may have its cause in the instability of the fluviatile environment as compared with the lacustrine, where the species evidently finds its environmental optimum. - Diaphanosoma brachyurum (Liévin).—Average number, 479, of which 154 are females, 49 females with eggs, and 276 immature. This species in our waters is monocyclic, with sharply defined seasonal distribution. With the exception of two records of young individuals in March—April, 1895. (and the identification of these individuals is questionable), all our records of occurrence in 1894-1899 fall between May 25 and October 19, the first vernal records appearing at temperatures of 55.8° to 72.3°, and the last autumnal at 52.5° to 65°. The one pulse in each year—except in 1894, when none was recorded—falls in a period of 3-6 weeks in July-September, the first record above 2,000 per m.* appearing July 26, and the latest (with one exception, 2,175 on September 27, 1895) on September 7. The pulse varies in duration in different years from 3 to 6 weeks, and attains a maximum on dates ranging from July 26 to August 31, and varying in amplitude-from 8,580 to 19,602 per m.* An analysis of the distribution of 61 recorded occurrences in channel plankton shows that of these only 13, or 21 per cent., occur outside of July-September, and that the records outside of the seven weeks of the pulse include less than 12 per cent. of the total individuals. A comparison of the seasonal curve of distribution with the annual thermograph reveals the fact that the pulse occurs toward the close of the period of maximum summer heat, and in every case at a temperature of 78° or above, and that the decline of the pulse often begins with declining temperatures, and is always accom- plished during the autumnal decline. The effect of summer heat pulses upon the Diaphanosoma curve is strongly suggested by the 248 data of the appended table, which gives the statistics of temperature, river level, and Diaphanosoma population during the periods of maximum development in 1895-1898. All these data except those of Diaphanosoma are shown graphically in Part I., Plates [X.—XII. The data for Diaphanosoma are less complete than the others, since all of the collections were not counted. In 1895 the Diaphanosoma pulse culminates at 19,602 on August 21, following immediately upon a heat pulse which culminates August 15 at 85.3°. The decline of the pulse occurs with a decline of temperature to 72° on September 7. The declines, both of Diaphanosoma and temperature,are hastened after September 3 by 1895 | 1896 | Date River | hemp: Dash. | Date River Temp. Dasph. gage | | anosoma | | Pe Eee y anosoma a | te ss te 40 —— a | —— | “ 10] 4.00 79.5 800 Aree || eon Meee 1250: || 79) eer July 23 | 5.20 so 424) “ 23 | 4.20 | 80 | 120 e959) 5 se." 75 Sen 240 || « 28 | 6.40 | 82 7,440 Aug. 1 | 4.20 78.5 |. 1,088 || Aug. 3 | 8.50 | 80.3 160 Sul 2663 79 988 || “ 8 | 3.40 | 86 | 14,260 | eo 84.8 9,800 Ilr © - 295) 1 720 | 82 2,240 LD I P05 Sits 19,662 | « 21 | 7.10 79 880 ON Dae 80 7,950 CS Vania 50 ToS 600 —— —= | S| 29'| 6.00 74.3 440 Sepia 5/256 70 74 189 || Sept. 16 | 4.10 73.5 663 Coe LDA 73200 79 1,053, ||" 528) $3 0snn 430 58. i 80 « 99 | 3.20 | 79 | 468 || 2 pes) ee Ce 0 (DESO AI evi 2 — uy 249 1897 | 1898 | | Date | ee | Temp. | Dap | Date ae Temp. Des ae | aAnOSOMaA anosomMma | eee July 14 | 6.30 | 79 160 || July 12 | 7.00 78 60 91. 5090 | Bird 960 (E10 || A701 84 40 “ 30] 4.60 | 84 | 4,720 96 | 2.90 | 80 8,580 as 10 | 2:30 | 80.8 | 7,600 || Aug. 2] 2.70 |. 78.3 6,960 “49 | 400 | 79 | 7,420 | Ue Oe|| 3220) 1) 983 360 ey oa\ 150.) 77-5 |. 5,120 || © <16.| 3.70: | 77 60 31] 1.80 | 80 | 11,000 SG) |e A201) 9 82 1,020 pee He) a ag, 3 00:. | Rss. |? 2, $90 Septyw |staso- | 80 7,600 || Sept. 6 | 4.70 | 79 240 sey etO0 | 83 4 | 4,500 le aig 2 a0. | 62e5 1,800 Cee 2.00r |) 71 | 2400 20u e428 23 960 meee | | Ce? yen oG) a (NK73 400 the rise in river levels. Prior to that date hydrographic changes are slight. With falling levels and higher temperatures after September 7 there is a slight recovery in Diaphanosoma—trom 189 perm. om the 5th, to 1,053 on the 12th. In 1896 a well-defined heat pulse culminates August 10 at 86.5°; and Diaphanosoma,on August 8 at 14,260, with an abrupt depression from 7,440, on July 28, to 160, on August 3, in flood waters. The decline of this pulse from the maximum on the 8th to 440 on the 29th is attended by a uniform decline in temperatures from 86° to 74.3° in fairly stable hydrographic conditions, that is, declining river levels. In 1897 there are two well-defined summer heat pulses, one culminating August 3 at 89°, and the other September 14 at 83°, separated by a depression to 77.5° on August 24. The crest of the Diaphanosoma pulse likewise has two apices, the first culminating at 7,600 on August 10, followed, during the decline in temperatures, 250 by a fall to 5,120 on the 24th, and, in the rising temperatures which then ensue, by a recovery to a second maximum of 11,000 on the 31st. Diaphanosoma then declines though temperatures continue to rise. These fluctuations all take place in comparatively stable hydrographic conditions. There is a suggestion in the records of this year that rising temperatures in midsummer conditions tend to accelerate, and falling -temperatures to depress, development of the Diaphanosoma pulse, and also that after the pulse has continued for some time (six weeks in this instance) rise in temperature ceases to be effective. The autumnal decline in Diaphanosoma may therefore not always of necessity be due tc temperature decline alone. In 1898 there are also two midsummer heat pulses, culminating on July 26 at 89°, and August 30 at 82.5°, separated by a depression which reaches 77° on August 16. The depression to 78.3° on August 2, with the consequent appearance of a third summit at 83° on August 9, is due mainly to the fact that the temperature was taken at 9:15 a. m., while all the others were in the late afternoon. The seasonal curve of Diaphanosoma shows likewise two apices, the first at 8,580 on July 26, and the second at 2,520 on August 30, separated by a depression to 60 per m.* on August 16, when temperatures are lowest. - In this year the flood of the middle of August doubtless plays a large part in depressing alike the thermograph and the seasonal curve of Diaphanosoma, but in the light of the evidence from 1897 in stable hydrographic conditions the direct influence of temperature is also possible in this instance. Diaphanosoma is thus a late summer planktont which in develop- ment is very responsive to changes in temperature. It appears in the plankton in small numbers shortly after the establishment of summer temperatures in May—June, but does not begin its maximum development until maximum summer temperatures have existed for six to eight weeks, and is apparently incited to this by a summer heat pulse. Males were recorded on July 18 and August 1, and ephippial females on August 1 and September 5. Dead individuals were most numerous during or subsequent to the maximum of the pulse. This species is reported by Apstein (’96) in the plankton of Dobersdorfersee, where it is also monocyclic, first appearing in May, and attaining its maximum in September, when the males first appear. In contrast with conditions in our waters the maxima Zor appear after the period of maximum summer heat. Zacharias (’97a) reports it from German carp ponds in July, and Stenroos (’97) lists it as a littoral species in midsummer in northern Russia. Scott (’99) finds it rarely in lakes of Scotland in August, and then only in the plankton, though many shore collections were examined. Burck- hardt (’00) reports it from the smaller and shallower Swiss lakes in isolated records ranging from May to November, and regards its absence from the deeper lakes as due to the low temperatures which at all seasons would surround its winter eggs, which sink to the lower levels. In Vierwaldstattersee (’00a) he finds this species in the plankton only in September—November, and then more abundantly near shore than in the middle of the lake. In Alpnachersee the period of occurrence extends from June to November with a maxi- mum in July. Fuhrmann (’00) gives the seasonal distribution in Neuenburgersee as extending from May to November, with a maxi- mum in September. Marsson (’00) finds a seasonal distribution from July to October in small lakes near Berlin. Seligo (’00) finds in Hintersee, near Danzig, a seasonal distribution in 1898 extending from June 6 to October 18, with a maximum of 225,000—under 1 sq. m., depth, 24 m. (?)—on August 9. Fri¢ and Vavra (’01) state that this species is very abundant in summer months in the plank- ton of the backwaters of the Elbe, especially in levels at depths of 0-1 meter. Cohn (’03), on the other hand, finds in waters near K6nigsberg that Diaphanosoma is present in greatest abundance in depths of 20-30 meters. It occurs in summer months, with large numbers in July-September and a maximum in August-September. It was not found in shallow waters. As a constituent of the potamoplankton Diaphanosoma has been reported by Schorler (’00) in the Elbe at Dresden as abundant in June-September. Steuer (’01) finds it in the backwaters of the Danube at Vienna in June-September, with a maximum in August, but never in great numbers. Meissner (’03) reports it sparingly from the Volga in July. In American waters Diaphanosoma is widely distributed. Forbes (’90) found it abundant below surface levels in Lake Michigamme in August. Birge (’94) reports it in the plankton of western Lake Erie but not in that of Lake St. Clair in September. In Lake Mendota, Wis., he (’95 and 797) has worked out its seasonal and vertical distribution with a fulness and care not equaled by any 252 European author previously quoted. Our results in Illinois waters are in striking confirmation of his conclusions. He finds the first scattering individuals in the plankton late in May, but numbers do not rise until late in July or early in August, increasing rapidly through August or even into September, then declining rapidly, and disappearing entirely before November 1. The active period is thus at a time when a considerable part of the lake is at or above 68°. In our waters these temperature limits are 78° or above, but the sea- sonal distribution is almost identical with that in Lake Mendota. He finds it more abundant in the upper strata, 0-2 meters, than in the deeper ones—just the opposite of Cohn’s (’03) results. Marsh (97) has also determined its seasonal and vertical distribution in Green Lake, Wis., with considerable care. Occurrences from the last of October to the last of June are very few, and maximum numbers appear from the middle of August to the middle of Septem- ber, when surface waters have a temperature of 65°-80°. It occurs in all depths (0-40 m.), but 70 to 80 per cent. of the individuals were taken within 10to 15 m.of the surface, the upper 5 meters being more densely populated by night than by day and in September-— October than in August. Diaphanosoma is a typical planktont, with strong antenne, and an active swimmer. Examination of the literature indicates its wide distribution in the plankton of lakes and streams, and its very marked seasonal limitation to seasons of higher temperature. It is thus, as Birge (’97) has stated, markedly stenothermous. The divergent conclusions concerning its limnetic habit and its vertical distribution will doubtless be found to rest in some cases upon insufficient data, and in others, upon its reactions to varying condi- tions of light and temperature. Eurycercus lamellatus O. F. Mill.—This species occurred spar- ingly and irregularly in the winter plankton at minimum tempera- tures from November 30 to March 28. It is evidently adventitious. Ilyocryptus spintfer Herrick.—Average number, 4. This species occurred sparingly and irregularly in the plankton during the warmer months. The earliest record was on July 23, and the latest October 11 at 65°. This species is evidently adventitious in the plankton. I have doubtfully referred our examples to Herrick’s species I. sprntfer, for the reasons given by Herrick and Turner ('95), rather than to J. longtremts, to which Birge (’91) would refer our ee 209 American form described by Herrick as J. spinifer. A larger amount of material exhibiting a fuller range of variation may, however, serve to connect the two. Leptodora hyalina Lilljeborg.—Average number, 3. This species occurred in small numbers and somewhat irregularly in our collec- tions of channel plankton in summer months. Our earliest record was June 28; and the latest, August 30. It is our largest crustacean planktont and a fairly active swimmer, and was often taken in our tow-nets, which had a larger mouth and coarser mesh (No. 12) than our plankton net. I took this species in great numbers in the upper meter of water at midday in May—June in Lake Meredosia with a seine of No. 000 silk. It may be that it is less abundant in the channel than in the backwaters, and the small number in the plank- ton collections from the channel may also be accounted for in part by the escape of Leptodora from the small orifice (10 cm.) of the plankton net, or to its negative rheotropism when stimulated by the currents of the plankton pump. Macrothrix laticornis Jurine was found in the plankton in May at 64°-73°, adventitious in flood waters. Moina micrura Kurz.—Average number, 261 per m.* In 1897 it was much more abundant, averaging 5,106 in the more stable conditions of that year. This is the most abundant of all our Cladocera, appearing in great numbers in periods of stable low water during maximum temperatures. It is exceedingly irregular in the extent of its devel- opment in different years, the average numbers per m.?* in 1894-1898 being respectively 21,844, 22,842, 188, 5,106, and 261. After mak- ing allowances for the irregularity in the number and distribution of the collections in the several years, it still remains apparent that Mozina is very uneven in its distribution. The seasonal distribution of Moima in channel plankton is con- fined to July-September with the exception of 9 occurrences in small numbers in the last days of June and the early part of October. The earliest record is June 19, in 1895, when the very large number of 329,448 per m.* were found,—a degree of development which implies a previous period of multiplication. The first records in subsequent years were all later than this date in June or early in July. After several recurrent pulses, each of 3 to 5 weeks’ duration, the numbers decline to a very low level, and the species disappears 254 from the plankton in September—October. In 1898 (Table I.) the last record was made October 11—the latest in any year with the exception of an isolated record October 26, 1897. Mozna micrura is thus distinctly a summer planktont. It appears in the plankton only after maximum summer tem- peratures of approximately 80° have been reached, and decreases rapidly as soon as the autumnal decline passes this point, and soon thereafter vanishes from the plankton. Its optimum temperature in channel waters is thus near 80°. The relation which hydrographic conditions bear to the ap- pearance of Moina in channel plankton appears upon a comparison of the Moina population and the movement in river levels in differ- ent years, as shown in the following table. Moina AND HyDROGRAPHIC CHANGES. June July August September October Year a8 2 ie 2 aie 2 Be ts a 2 ° ° ° ° 5° Z 3 5 Zo E Z 9 E Z 9 E Zo g » | 3 a ca) mes lacs nn ae a Nes . 9° a ° > ° 2 e) a ° ) 2. 1129" SSOr 2 eOn ramon eer, 0 Shel 1895329 448 | 2.7 | 91,318 | 7.3 2 OT Neon 87 | 8.8 10 PT | 1896 OF 324 SZ eS PPA | ZB OF) -sa2 0 4.6 1897 On 6e3 1 oe Se |) Sone 1,280] 2.6 | 70;0407);-036 (|) 605 0.6 1898 dd~ | 240 660 | 7.4 1,496 | 7.5 LEON “62 40 329 While the correlation is not proportionate between the extent of movement in levels and the Mozna per m.',it is still very evident that in years of continued and more stable low water Mozina is found in much greater numbers, as appears on a comparison of 1897 and 1898. It is also confined largely to the more stable part of the year, appearing in 1895 in June—July in large numbers, but falling off when the minor floods of August-September occur, while in 1897 the large numbers are found in the stable levels of August. 255 The cause of this limitation of Morna to periods of low levels in maximum temperatures appears to lie in the food relations of the species. Moima abounds in waters approaching stagnation. The slackened current, increased sewage contamination, and excessive growth of the smaller alge and chlorophyll-bearing flagellates at such seasons 1n the channel of the Illinois furnish an environment favorable to the great increase in Moina, such as was recorded in the low water of July-August, 1894, of June-July, 1895, and of september, 1897, exceeding in each instance that of any other species of Entomostraca in the plankton. The relatively smaller numbers of Momma at the same seasons in the less contaminated backwaters lends additional support to the view that these condi- tions approaching stagnation are in a measure responsible for its unusual development in channel plankton. Of the total Moma population, over 65 per cent. are young or immature, 7 per cent. are egg-bearing females,—embryos are often freed from the parent on application of the preserving fluid,—11 per cent. are males, and the remainder,’ females without eggs. Males appeared with the maximum or decline of the major pulse for the year in 1894 (August), 1895 (July), 1897 (July and September), and 1898 (September), but ephippial females were recorded only in June—July, 1895. The seasonal distribution of Mota conforms to the type of a series of recurrent pulses wherever the numbers are considerable and the collections sufficiently frequent to delineate their courses. Even in the small numbers of 1898 (Table I.) there are suggestions of such pulses. Moima micrura seems to be a species characteristic of the pota- moplankton. It is not mentioned as a constituent of the plankton or littoral fauna by any of the various investigators quoted else- where in this paper who deal with lakes or ponds 1n Europe or North America; nor does it appear as a frequent constituent of the potamoplankton elsewhere. Skorikow (’02), indeed, makes the statement, “ Bemerkenswert ist ftir die Flusse vollstandiges Fehlen der Gattung Mona.” ‘This, however, is hardly the case, for Sowinski (’88) finds it in the plankton of the Tetérew, a tributary of the Dnieper, and Fri¢ and Vavra (’01) report it from the Elbe in O-1 m. strata in July-September, males appearing in the latter month. Meissner (’02 and ’03) also finds it in the Volga at Saratoff, 25,0 where it ‘appears almost constantly in the plankton.” His investi- gations, however, appear to cover only the months of May—August. Maximum numbers appeared in July, and considerable differences were noted in two successive years. I find no previous record of the occurrence of Moina micrura in American waters. Pleuroxus denticulatus Birge.—Average number, 5. Occurs in small numbers and irregularly during the autumn and spring months during declining or rising temperatures. The earliest autumnal record is November 2, and the latest, December 15; the earliest vernal is March 8, and the latest is May 31. Egg-bearing females appear in the earlier occurrences in each season. It is evidently adventitious. Pleuroxus hamatus Birge was found once—March 29, 1898. Scapholeberis mucronata O. F. Mill. was recorded in small num- bers in May and August—December through the seasonal range of temperatures. It is apparently adventitious in channel plankton, though not attending flood invasions. Sida crystalina O. F. Mull. is rare in the summer plankton. Simocephalus serrulatus Koch.—Average number, 261. This species appears irregularly in the plankton, generally in small numbers and in isolated occurrences. An exception to this 1s found in May—June, 1898 (Table I.), when it is found continuously May 10—June 14 in numbers which furnish 61 per cent. of the total for all years. There is a slight preponderance of occurrences in May and September, 12 of the 26 recorded appearing in these months. Their irregular appearance in the plankton in general suggests that they are adventitious from the littoral area, especially at times of their maximum development there. The period of their occur- rence in the channel plankton in 1898 was one of rising water, 10 to 14 feet above low-water mark—a stage permitting free communica- tion between the channel and large areas of slightly submerged bottom-lands. Simocephalus vetulus O. F. Mull. appeared irregularly and in small numbers in the plankton in April-June (4 occurrences) and September—December (5 occurrences). It is evidently adventitious in the plankton, coming from the littoral area, though not confined to flood waters. ne ciegh iS) Ca ~~] OSTRACODA. The species of this order are in the main, during adult life, limicolous forms found in the littoral or bottom ooze or amid the decaying organic matter which accumulates in these regions. The current, the movements of fish and other large aquatic organisms, the action of waves along shore and in shoal regions, all tend to bring these animals into the limnetic fauna. Their centers of distribution are thus in littoral or bottom regions, and in the adult stage they are almost wholly adventitious in the plankton of our waters. In 1898 the average number per m.* was 191, but in 1897, a more stable year, only 97. The seasonal distribution of their occurrences in the plankton indicates a decided predominance in March—October, in which months all but 6 of the 73 records were made. In these months from 23 to 82 per cent. of the collections contained Ostracoda, while in December—February only 8 to 20 per cent. The percentages in April-September are all above 45 per cent., and the numbers per mi. are also larger in this period (see Table I.). The tendency toward a vernal increase is apparent in the records of each year in much the form in which it occurs in 1898 (Table I.). The numbers are always small at all seasons, not exceeding 1,600 per m.* even in the vernal season. The seasonal distribution is such that the greater part of the occurrences ‘and the greater number of individuals appear in the plankton during the warm season, that is, above 50°. Thus, in 1898 all but 4 of the 24 occurrences and 99.5 per cent. of the indivi- duals appear after the vernal rise passes 50° and before the autumnal decline reaches that point. The Ostracoda are plank- tonts of the warmer season. It is significant that the Ostracoda in our plankton collections are largely young or immature individuals. In 1898, for example, 74 per cent. of individuals observed were not adult, and most of these appeared in April-June. Their occurrence in the plankton can not be traced to the action of flood waters. It thus seems probable that the young Ostracoda may temporarily adopt more of a limnetic habit than the adults. No attempt was made to systematically identify the Ostracoda of the plankton catches. The list of species and the notes thereon 258 which follow, are drawn in the main from Sharpe (’97), to whom I am also indebted for assistance in identifications which I have made. A few supplementary notes are based on my plankton records. DISCUSSION OF SPECIES OF OSTRACODA. Candona sigmotides Sans is rare in shore collections below the plankton station. Candona reflexa Sharpe was taken but once in the river—on November 11. Candona simpsoni Sharpe appears commonly in April-May, and again, in smaller number, in October-November in shore collections on the west side of the river at the plankton station. It is occa- sionally adventitious in the plankton at these seasons. Cypria exsculpta Fischer appears rarely in the channel plankton and in shore collections in April-October. Cypria ophthalmica Jurine is found frequently in the plankton throughout the year, but more abundantly in May—September, and especially in late summer and early autumn. Cypria pustulosa Sharpe was taken rarely in channel plankton in July and September. Cypridopsits vidua O. F. Mull. was perennial in the plankton, though present in greater numbers in May—October. It is the commonest of the Ostracoda in the plankton, and it seems probable that many, though not all, of the young and immature forms belong to this species. Limnicythere illinoisensis Sharpe was taken in the plankton in March, August, and November in 1898, in two instances in flood waters. COPEPODA. This is the most abundantly represented order of the Entomos- traca in channel plankton. Though the species number but 12 to the 25 Cladocera, the individuals among the Copepoda outnumber the Cladocera over fivefold in the grand totals, the ratio varying in - individual years from twofold in 1894 to almost sevenfold in 1898. The average number in 1898 was 40,608 per m.*; in 1897, in more stable conditions, 80,632; in 1896, a year of recurrent floods, 43,764—approximately the number in 1898; in 1895, a year of low water in spring, 116,264—the highest average of any year; and in eS . 299 1894, 53,149. On June 19, 1895, the Copepoda attained a vernal maximum of 1,022,476 per m.*—more than twice the maximum record for any other year. The Copepoda occur in every collection examined, and throughout the whole seasonal range in temperatures. As shown in Table L., the copepodan population during minimum temperatures in De- cember—February is at a minimum, the number per m.° rising above 10,000 per m.* in but 6 instances in 44 collections in these months, and falling below 1,000 in but 5. In March—April, as temperatures rise, the numbers increase rapidly, especially after 50° is passed, to a vernal maximum in the last days of April or early in May, usually at the time of the vernal volumetric maximum or very shortly thereafter. In fact, volumetric maxima are generally accompanied by copepodan maxima culminating at the same time or a week later,—as in May, 1898, when the volumetric is on May 3 and the copepodan on May 10. Numbers continue to be large during the period of summer heat, declining somewhat tardily with the autumnal decline in temperatures. In midsummer in 1898 numbers fall below 20,000 in 9 instances in disturbed hydrographic conditions, but in all previous years in April—September there are only 9 such records in a total of 63. The decline to the winter minimum is usually com- pleted in November, though in 1897, 20,000 is not permanently passed until December 21, at 32°. The Copepoda are thus perennial in the plankton, and the fact that they exhibit a larger winter population than the Cladocera 1s due to the fact that a number of species,—the Harpacticide, Cyclops bicuspidatus, C. prasinus, C. serrulatus, and C. modestus appear to be planktonts belonging to the colder part of the year. Asa whole, however, the Copepoda reach their greatest quantitative develop- ment in the warmer part of the year, with a major pulse in April— May and an occasional autumnal pulse, as in 1897, of equal or greater proportions. The whole course of the seasonal occurrence of the Copepoda as revealed by collections at frequent intervals, exhibits the phenome- non of recurrent pulses at intervals of 3 to 6 weeks, and more clearly defined in stable conditions. Owing to their relatively smaller numbers the adult Copepoda do not show the pulse phenomenon (18) 260 as clearly as the naupli and immature forms. In 1898 the adults form only 10 per cent. of the total. The relation which hydrographic conditions bear to the copepodan population may be inferred in part from the comparison of years given above, and from the following table, in which are given the average number of Copepoda per m.* and the total monthly movement in river levels in July-December, 1897 and 1898. July August September el SOi7 1898 1897 1898 1897 | 1898 Average Copepoda josie Saat ol GS 81,543 LA 2O0 | W210 707 SM OSON| ZO 879 esOno20 | ; Total movement in | levels, in ft..... Se ee: 26 G25 0.6 | 6n2 October | November December | | | 18975 | 1898 4) son = sso 1897 | 1898 Average Copepoda | | | | perm........ 128,093 | 28,285 | 49,240 | 10,692 | 15,740| 7,908 | Total movement in levels; imttes.. 0.6 39 Pe); | 216 OFS 2.4 With a total movement of 11.7 ft. in July-December in 1897 and nearly three times as much (30 ft.) in 1898, we find copepodan population falling off to less than one sixth that of the more stable year. Of the total Copepoda in our records for 1894-1899, 78 per cent. are nauplu of Cyclops and Diaptomus, 13 per cent. are immature Cyclops, and the remaining 9 per cent. are Harpacticide, Diapto- mus, and adult Cyclops. Of the twelve forms, Cyclops viridis var. imsectus is the most important quantitatively, and includes one fourth of the total adult copepodan population, exceeding the next in importance, C. viridis var. brevispinosus, by over threefold. 261 af The following forms are of numerical importance in the order named: C. bicuspidatus, young Diaptomus, Cyclops edax, Diaptomus siciloides, D. pallidus, Canthocamptus spp., and Cyclops albidus. Cyclops prasinus, C. modestus, C. phaleratus, and C. serrulatus are also found, but in such small numbers as to be of no quantita- tive consequence. DISCUSSION OF SPECIES OF COPEPODA. Argulus sp.—A small and apparently young argulid was found in the plankton on August 10, 1897. Members of this genus are abundant upon Amza calva and both species of Lepisosteus, all very common fish in channel waters. Canthocamptus spp., including C. allinotsensis Forbes.—Average number, 78. Canthocamptus was found in the plankton in every month of the year but June. The percentage of collections contain- ing Canthocamptus is greatest (44 to 63 per cent.) in March—May and November, and the numbers per m.* are highest in March—May, when females, females with eggs, and nauplii all occur in their maximum numbers. All records of totals in excess of 400 fall in this vernal period with the single exception of one collection in August, 1897. The largest number, 3,058 per m.°, was found April 29, 1896. Canthocamptus occurs throughout the whole seasonal range in temperatures, with smallest numbers and least regularity during maximum summer heat in June-August. It is thus a planktont of the colder rather than the warmer part of the year. The relations which hydrographic conditions bear to the occur- rence of Canthocamptus in the plankton may be inferred from the fact that of the 48 records in 1894-1899, 24 were made in rising flood waters, 14 in falling flood stages within a few days after the culmination of the rise, and but 10 in stable conditions or in declining levels when flood waters of recent origin did not fill the channel. From these facts it seems probable that Canthocamptus is in the main adventitious in the plankton from its normal habitat in the slime at the bottom and margins of the river and its backwaters. Over 88 per cent. of the total Canthocamptus recorded in the plankton consists of nauplii. It may be that—as is the case with the young Ostracoda—they enter the area of the plankton more readily than the adults. Adults were found in the plankton only in 262 November-May,; females with eggs, only in February—April; and a female with attached spermatophore, in March. Nauplu appear in greatest numbers in April-May, attaining 2,862 per m.* April 24, 1896, but they rarely rise above 400 per m.* outside of this vernal period, and are found only in very small numbers in December— March. It appears from our data that the breeding season is prin- cipally in April-May. Cyclops albidus Jurine.—Average number, 113; in 1897, 136; in 1896, 33; and in 1894, but 10. A discussion of the variation and synonymy of this species has been published by E. B. Forbes (’97). The species is numerically least important of the dominant members of the genus in our plankton. It was recorded in all months but December and February, but its season is practically confined to April-October, the only exceptions being three records in small numbers in January, March, and November, and two of larger numbers (300 and 200) in the higher temperatures of the delayed autumn of 1897. There is a tendency toward a summer minimum in June—July, with pulses of greater amplitude in April-May and again in August-October. In these months the percentage of collections containing C. albidus is highest, being respectively 55, 50, 38, 56, and 53 per cent., and these are the only months in which the numbers per m.’ rise above 600. The highest numbers recorded, 2,862 and 2,400, occurred respectively on April 24, 1896, and October 5, 1897. Although C. albidus is found in the extremes of temperatures, it shows a decided increase after temperatures pass 60° in the vernal rise, and falls off immediately after the autumnal decline passes this point. With high temperatures continued into October, in 1897 we find it continuing in larger numbers. On the other hand, during maximum summer heat (about 80°) numbers, as a rule, fall below 300 per m.* The temperature optimum thus appears to be in the neighborhood of 70°. The three greatest pulses re- corded, occur respectively on April 24, 1896, at 72°; on April 26, 1898. at 57°, and on! October Ss: 1897 .an mle The numbers are too small to exhibit very clearly the phenome- non of recurrent pulses, though the vernal and autumnal pulses are usually well defined, and in the stable conditions of 1897, August, September, October, and November pulses may be traced. nt ee 263 Hydrographic conditions appear to affect C. albidus as they do other Entomostraca. In July-December, 1897, in stable low water the C. albidus population exceeds by over threefold that of these months in 1898. Of the totals of all records in 1894-1899, 74 per cent. are fe- males,—4 per cent. with eggs and 70 per cent. without,—and the remaining 26 per cent. are males. Immature forms and nauplii were not distinguished from those of other species. Egg-bearing females were recorded only in May and August—October, at times of maximum pulses. Over 82 per cent. of the males were found in August—October—a period of declining temperatures and decreas- ing food supply. This is a widely distributed species, though it seems generally to be present in relatively small numbers in the plankton. It occurs in many European lakes. Stenroos (’98) finds that it 1s the most abundant species of Cyclops in Nurmijarvi See, occurring in both the plankton and littoral fauna throughout the summer. Scourfield (98) finds it common in the waters of Epping Forest, where it is perennial in ponds and small lakes; and Burckhardt (’00) also finds it in the smaller lakes of Switzerland. It appears to be more generally reported from European streams. Thus, Schorler (’00) finds it to be rare in the plankton of the Elbe at Dresden in May; and Fri¢ and Vavra (’01), perennial in the littoral fauna of the same stream at Podiebrad, while Volk (’03) reports it in the plankton at four of seven localities examined at Hamburg. Meissner (’02 and ’03) finds it in May—August in the Volga at Saratoff, where it is abundant in the littoral zone or among vegetation and in quiet backwaters. Under a variety of synonyms this common and variable species has been reported from many American waters by Herrick (’84) and others. It was described by Professor S. A. Forbes (’90) as C. gyrinus, from the plankton of Lake Superior. With the exception of Marsh’s record (’95) from Lake St. Clair, it does not elsewhere appear to have been found in the plankton of the Great Lakes. Marsh (’93 and ’95) finds it generally in the plankton of smaller bodies of water in Wisconsin and Michigan, and E. B. Forbes (97) reports it as generally distributed in American waters of a permanent character. Brewer (’98) reports it (as C. signatus) in the vernal plankton of deep pools near Lincoln, Neb. No statistical 264 data on its seasonal distribution are given by any of the authors cited. C. albidus appears thus to be adapted to both the littoral and limnetic areas, but seems never to attain great numbers in the latter. Cyclops bicuspidatus Claus.—Average number, 373; in 1897, 206: in 1896, 1452 in 1895, 312. and in 1894 only 2.) walle cussion of the variation and synonymy of this species has been published by E. B. Forbes (97). This species shows sharply marked seasonal limitations. Every one of the 68 records, with the exception of one of a single female found September 30, falls within November—May, and all of the May records were made in the delayed low temperatures of the spring of 1898. The general distribution of this species during this period is indicated by the high percentage of collections in which it was found, viz., 63, 71, 67, 73, 93, 53, and 40, respectively, for November— May. The numbers per m.* are, however, high only in November and April-May, reaching 8,000 in 1895 and 1898 in this vernal pulse, and 3,560 in November, 1897, in the autumnal pulse. In Decem- ber-—March numbers do not rise above 500 per m.* save once in December and on March 24-30, 1896. C. bicuspidatus is thus a winter and early spring planktont in channel waters of the [linois. The temperature adaptations are exhibited by the fact that only 13 of the 68 occurrences are in temperatures above 50°, only 5 above 60°, and but 1 above 70°—that of May 24, 1898, at 73°. On the other hand, the greater developments in numbers take place during these higher temperatures of 50°-70°, the only rises above 1,000 per m.*at temperatures below 50° being those of March 30 and April 10, 1896, at 48° and 46.4°, and of November 15, 1897, at 47°. Muini- mum numbers thus prevail below 45°, and the temperature opti- mum in channel waters of the Illinois appears to lie near 60°. The seasonal routine in channel waters begins with the appear- ance of small numbers about November 1, with an occasional pulse of some amplitude in that month followed by a continuance of small numbers through the minimum temperatures of December—Feb- ruary, and a rise with the temperatures in March to a maximum vernal pulse toward the end of April or the first of May, and a complete disappearance of adult individuals after temperatures pass 70° during May—October. 265 Stable hydrographic conditions appear to favor the increase in C. bicuspidatus, as is seen in the large pulse of November 15,-1897 (3,560),and the shght pulse (240) during declining levels in February, 1899. The vernal development of 1898 (Table I.) is distinctly pulse-like, and there are traces elsewhere of similar phenomena, but 1n general the numbers of C. bicuspidatus are too small to exhibit clearly the phenomenon of recurrent pulses. Of the totals of all individuals recorded 1n 1894-1899 I find that 37 per cent. are males, 16 per cent. egg-bearing females, and 47 per cent. females without eggs. Immature forms and nauplii were not distinguished from those of other species. With the exception of a few stragglers, the egg-bearing females were limited principally to March-May. In exceptional cases the males greatly outnum- bered the females, as on November 15, 1897, when the ratio was 2,820 to 680. Though apparently widely distributed, this species does not appear frequently among the planktonts reported from European lakes. Scourfield (’98) reports it as a common species in the waters of Epping Forest throughout the year with the exception of a period of absence or depression in July—August, and Scott (’99) finds it in shore collections made in various months of the year in Scottish lakes, and more abundantly in the warmer months. It has been reported in the potamoplankton in Europe only by Rossinski (’92) from the Moskwa, by Zernow (’01) from the Schoschma, and by Volk ('03) from but one of seven localities in the Elbe at Ham- burg. In American waters, on the other hand, C. bicuspidatus is more abundant, and in the Great Lakes it forms a very important part of the plankton. Forbes (’82) finds it (as C. thomast) to be the dominant Cyclops in the summer plankton of Lake Michigan and (790) also abundant in that of Lake Superior. Marsh (’93 and ’95) finds it in the summer plankton of the Great Lakes, near Charlevoix, in Lake St. Clair, the Detroit River, and Lake Erie, but only rarely and in small numbers in the smaller bodies of water in Wisconsin and Michigan. E. B. Forbes (’97) extends its recorded range to Massachusetts and to the lakes and rivers of Wyoming, and states that it is widely distributed in America and occurs in large ponds and rivers. Brewer (’98) reports it in the vernal plankton of deep 1 (An ( OF cial Re Va ty 266 pools near Lincoln, Neb. None of the investigators quoted give statistical data of the seasonal limitations of C. bicuspidatus. The absence of this species from the summer plankton of the Illinois River and its abundance in that of the Great Lakes is perhaps explained by the temperature conditions. Surface waters in Lake Michigan are reported by Ward (96) to range from 62° to 67° August 11-29, while deeper waters at and below the thermocline reach a minimum of 42°. The warmest waters there (62°-67°) are thus considerably cooler than the coolest in the waters examined by us (which are usually above 70° and often above 80°) during the months in which C. bicuspidatus is not found in our plankton. That its absence is not due to sewage contamination in low water which usually prevails during the warmer months is shown by the prompt reappearance of the species in the autumn; as, for example, in 1897, when sewage was even more abundant than usual. It may be that temperature is also one of the factors limiting its distribution elsewhere. Cyclops edax Forbes.—Average number, 49; in 1897, 194; in 1896, 159; in 1895, 321; and in 1894, 187: This is the third species of Cyclops in numerical importance in channel plankton of the Tlhnois. With the exception of a single record on November 2, 1897, all occurrences of this species in channel plankton are confined to April-October, and all but 9 of the 48 occurrences are in July— October, and 32 of them in July-September—the period of maxi- mum summer heat. During these three months the percentage of collections containing C. edax is highest (44 to 75 per cent.), and they are the only months in which the C. edax population rises above 1,200 per m.*1n channel waters excepting a single instance on October 5, 1897, in the high temperatures of that delayed autumn. In other months the records are all below 800 and generally below 400 per m.* The highest number recorded was 3,600 on October Sy LSoi. The seasonal distribution, with maximum numbers in July— September, exhibits a temperature adaptation on the part of C. edax to maximum summer temperatures (70° to 80°) in channel waters. An examination of the records shows that only 13 of the 48 records of this species fall in temperatures below 70°, and these were all in the months of April, May, September, October, and November, at ere ae net 267 times when occurrences were scattering and numbers few; that isdunine the rise or decline of the species to or from the summer maximum. Of the 13 records below 70°, there were 5 between 60° and 70°, 7 between 50° and 60°, and but 1 below 50°. Cyclops edax in channel waters of the Illinois is thus stenothermic in narrow limits near the maximum temperatures of the year. The relation which hydrographic conditions bear to the seasonal development of C. edax may be inferred from the fact that the July—October population of this species in the disturbed waters of 1898 was only 35 per cent. of that in the more stable months of the preceding year. ; The occurrences of C. edax take the form of pulses, though less distinctly recurrent and less clearly defined than in species present in larger numbers. Such pulses appear in July, August, and september, 1895, and in August and October, 1897. In 1898 (Table I.) the numbers present are too small to clearly indicate recurrent pulses, though suggestions of the phenomenon appear in the records. In general these pulses tend to coincide with those of other Eutomostraca. Of the totals of all our records of C. edax in 1894-1899, 60 per cent. are females without eggs; 11 per cent., females with eggs; and 29 per cent., males. Young and nauplii were not distinguished from those of other species. Egg-bearing females were found in April and in June—October, but in greatest numbers in July—August. Males occur in June—November, with no marked predominance in any period. This species has not’ been separated from C. leuckartt by other investigators of the plankton, though E. B. Forbes (’97), after a careful comparison of American forms with C. leuckartt of Europe, concludes that edax is specifically distinct, and that leuckarti also occurs in American waters, though apparently not in numbers com- parable with those in European waters. C. edax appears in a measure to replace it in our plankton. He reports it as widely distributed in American lakes and streams and in the plankton of our Great Lakes. Cyclops leuckartt Claus.—A single dead specimen was recorded in channel plankton August 26, 1898. E. B. Forbes (’97) records it from the Fox and Sangamon (tributaries of the Illinois), from the Illinois and Mississippi rivers, and from Quiver, Flag, and Dogfish 268 lakes, backwaters of the Illinois at Havana. It is not, however, at any time a factor of any importance in channel plankton of the Illinois at Havana, being confined to the spring-fed lakes or those shaded by vegetation, where regions of lower temperatures may be found. This is a widely distributed form in the plankton of European waters. Stenroos (’98) finds it abundant in the plankton of Nur- mijarvi See, Scourfield (’98) reports it as common in the waters of Epping Forest in February—October, and Scott (’99) as rare in that of Scottish lakes. Fuhrmann (’00) states that it is always rare in Neuenburger See except in April, and is absent in November— December, while Burckhardt (00a) finds it to be perennial in Vierwaldstatter See, with breeding season in May—September and maximum in August or September. It has been generally reported from European streams. Schorler (’00) finds it in the Elbe at Dresden in May—October, with greatest numbers in July-September, and Volk (’03) reports it from four of seven localities in the same stream at Hamburg, though Fri¢ and Vavra (’01) do not find it at Podiebrad. Zykoff (’03), Zernow (’01), and Meissner (’02 and ’03) find it in the plankton of Russizn rivers. The last author states that it occurs in both channel plankton and littoral fauna among vegetation where breeding females abound during the maximum in May. The young only appear in the chan- nel plankton. | In American waters this species has often been held to include C. edax, and the data here quoted from Birge and Marsh refer to the combined species. Marsh (’93 and ’95) finds it generally distributed in the lakes of Michigan and Wisconsin, and in the plankton of lakes Erie, Michigan, and St. Clair. Buirge (’97) finds it in the summer plankton of Lake Mendota, where it is even more abundant than C. viridis var. brevispinosus. Cyclops modestus Herrick was recorded in channel plankton only in November, December, and March, in small numbers and isolated occurrences at temperatures of 41° and below. E. B. Forbes (’97) states that this species lives in shallow, weedy water, and has never been found in large numbers, though widely dis- tributed. On account of its relative rarity it may have been over- looked by me and have a wider seasonal distribution than my scanty data indicate. 269 Cyclops phaleratus Koch was recorded in channel plankton only ~~ in small numbers in November—December, 1897, at minimum tem- peratures. EB. Forbes (97) states that it is a littoral form, confined to marginal vegetation. Cyclops prasinus Fischer.—Average number, 2. This species occurs sparingly and irregularly in September—March in channel plankton, appearing in largest numbers in the early autumn of 1895 and most continuously in the winter of 1898-99. The numbers are always small, never reaching 400 per m.°%, and in 12 of the 17 records falling below 100 per m.* The percentage of collections containing C. prasinus in the totals rises above 20 per cent. only in December (24 per cent.). The seasonal distribution in channel plankton indicates a limitation to the colder part of the year, all records but 5 being below 40°. Nevertheless, in September—October, 1895, the species was recorded in 56°-79°. This fact and its relatively small numbers generally, make it probable that inferences from our scanty data concerning its seasonal distribution can not be con- clusive. Of the totals in all years, 86 per cent. are females without eggs, 6 per cent. females with eggs (found in February and November), and 8 per cent. males. E. B. Forbes (’97) finds the species widely distributed in American waters from the Great Lakes to roadside pools. Marsh (793 and ’95) finds it (as C. fluviatilis) in the larger bodies of water in Wisconsin and Michigan, and in lakes Erie, Michigan, and St. Clair.: In Green Lake he (’97) finds it to be the most abundant species of Cyclops, and perennial, with maxima in September-— November. His statistical data exhibit somewhat irregular numbers which contain suggestions of recurrent pulses such as appear in our records of other species of Cyclops. Brewer (’98) finds the species in the plankton of pools near Lincoln, Neb. Cyclops serrulatus Fischer.—Average number, 3. This species was taken sparingly in channel plankton, exhibiting only isolated occurrences in December, January, March, and May, in flood waters at temperatures of 32°--75°. It is much more abundant in Spoon River, where it is sometimes the dominant species of the genus, appearing in May—September, and in small numbers in colder months. It appears to be adventitious in channel plankton of the Tllinois River. 270 This widely distributed Cyclops appears but rarely in the records of the plankton of European lakes, and then only in the smaller ones. Stenroos (’98) reports it as abundant in the littoral zone of Nurmijarvi See; and Scourfield (798) finds it perennial and the most abundant species of Cyclops in the waters of Epping Forest. On the other hand it has been found generally in the plankton of European streams. Zimmer (99) finds it in the Oder, and Schorler (’00) states that it is abundant in April-June in the plank- ton of the Elbe at Dresden; Frié and Vavra (’01) find it only in the littoral fauna at Podiebrad; and Volk (’03) in the plankton in four of seven localities in the Elbe at Hamburg. Sowinski (’88) found it in the plankton of the Dnieper, Rossinski (’92) in that of the Moskwa, Zykoff (’00) in the summer plankton of the Volga, and Zernow (’01) in the winter plankton of the Schoschma. Meissner (702 and ’03) reports it in May—August as not abundant in the back- waters and vegetation of the Volga at Saratoff. In American waters Marsh (’93 and ’95) finds it in smaller lakes of Wisconsin and Michigan but not in the Great Lakes, and E. B. Forbes (’97) states that it is one of the most common and widely distributed species in American waters. It appears, however, not to be quantitatively an important element in lake or river plankton. Brewer (’98) finds it to be the most abundant vernal Cyclops in the small bodies of water near Lincoln, Neb. Cyclops viridis Jurine.—-A synonymy and a discussion of varia- tions in this the dominant and most variable of all the Cyclops in our channel plankton, has been given by E. B. Forbes (97). I have grouped the individuals in our plankton under two varieties, brevispinosus Herrick and insectus Forbes. The two varieties inter-_ grade, and in my separation I have followed only a single character readily visible without dissection or manipulation, namely, the outer terminal spine of the stylet, which is short, broad, and lance- shaped in brevispinosus, and more spine-like in imsectus. Judging from the results of this method of separation, it appears that this lance-shaped spine is a character of the male in many instances, though not found in all males or limited to this sex. Cyclops viridis var. brevispinosus Herrick.—Average number, 124: in 1897, 447- in 1896,622- im 1895-8502 andim1894-63adehs form occurred in all months but January, but predominantly from the last days of April to the first week in October, the percentage AR ee? a Pee De ° 5 271 of collections containing brevispinosus in these months being 27, 80, 62, 67, 48, 75, and 59 per cent, respectively, while in other months it does not rise above 20 per cent. The number of individuals is also greater during the warmer season. No record between October 15 and April 20 exceeds 200 per m.*%, while between April 20 and October 15 the pulses often culminate at 3,000—5,000 per m.*, and over 98 per cent. of the total individuals were recorded. This variety appears throughout the whole seasonal range of temperatures from summer’s maximum to winter’s minimum, but predominantly during the warmer season. Only 15 of the 71 occurrences and 2 per cent. of the individuals were recorded at temperatures below 60°. As soon as the vernal rise in temperatures passes 50°-60°, the minimum numbers and scattered occurrences of the winter months give way to a vernal pulse of considerable mag- nitude in April-May, attaining 4,452 on April 25, 1895, and 4,960 on May 25, 1897, but only 2,600 on June 7, 1898. This is followed by a period of depression in July, when the summits of the pulses did not often surpass 1,000 per m.* In the late summer and autumn of 1895 and 1897, and to a less extent in 1896 and 1898, a second period of maximum pulses appears, attaining 9,711 September 12, 1895, and 4,800 October 5, 1898. When temperatures decline in September—October below 50°, this variety falls at once to minimum numbers. The records of brevispinosus in channel plankton exhibit some- what clearly the phenomenon of recurrent pulses whenever collec- tions at brief intervals make it possible to delimit the pulses. Thus, in 1895 there are pulses culminating in July, August, September, and October; in 1896, in April, May, June, July, August, and September ; in 1898, in July, August, and October; but in 1898 (Table I.) the numbers are too small to exhibit fully the phenomenon of recurrent pulses. The relation to hydrographic conditions may be inferred from the fact that while in the stable conditions of July—October, 1897, pulses culminated at 800—4,800 per m.%, in the same period in the disturbed hydrographic conditions of 1898 no pulse rose above 200 per m.*, and the total of all records in those months is only 8 per cent. of that in 1897. Evidently brevispinosus does not thrive in flood waters. 212 The surprising fact derived from the examination of our records of this variety of C. viridis, is that the individuals referred to it are predominantly of the male sex. Out of a total of 74,308, 64,883, or 88 per cent., are males, 8,542, or 11 per cent., females without eggs, and only 883, or one per cent., egg-bearing females. In so far as these data go, they indicate that this so-called species, or even variety, of C. viridis, in so far as it is based on the lance-like spine of the stylet, is not well founded. This is, it seems, predominantly a male character, though not exclusively so, since females, and even egg-bearing females, are found which exhibit this structure. C. viridis var. brevispinosus appears to be confined to American waters. Marsh (’93 and ’95) reports it from the larger lakes of Wisconsin and Michigan, and from the Great Lakes, except Lake Michigan. Birge (’95 and ’97) finds that it is the most abundant species of Cyclops (except in summer, when C. leuckartt abounds) in Lake Mendota, and the only one reproducing under the ice. His data exhibit a major pulse in May, and a second one, of less ampli- tude, in October, with slight indications of recurrent minor pulses in midsummer, obscured possibly by the massing of his data in fortnightly averages. The seasonal distribution in Lake Mendota is thus much lke that in the Illinois River. Marsh (’97) finds the maximum in Green Lake in June at 68°-69°, and only scattering occurrences at other seasons. E. B. Forbes finds this variety widely distributed in American waters, but never especially abun- dant. Cyclops viridis var. tnsectus Forbes.—Average number, 539; in 1897, 2,115; in 1896, 949; in 1895, 2,966; and in 1894, 905. It is thus more abundant by two- to threefold in the stable years of 1895 and 1897 than in the flood-swept years of 1896 and 1898. This variety was found in every month of the year, though predominantly in April-October, when the percentages of the collections containing it were respectively 64, 100, 85, 100, 100, 87, and 76 per cent. In November—March the percentages were only 44, 6, 17, 7, and 13.. The numbers of individuals are very small, however, from October 1 to April 20, excepting in the autumn of 1897, when, with the delayed high temperatures and the great impetus given to plankton development in the stable conditions of low water, the maximum pulse of all our records, 30,800 per m.*, was reached on October 5, a pulse of 1,200 following in November. With 213 these exceptions no record exceeding 600 per m.* was made between the dates named. Between April 20 and October 1 the minimum records rarely fall below 600 per m.*%, except in 1898, and the pulses often culminate at 2,000-8,000. C. viridis var. insectus is thus a planktont of the warmer season, and its seasonal distribution is strikingly similar to that of the so-called var. brevispinosus. This form occurs in our plankton throughout the whole seasonal range in temperatures, but only in small numbers and irregularly Isclow 60°. Only 21 per cent. of the collections containing ansectus were made at temperatures below 60°, and these contained less than 3 per cent. of the total individuals. With the exceptions of the pulses culminating at 43° November 23, 1897, at 1,200 per m.?, and at 57° April 26, 1898, at 4,160 per m.*, no development of this species exceeding 600 per m.* occurs below 60°. All pulses of more than 3,000 per m.*, excepting only the April pulse of 1898, occur at temperatures above 70°. The species reaches its greatest development in channel waters during the period of maximum temperatures, 70°-80°. The seasonal distribution of this form shows a few straggling individuals in November—March during temperatures below 50°, and a meteoric rise to a vernal pulse in April-May as this tempera- ture is passed and 60°-70° arrives. This is followed by a series of recurrent pulses, often of considerable amplitude, through Septem- ber or until temperatures fall below 60°, as in October, 1897. With falling temperatures the drop in numbers to the winter minimum is quickly accomplished. A comparison of the distribution in 1897 and in other years, shows a close correlation between the decline in temperatures and the falling off in numbers of znsectus. The relations which hydrographic conditions bear to the develop- ment of zmsectus in channel plankton may be inferred from the hydrographs on Plates IX.—XII, Part I.,and from the data sum- marized in the following table,—1894 being omitted because of the incompleteness of the seasonal representation. In 1895 levels were low, unusually so in the spring, and the flood-free intervals of the year were of more than the usual extent. About 10 feet of the total movement in levels (51.9 ft.) 1s found in whe late December rise. If this is excluded, the total movement falls to 42 feet, and the range in levels to 6.5 feet. Under conditions, Die Ree Total Average height, | Average number Year 1 Seo o movement, in ft., of stage of znsectus Bees oat in 6, of river Perens 18905. ss. 22: nee {2 51.9 3.61 2,966 (SOG, Ric Sane 10.1 45.7 6.98 949 OOS Ars oe 14.3 44.8 6.90 A ANAS) 1SOSR aac eee 155 Ovnae 8.02 539 then, of lowest levels, least range, and total movement, we find the largest development (2,966) of zmsectus in channel plankton. In 1896 the average river level is much higher, affording in- creased current and more silt. A series of recurrent floods also flush the channel, though the total movement and range in levels within the limits of the year are not greatly increased. Neverthe- less, the changes, which appear mainly below bank-height, affect channel plankton profoundly, and the production of znsectus falls to 949 per m.* In 1897 the population rises to 2,115 per m.’, largely as a result of the stable conditions of flood-free waters at low levels and with slight current in the last half of the year. In 1898 the total movement (67.2), range in levels (15.5), and average stage (8.02) reach the extremes in the four years under comparison, and the znsectus population falls to the lowest level—539 per m.3 A detailed comparison of the July—November period of the two years follows. Mont tai oc.eete etree July August | September MiGaieenctch oe Soret | 1897 1898 1897 1898 | 1897 1898 Total movement....... |e! 7.4 2.6 Los NW Os On Average stage......:.. 6.05 By (AO) = Bao) 3.66 2aOn 4.44 Average number of C. viridis var. insectus...| 5,093 210 2,030 304 2 Dis 325 | | « So or ww) IPT vt! ® «@ b' at) @ OS SS iw ay es 215 IMomiGhie yore toe tee A October November Average WiGAIr fs ala Peps ase hash jou 1897 | 1898 1897 1898 1897 1898 Hotalamovement.. 2.2.4) O66 | - 3-9 2D 20 BGP |) sys) Average stage......... 2200 | 24286. (2282 TAs | 32 04"|| 35 00 Average number of C. | viridis var. insectus...| 8,625 200 520 68 Be 7098 |) 22 In 1898, with two and a half times the movement in levels found in 1897, the development of tusectus attains less than 6 per cent. of the numbers reached in the latter year. The occurrences of imsectus in channel plankton exhibit the phenomenon of recurrent pulses during the season of its occurrence in large numbers whenever collections are sufficiently frequent to delimit the pulses. Thus, in 1895 there are such pulses in July, August, September, and October; in 1896, in April, June, July, August, and September; in 1897, in July, August, September, October, and November; and in 1898, in April, May, June, July, August, and September, though of slight amplitude in the last three months. Some of the seeming gaps and irregularities in the series of pulses of brevispinosus and insectus will be eliminated if the statistics of the two forms are combined in a single series,—a fact which lends support to the view that the two forms belong to the same species, and are parts of a common group of variable organisms. Steuer (’01) concludes from his examination of the plankton of the Danube at Vienna, based on 19 (?) collections in 15 months, that Cyclops has usually two maxima and two minima in each year, and that in the same body of water, owing to various meteorological influences, the two maxima do not in any year fall near each other. The more extensive data at my command show the limitations of such a general conclusion. An examination of the records of ind1- vidual species of Cyclops and of the total Cyclopide in our waters, make it clear that the major pulses may follow each other at about a monthly interval. For example, in 1897, the total Cyclopide (19) 276 have their major occurrences in our records as follows, the pulses _appearing September 14 and October 5: jilyss Ole 8,080 Sépt. dG, a oon) Avice IO ta oe. A0', 360) % GOepieedie st erher ae 15,260 TN Saag Memes Me 17,120 Sept es. eee 14,400 NUGe Dade ees 205320 WEES -Diiecta cere 101, 600 NS Sil Mara fe ae 67, 200 Octet oeeaae 3,400 DEMb ss dean eer 107,200 Again, in 1896, the two major pulses of the year are on June 19 (928,984) and July 18 (563,815). Steuer’s conclusion seems to be founded upon insufficient data, and can not have general applica- tion. Of the total 240,830 individuals of C. viridis var. insectus in our records in 1894-1899, 117,166, or 49 per cent., are males; 109,460, or 45 per cent., females without eggs; and 14,204, or 6 per cent., females carrying egg-sacs. If the brevispinosus totals are included, the percentages change to 42 per cent. of females—of which 37 per cent. and 5 per cent., respectively, are without and with egg-sacs— and 58 per cent., males. The apparently high proportion of males may be due to the fact that in the enumeration more young females than males were included in the “young” Cyclops. The egg-bearing females were generally more numerous in April-July. No marked predominance in the proportion of males appears at any season in our records. Cyclops viridis does not appear extensively in the plankton literature of European lakes. Stenroos (’98) finds it not rare in the littoral fauna of Nurmijarvi See. Scourfield (’98) reports it as next in abundance to C. serrulatus in waters of Epping Forest, where it is perennial. Scott (99) finds it at all seasons in both littoral and pelagic collections in Scottish lakes, and Amberg (’00) lists it for Katzensee. It appears but infrequently in the investigations of European streams. Neither Schorler (’00) nor Fri¢ and Vavra (’01) report it from the Elbe, though Volk (03) lists it from six of seven localities in this stream at Hamburg. Sowinski (’88) finds it in the littoral fauna of the Dnieper, and Zykoff (’03) in the summer plankton of the Volga, though Meissner (03) states that it is never found in the plankton of that stream at Saratoff, being confined to the littoral Dahl zone and to vegetation. No statistical data concerning its seasonal distribution are given by any of these authors, though Meissner states that it reaches its maximum in May in the Volga. In addition to the species of Cyclops here listed for the channel plankton of the Illinois, E. B. Forbes (’97) records in May-—Septem- ber, 1896, C. varicans Sars as common, and C. fimbriatus var. popper Rehberg and C. bicolor Sars as rare. Owing to the impossibility of separating with certainty the nauplii and young of the various species of Cyclops they were al] recorded together under the head of“ nauplii” and “ young Cyclops.”’ The former includes also the nauplii of the two species of Diaptomus occurring in our plankton. Young Cyclops.—Average number, 4,780; in 1897, 16,035; in 1896, 10,196; in 1895, 21,960; and in 1894, 5,960. With two ex- ceptions in January and February they occur in every collection examined. Numbers are, however, at a minimum in November— March, only 9 instances of more than 1,500 per m.* appearing in our records in this season. With the exception of two pulses in the autumn of 1897, and two in this season in 1895, all pulses of an amplitude exceeding 8,000 per m.* are confined to the interval between April 20 and October 1, practically to temperatures above 70°. They also exhibit relations to hydrographic conditions of the same nature as those found in case of the adults of the various species of Cyclops, and manifest. likewise the phenomenon of re- current pulses (Table I.). The totals of all young Cyclops in 1894— 1899 are almost five times those of all adults of the genus. This ratio gives an index of the extent of the decimation by enemies and inimical factors of the environment which exists after the nauplius stage has passed and before that of the adult is reached. Nauplii of the Copepoda (excluding the Harpacticide).—Average number, 36,707; in 1897, 53,786; in 1896, 24,560; in 1895, 88,442; and in 1894, 45,648. Nauplii were recorded in all collections ex- amined with but two exceptions. As in the case of the adults and young, the large numbers are, however, confined to the warmer season between April 15 and October 1. During the colder months the pulses rarely rise above 20,000 per m.*, and those in excess of 35,000 during these months are with one exception confined to the delayed high temperatures of the stable autumn of 1897. During 278 the warmer season, on the other hand, the pulses frequently attain 100,000 or over. The maximum record of 928,984 was made in the stable low water of June 19, 1895. All large developments thus lie at tem- peratures above 70°. The nauplii bear much the same relation to hydrographic condi- tions as that found in the adults; for example, in Cyclops viridts. This is seen in the fact that in unstable years such as 1896 and 1898 the numbers are on the average only 28 and 68 per cent. of what they were in the more stable conditions of 1895 and 1897, and the average monthly population in July-December in the unstable conditions of 1898 is only 18 per cent. of that in the same months of the previous year. The relative numbers of adult, young, and larval stages of the Cyclopide are given in the accompanying table. Nauplii Young Cyclops Adult Cyclops Year No. | Ratio No. Ratio No. Ratio | 1804) A) ce. 456,483 | 38 59,598 5 11,726 1 LE OSHS See DAN ps ero 680,749 | 5 140,779 1 1S OGR ae ete 145 524 | 17 428,211 | 5 84,786 1 TSOUR. eee. ae 1,828,720| 18 545,200 | 5 LOPES Owl ed MSO See 1,908,780 30 248,576 4 62,735 1 SOO ean oe eee 121,345 | 61 5,422 3 Totals....| 8,508,570 / 2 | 1,967,756 5 | 404,749; 1 The ratios between total adult and young, 1 to 5, are fairly constant in the different years, falling to 1 to 3 in January—March, 1899, and to 1 to 4 in 1898,—a year in which the colder part of the year was most fully represented. This ratio probably represents more truly the relationship of young and adult in the total yearly production. The ratios of adults to nauplii in the several years vary considerably from the totals of all years (1 to 21), rising to 1 to 279 61 in winter conditions of 1899 (January—March), and falling as low as 1 to 171n 1896. This was a year of recurrent floods, but its ratio is in sharp contrast with that of 1898 (1 to 30), also a year of con- siderable hydrographic disturbances during the summer. The adult population was reduced during this year, and especially during the summer floods, but the nauplii do not fall conspicuously below those of other years. It would therefore seem that the deleterious action of flood conditions operates more effectively upon the adult and young than upon the nauplii. This fact may be due to the relative absence of spines and hairs on the nauplu, structures which gather silt and load down the larger forms in the flood waters. The greater number of young and adults in 1896 as compared with 1898 may be due to the more gradual rise of the floods of the former year (see Pl. X. and XII., Pt. I.) land the proportionally greater amount of silt in the more sudden floods of the latter. The ratios given in the table are of course subject to the error arising from the uneven seasonal distribution of the collections in some years, and to that arising from varying location of the collec- tions on the pulses, especially on those of greatest amplitude. An ad- ditional error arises from the leakage of the smaller nauplii through the meshes of the silk net. I have found on experiment that they will thus escape under pressure of a column of water only 3-4 cm. 1n height. Their dimensions are such that the smaller individuals can pass through the meshes of even the No. 20 silk. It seems probable that ratios of nauplii to adults are actually greater than our records indicate. The relationship which the pulses of nauplii bear to those of the adult Cyclopide may be inferred from an examination of the data of Table I. An analysis of the seasonal distribution of the total young and adult Cyclopide and of the nauplii reveals the fact that in all seasons in which collections at approximately weekly intervals were made, their pulses coincide in a majority of cases in their maxima, and when the coincidences do not occur the maximum of the nauplius pulse appears in the collection of the week following that of the young and adult Cyclopide. This appears less constantly and clearly in the disturbed hydrographic conditions of 1898 (Table I.) than in the records of more stable years. Apstein (96) finds that nauplii of Copepoda are most abundant when eggs are most common, and that this bears no constant relation 280 to the abundance of adults. Our collections, extending over longer periods and being at briefer intervals, indicate, however, that this relation does exist. As above stated, the larve are most abundant at or shortly after the times of greatest abundance of adults—that is, the maxima of the recurrent pulses. Apstein also states that reproduction is periodic and development rapid. Maximum numbers are reported by him in May and September. Cohn (’03), on the other hand, maintains that the “innere Logik”’ and his data show him that the nauplii reach their greatest numbers just prior to the appearance of largest numbers of young and adult Copepoda. His data are from 12 collections between May 1 and October 1, and favor his contention in 2 out of 3 cases (of maxima), and both of these lie in collections at intervals of 15 to 16 days. In the light of our data obtained at briefer intervals and the conclusions therefrom that the pulses of larve tend to coincide or follow at a brief interval those of the adults, it becomes questionable whether his data are sufficient for his conclusion. His logic also overlooks the fact, apparently, that smaller numbers of larve might lead to coincident maxima of grown forms during a period of abundant food, on which all pulses must be based, since the larval stage may be at such times a brief one and the adult a relatively longer one, and the cumulative effect of this relationship would make the conditions shown im our data logically possible. Furthermore, Cohn used a No. 12 silk in his plankton net, and this allows many nauplii to escape, and probably accounts for the fact that the ratio of larvee to grown forms in his figures is only 1.3 to 1, while in our records it is 3.5 to 1. The discrepancy arising from this leakage may further tend to weaken his data for his conclusions concerning the relations of larvee and adults. | Steuer (’01) finds that the nauplii in the Danube at Vienna reach maxima in June and in August, but his data are too scattered to fully delineate their fluctuations. Two out of three of his max- ima coincide with those of all Cy clops, and the third ene Hrs it (monthly intervals of collection), as in Cohn’s data. Diaptomus pallidus Herrick.—Average number per m.’, 11; in 1897, 367 + 1m. 1896, 87> 101895. 152- and msrsote 146: This species was recorded in all months of the vear but February, though in a larger percentage of the collections and in larger numbers in July-December. Prior to this season the percentage does not ie nk 281 rise above 31 per cent., the occurrences are irregular, and the num- bers are small. Thus in 1896 and 1898, years of numerous winter and vernal collections, there were but 4 occurrences in each prior to July 1, and all but one of these was of numbers less than 100 per m.* Only 12 of the 72 occurrences and 8 per cent. of the total individuals were recorded in the first and less stable half of the years. In July—December numbers rise in feebly outlined pulses which attain at the most 800-2,400 per m.* The percentage of collections con- taining the species rises to 33-75 per cent., and in stable autumns such as 1895 and 1897 the occurrences are but little interrupted. In its seasonal distribution in channel waters it 1s thus largely confined to the last—and more stable—half of the year. Its relationship to hydrographic conditions here suggested also appears in a comparison of the yearly averages given above. The average numbers per m.’ in 1896 and 1898, 87 and 11, are greatly exceeded by those of 1895 (152) and 1897 (367). The total number recorded in July-December in 1897 is 29 times that in 1898. This well-defined predominance in stable seasons, which appears also in the case of the closely related D. stctloides, exceeds that of the other Entomostraca, and indicates a greater sensitiveness on the part of these species to the deleterious effects of flood waters. The long antenne and great development of the feathering of the caudal stylets afford a large area for the attachment of the silt and debris of flood waters, and accordingly facilitate the destruction or removal of Diaptomus from the plankton more quickly than in the case of Entomostraca in which these processes are less developed—as in Cyclops or Bosmina. The numbers of individuals are too small to delineate accurately the recurrent pulses which are suggested in the data of distribution. In the autumns of 1895 and 1897, when the occurrences are most continuous, the larger numbers tend to fall at the times of the maxima of pulses of other Entomostraca. There is no marked limitation placed upon this species by the seasonal changes in tem- perature. It is found throughout the seasonal range in tempera- tures, though numbers are slightly smaller in channel waters in November—December. Nevertheless it occurs in considerable num- bers in the backwaters in breeding activity under the ice at mini- mum temperatures in December. 282 Of the total individuals, 40 per cent. were males; 45 per cent., females without eggs; and 15 per cent., females with eggs. The sexes show no marked or constant seasonal differences in distribu- tion. Females with eggs are more abundant in August—October, and with spermatophores in the same months. Detached sperma- tophores were found until December. This species is stated by Herrick (’84) to be distributed in the entire Mississippi Valley. Marsh (’93) finds it in Wisconsin, but it appears nowhere in the plankton of the Great Lakes. Brewer (98) reports it in the backwaters of the Platte in Nebraska, and Schacht (’97) states that it is an exceedingly common species in central Illinois, and that it has been reported from Wisconsin, Ohio, and Minnesota. It thus appears to be limited to the shallow and relatively warm waters of the prairie regions of the Mississippi basin. Inaptomus siciloides Lilljeborg.—Average number, 10; in 1897, 350; in 1896, 56; in 1895, 282: and in 1894, 23. As will be ‘Seen on comparison, these yearly averages are very similar to those of the preceding species with the exception that the development of D. siciloides is about twice that of D. pallidus in 1895. In other particulars its seasonal data so resemble those of D. pallidus as to make their discussion in large part a repetition. Its seasonal- distribution relations to temperature and hydrographic conditions, breeding season, and its tendency toward a pulse-like recurrence in coincidence with other Entomostraca are all very similar to these features in D. pallidus. The proportions of the sexes differ slightly, the males being less numerous (31 per cent.) and egg-bearing females more abundant (18 per cent.) than in the previous species. This is also an American species, reported thus far only from Lake Tulare, Calif., the Illinois River, and waters of Indiana and Iowa (Schacht, ’97), and by Brewer (’98) in lakes and pools of Nebraska. It is thus confined largely to shoal and warm waters. Diaptomus spp., immature.—Average number, 19; in 1897, 560; in 1896, 158; in 1895, 336; and in 1894, 120. The immature individuals of D. pallidus and D. siciloides were not distinguished from each other in the records. Young Diaptomus presumably belonging to these two species occur in every month but March, though but 10 of the 74 records were made in January— June. The percentage of occurrences and the numbers per m.? are lowest in these months, not rising above 33 per cent. and 500 per 283 m.* save in two instances. Occurrences of small numbers continue through July, but from August 1 to October 15 appear the major pulses of the year, attaining an amplitude of 1,000 to 8,800 per m.° With the decline of temperatures in October, numbers fall to levels below 400 per m.’, with one exception (December 14, 1897) at 700. The percentage of occurrences is, however, high (41 to 44 per cent.) and declines only to 33 per cent. in January. The period of greatest numbers of young thus coincides with that of greatest abundance of adults, and lies at temperatures of 70°, and above, in channel waters. The effect of hydrographic changes upon the occurrence of young Diaptomus appears in striking form in the annual averages above quoted. In 1898, a year of sudden changes, the average per m.* is only 19, while in the stable conditions of the previous year it is 560. The July-December production in 1897 is 28 times greater than that of 1898. In 1896, a year of recurrent but less sudden floods, the average (158) is less than that of 1895 (336), a more stable year. The great reduction of adults noted in 1898 and 1896 is thus paralleled by an even greater reduction of the young. Osphranticum labronectum Forbes occurs in the plankton of Quiver Lake in small numbers (see Schacht, 98), and was found once in channel plankton in June, 1896. AMPHIPODA. Allorchestes dentata (Sm.) Faxon.—This is an abundant littoral species found amid vegetation, especially in the vegetation-rich backwaters, such as Quiver Lake. It was not often found in channel plankton, being taken only in the summer of 1895, when the July— August floods carried away the vegetation which had accumulated during the antecedent low water. ATROAC TH NDE. ACARINA. In vegetation-rich backwaters members of the family Hydrach- mide were frequently taken, along with other adventitious or- ganisms, with the plankton. In channel waters they are less frequent, and are represented principally by Atax, which is parasitic 284 in great numbers (see Kelly,’99) in the Untonideé which are found in the bottom of the channel. Occurrences in the plankton were limited to the months of May—August, and may be due in part, especially in the warmer months, to the release of the parasites by the death and flotation of their hosts. Flood waters in warm months were often disastrous to the Unionide because of the load of silt, sewage, and industrial wastes which they carry in channel confines at the lower river stages often prevailing in these months. Other small aquatic Acarina were also present, probably adven- titious from the littoral or bottom ooze. With two exceptions their occurrences in the plankton were all in warmer months, April— September, though not in flood waters. During the period of the migration of waterfowl, parasitic Acarina were noted in plankton collections in a few instances. TARDIGRADA. Macrobiotus macronyx Duj.—Average number, 11. This species is found principally in the colder part of the year, from October to May. The earliest autumnal record was October 30, 1895, at 45°, and the latest vernal one, May 1, 1896, at 68.8°, and the maximum number (2,980 per m.*) was recorded on April 10, 1896, at 46.2°. Of this number, one sixth were females with eggs. Females with eggs were also found in November, February, and March. Because of its seasonal distribution it is found principally, though not solely, in disturbed hydrographic conditions, and its occurrence in the plank- ton is largely adventitious. Jal, D:< JV 12(@) 1D aN Owing to the shoal waters, relatively narrow confines, and the hydrographic fluctuations in our fluviatile environment, the aquatic insects, both larval and adult, have many points of contact with the plankton. They constitute a large element in the total volume of the animal population of shore and bottom, and are all connected by chains of food relations, more or less complex and remote, to the plankton organisms or their sources of food. With the single exception of the larvae of Corethra they are all in the main adventt- tious members of the plankton assemblage, and are much more abundant in the vegetation-rich backwaters than in the channel. ee ee 285 Since the aquatic insects of these collections are being studied by others, with reference to publication in this Bulletin (see Hart,’95, and Needham and Hart, ’01), only passing notice of the more important representatives appears in this connection. EPHEMERIDA. Ephemerid larve, as a rule in early stages, were found singly or in small numbers in the channel plankton in the warmer months, April—October, at temperatures above 56°. Since these occurrences were with few exceptions in stable hydrographic conditions, it seems probable that the younger larve of this order may adopt, at least temporarily, a limnetic habit. Specific identifications of these larvee were not made. HEMIPTERA. Corisa (?) sp.—Average number, 37. A small hemipterous larva doubtfully referred by Mr. C. A. Hart to Corisa, was taken with some frequency but in relatively small numbers in the plankton during the summer months. Of the 36 occurrences 27 fall in June-August, 2 in May and 3 in September, 2 in January, and 1 each in October and November. It thus appears in the temperature extremes, but exhibits a great predominance in the season of maxi- mum heat. There is no marked increase in its frequency or numbers in years of more disturbed hydrographic conditions. Its numbers are always small and somewhat erratic. Adult Corisa, as well as many other aquatic Hemiptera, were found in plankton collections singly and infrequently. DIPTERA. This group of insects is abundantly represented in the plankton, but in all cases by larval or pupal stages. Chironomus spp., larval stages.—Average number, 124. Larve in various stages of development from that immediately after hatching to that approaching pupation were found in channel plankton. They occur in considerable numbers in the ooze in the river bottom, but appear to abandon the limicolous for the limnetic habit, temporarily at least, as a result of hydrographic or other disturbances. There is evidence from their relative numbers in 286 years of different hydrographic conditions that these have consider- able influence in bringing them into the plankton. Thus in 1897, in stable conditions, there were only 5 occurrences in 31 collections examined, averaging 88 per m.°, while in 1898, in more disturbed conditions, there were 29 occurrences in 52 collections, averaging 124 per m.? There is also a marked seasonal distribution. The larvee appear in the plankton in March—December through the seasonal extremes of temperature, but the numbers in March and November—December are always small. Only 15 per cent. of the occurrences and 5 per cent. of the individuals were found at tem- peratures below 45°. The percentage of occurrences in the collec- tions 1s highest in March—September, the percentages being 53, 73, 80, 47, 78,52, and 50, respectively, to:.8 to 35 per cent. during the remaining months. Corethra sp., larval stages.—Average number, 6. These semi- transparent and active larve have the characteristics of limnetic organisms, and may be reckoned among the autolmnetic planktonts of our waters. Because of their activity. it seems probable that they escape the drawn net,—especially the small model used by us,—and also, because of their negative rheotaxis, elude the suction of the plankton pump to an even greater extent. Thus, in 1895, in net collections, there were 8 occurrences averaging 32 per m.? to 4 in 1898, in pump collections, averaging 8 per m.* Corethra larve were never abundant in our plankton, probably in part for the reasons just cited. With two exceptions all the occurrences lie in the period of maximum temperatures in June-September, 7 of the 14 occurrences and one third of the individuals being recorded in August. Dixa sp., larval stages.—Average number, 8. Larve were recorded singly in scattered occurrences in all months but February and October-December, though most of them appear during maxi- mum temperatures. Larvee of Tanypus and Odontomya were also recorded in May and June in isolated occurrences. In addition to the larval stages of these aquatic insects there occurred in the plankton a considerable number of insect eggs, principally those of Diptera and Ephemerida. These were generally isolated, though sometimes fragments of the egg-string of Chirono- cite < 287 mus appeared. They were recorded in all months but February and December, though 20 of the 30 records and 81 per cent. of the individuals appeared in May—August. The numbers are never very large, the maximum record, 5,424 per m.* on June 29, 1894, being due to a number of fragments of egg-strings. MOLLUSCA. GASTROPODA. The adults and young of many of our aquatic gastropods have the habit of gliding on the under side of the surface film of water, and they are also frequently dislodged from their foothold on aquatic vegetation, and thus enter the habitat of the plankton temporarily. This is especially true in vegetation-rich backwaters. The smaller forms, such as Ancylus, Amnicola, and Planorbts parvus were occa- sionally taken in the summer plankton of the channel. LAMELLIBRANCHIATA. This group is represented in the plankton by the larval stages, or glochidia, of the Untonide, which form an important part of the bottom fauna of the stream and its tributaries. Anodonta corpulenta Cooper.—Average number of glochidia, 21. The seasonal distribution of the glochidia in the plankton is very well defined. With but two exceptions the 48 occurrences all fall in October—April, and 40 of them in November—March. The occur- rences are thus during the period of minimum temperatures; indeed, 31 of the 48 are at temperatures not exceeding 35° in surface waters, and only 9 are above 45°. The earliest autumnal record 1s Septem- ber 30, at 58°, and the latest vernal one, June 6, at 79°. Generally the earliest records are in the closing days of September or the early ones of October, and the latest records are about the first of April. The occurrences are more frequent in December—March, the glo- chidia appearing in 64, 50, 53, and 60 per cent. of the collections, respectively, in these months. Their numbers are also several fold greater at this season than in the earlier and later months of their occurrence. The period of minimum temperatures is thus the season of greatest discharge of glochidia. The numbers are always relatively small, 520 on December 28, 1897, being the max1- 288 mum record. Their fluctuations are erratic, and show no apparent relation to hydrographic or other environmental changes. Lampstlus anodontoides (Lea) Baker.—Glochidia referred with some uncertainty to this species appeared somewhat irregularly in the plankton in small numbers in September—December and again in June-July. The seasonal distribution in two periods suggests the inclusion of two species. Arcidens confragosus (Say) Simpson.—Glochidia of the type referred by Lea to the old genus Margaritana, and presumably belonging to this the commonest member of this genus (as formerly understood) in our locality, were taken in the plankton December 18, 1895, in small numbers. BRYOZOA. This group is represented in our plankton by the floating stato- blasts, when these occur, as in Pectinatella and Plumatella, by detached and floating fragments, as in Urnatella, or by natant colonies, as in Lophopus and Cristatella. Genera such as Fredericella and Paludicella, whose statoblasts sink, fail to appear in the plank- ton, though in some cases they may be abundant in the bottom fauna. The Bryozoa are plankton feeders, and play an important role as plankton reducers in vegetation-rich backwaters. DISCUSSION OF SPECIES OF BRYOZOA. Cristatella mucedo Cuvier.—This species was found in the back- waters in summer months, especially in Quiver Lake. Statoblasts probably referable to this species occurred sparingly in May and August. Lophopus cristallinus Pallas.—This rare bryozoan occurred in the channel plankton, though not in our quantitative collections, in July, 1897, in that part of the channel containing the discharge from Quiver Lake. Small, free-swimming colonies of 5-50 zooids were taken in surface waters. Pectinatella magnifica Leidy.—Statoblasts of this superb bryo- zoan were not uncommon in the backwaters, and were seen several times in the vernal plankton of the channel. The large floating colonies are found near the surface in July—October in the open backwaters, and more rarely in the river itself. The translucent Goa = 289 gelatinous ccencecia are spherical, ellipsoidal, or often somewhat flattened. The longest diameter of these floating masses often exceeds 30! cil. Plumatella repens 1..—This is by far the most abundant bryozoan in our locality, being found everywhere on submerged vegetation in the backwaters. It often develops with surprising rapidity on the submerged stems of plants, where, as in 1896, summer floods reinvade the vegetation-covered margins of reservoir backwaters. It is represented in the plankton by its floating statoblasts. Their seasonal distribution shows some correlations with temperature, hydrographic conditions, and the seasonal cycle of the parent organisms. During the period of minimum temperatures (Decem- ber—February, inclusive) they are relatively rare in the plankton, appearing in 30, 8, and 20 per cent., respectively, of the plankton catches. They are rare in high- as well as low-water conditions, as, for example, in the floods of 1895-96 and 1898, when they appear in but one of 15 collections. With the rise of temperature in March they occur more frequently, as, for instance, in 1898 (Table I.), and continue during the run-off of the spring flood. The occurrences rise in March—May to 60, 46, and 50 per cent. of our total collections in these months, and the numbers also are larger. For example, in 1898, 81 per cent. of the total individuals for the year were found in these months. The discharge from impounding backwaters, the principal breeding grounds of the parent organisms, doubtless tends to increase the numbers of statoblasts in channel plankton during this season. During the remainder of the year, June-November, the percentage of occurrences again falls to 30, 50, 24, 32, 18, and 44 per cent., respectively. The 50 per cent. in July is due to the summer flood of 1896. If this year is omitted the record falls to 33 per cent. The large percentage for November is probably due to the predominantly higher levels of this month, to the invasion of lake margins seeded with statoblasts, and to the increased activity in the fishing industry, which tends to disturb the summer’s growth of vegetation in tributary backwaters. The relations to the seasonal cycle of the species are patent. The summer months, June— September, are the season of growth and spread of the parent organisms and of the formation of statoblasts, especially as receding levels expose the water margins. Hydrographic or other disturb- ances tend to increase the number of statoblasts in the plankton 290 until minimum temperatures are reached, when minimum numbers appear in the plankton. As temperatures rise, the statoblasts tend to float and become more abundant in the plankton, asa result, perhaps, of the physiological and accompanying physical changes in the contents of the statoblast. The declining phase of the major flood of the year is thus the period of greatest flotation and dispersal of the statoblasts. Urnatella gracilis Leidy.—This unique species is found in some abundance on the projecting margins of the shells of the Umonide which line the river bottom in many reaches of the channel. Small fragments of the colonies containing only several polypides were found in the plankton in May—August and October. The earliest record was May 25, and the latest, October 25, at 48.5°. oe a THE PERIODICITY IN THE MULTIPLICATION OF THE ORGANISMS OF THE PLANKTON. One of the most obvious conclusions brought to light by the detailed study of the volumetric fluctuations of the plankton pub- lished in Part I. of this report, and most strongly reinforced by the statistical data showing the fluctuations in the numbers of the indi- viduals of the various species and in the sums total of the various biological groups represented in the limnetic fauna and flora, is that plankton production is fundamentally rhythmic or periodic in character, viewed either in its constituent elements or as a whole. This total result is simply the sum of a like phenomenon pervading more or less completely and coincidently the reproductive cycles, the rise and decline in the numbers of the typical constituents of the plankton. The exceptions to this rhythm are usually found in those organisms which are adventitious in the plankton and have their centers of growth and distribution in other regions than the open water. Many illustrations of this periodic movement in the multiplica- tion of organisms of the plankton have been cited in the preceding pages and may be seen in the accompanying plates. As an illustra- tion for discussion in detail we may take the pulse of July, 1898, shown in the volumetric data of Table III. and Plate XII. of Part I. The fluctuations in the biological population during this period are also tabulated in Table I. of this paper, and graphically presented in Plates II. and IV., which exhibit the movement in the totals of the Chlorophycee, Bacillariacee, and chlorophyll-bearing Masti- gophora, and of the Rottfera and Crustacea. In the volumetric data the pulse rises from a minimum of .14 cm.* per m.%on July 5 to a maximum of .88 cm.’ on the 19th, declining again on the 26th to the second minimum, of .67 cm.? Its duration is thus four weeks and its amplitude, in comparison with many other pulses in the records, relatively slight. It occurs in the more stable conditions of declining river levels and midsummer tempera- tures. The following list gives the names of the more or less typical planktonts considered in the discussion of this pulse. Others, largely adventitious or insignificant in numbers, might be added (20) 291 292 to the list. Forms whose antecedent minimum does not fall on June 28 or July 5 are designated by a superior 1; those whose maximum does not fall on July 19 or 26, by a superior 2; and those whose subsequent minimum is not on July 26 or August 2, by a superior 3. The component forms and groups are Crenothrix, etc.', total Schizophycee, Microcystis tchthyoblabe', total Chlorophycee, Actinas- trum hantzschu, Crucigenia rectangularis, Pediastrum boryanum':? 8, P. pertusum*, Raphidium polymorphum', Scenedesmus genuinus, S. obliquus, S. quadricauda, Schroederia setigera, total Bacillariacee’, Cyclotella kuetzingiana, Diatoma elongatum', Fragilaria virescens’, Melosira granulata var. spinosa’, M. varians’, Navicula spp., Synedra acus, total Conjugate’, Closterium acerosum, C. gracilts, total Protozoa, total Mastigophora, Eudorina elegans, Euglena acus, E. oxyurts, E. viridis, Glenodinium cinctum, Lepocinclis ovum, Pandorina morum®, Phacus longicauda':**, P. pleuronectes? *, Platydorina caudata, Pleodorina californica, Trachelomonas acuminata‘, T. hispida, T. volvocina, total Rhizopoda, Difflugia globulosa, total Ciliata?, Codonella cratera*, Halterta grandinella® *, Tintinnidium fluviatile? *, total Rottfera, total Bdelloida’ *, total Plowma, Anurea cochlearts and var. tecta, eggs of A. cochlearis and var. tecta, A. hypelasma, Asplanchna brightwellii*.*, Brachionus angularis and var. bidens, eggs of B. angularts and var. bidens, B. baker and vars. 9 cluniorbicularis!, melhemi, and tuberculus' ?, total of all varieties of B. bakert, B. budapestinensis, B. militaris':*, B. pala and var. amphiceros, B. urceolaris var. bursarius, B. variabilis® *, Mastigocerca carinata!, Monostyla bulla, Polyarthra platyptera, eggs of P. platyp- tera?’*, Rattulus tigris?, Syncheta pectinata', S. stylata, eggsof Syncheta', Triarthra terminalis *, Pedalion mirum’ °, total Eutomostraca’ °, total Cladocera’ ? *, Bosmina longtirostris': *, Certodaphma scitula, Chydorus sphericus, Diaphanosoma brachyurum*, Moina micrura\* 8, total Copepoda » * 3, Cyclops viridis var. brevispinosus and var. insectus, C. edax, young Cyclops', nauplii of Copepoda’ *. An examination of the preceding list and of the qualitative data of Table I., reveals the fact that 71 of the more typical planktonts are found in appreciable numbers in the plankton during this month. To this number we may add 6 immature forms separately listed in the table and 14 group totals, making in all 91 sets of statistical data bearing on the components of this pulse. An analy- Z93 sis of the behavior of the constituent species shows that 43 of the 71 species (including varieties and forms), 4 of the 6 immature forms, and 10 of the 14 group totals reach their greatest amplitude on the 19th, coincidently with the volumetric maximum. Thus, in all, a total of 57 out of 91, or 63 per cent., of the sets of data are in pre- cise agreement as to the time of maximum development. Fur- thermore, of the remaining 35, there are 10 culminating in the collection prior to the 19th (on the 12th), and 16 on the next subse- quent one (on the 26th,) in all, 26 or 29 per cent. which culminate on immediately contiguous dates of examination. This leaves a residuum of only about 8 per cent. which do not exhibit precise or substantial agreement as to the time of maximum development. In the matter of the location of antecedent and subsequent minima the agreement is less pronounced, possibly because the enumeration error is relatively greater in the case of minimum numbers. We find, however, that 65, or 72 per cent., of the antecedent minima of the pulses occur on June 28 or July 5, and 71, or 79 per cent., of the subsequent minima are on July 26 or August 2. Nineteen, or 20eper cent., of the antecedent minima are-on July 12; and 10, or 11 per cent., of the subsequent ones are on August 12. There is thus a residuum of not over 10 per cent. of instances where the data of species or group totals do not coincide or approximate to this pulse, as described, 1n position of maximum or one or both of the limiting minima. Considering the necessarily large error entering into our data, it is not surprising that exceptions should occur. Some exceptions—as, for example, that of Pedzastrum pertusum (Table I.)—are plainly not due to insufficient data, but are appar- ently normal dislocations; that is, the rhythm of this species at this time is not in harmony with that of the majority of the components of the plankton. But this is only a temporary derangement, and is not the habitual relationship which movement of production in Pediastrum bears to that of the plankton as a whole. So, also, many of the Entomostraca are much delayed in the culmination of their increase, running over to August 2 or 9, while the most of the other planktonts culminate on July 19 or 26. This lag on the part of the Entomostraca is not, however, habitual, as will be seen on examination of Plates II. and IV. This tendency toward a coincident rhythmic movement in production on the part of the constituent organisms of the plankton will be found throughout all 294 the data where collections are of sufficient frequency to adequately delineate the curve of production, that is from July, 1895, to Oc- tober, 1896, and from July, 1897, to March, 1899, a total of 37 months, and suggestions of a like phenomenon appear in the less complete data of other years. The degree of agreement indicated in the pulse of July, 1898, will be found,on examination of the data in Table I. and in the plates of this paper, to vary with the environ- mental conditions. Times of rapid change in hydrographic condi- tions or in temperature generally show less agreement, and more stable conditions will exhibit an equal or even greater uniformity in the prevalence of the pulse-like rise and decline of the component organisms. In order to show the course of these recurrent pulses in the chlorophyll-bearing planktonts, the total Chlorophycee, Bacal- lartacee, and chlorophyll-bearing Mastigophora on the one hand, and of the Rotifera and Entomostraca (‘‘Crustacea”’ of the plates), I have presented the data graphically on Plates I.-IV., and in the table on pages 296-299 have drawn up a list of the pulses, indicating the dates of the collections which in the main enter into the respective pulses, and the dates of the maxima or culminations of the five groups named. Owing to the irregularities in the data, there are some instances in which several possible dates might have been chosen. Reasons for the choice are in several important instances given in the foot-notes to the table. It is evident from the data here presented in graphic and tabular form that the pulses of the five groups of organisms tend in the main to coincide. This is shown in Plates I.-IV., and in the fact that the average divergence of 175 group pulses listed in the table is 6.4 days, or, if 5 aberrant instances are omitted, only 4.8 days. In other words, the pulses of the totals of the 5 groups included in the table culminate on an average within an interval of 6.4 (4.8 in 170 cases) days. The average of the extreme limits between maxima of group pulses in the 36 periods of movement listed in the table is 11.7 days. It is apparent that the pulses would be more completely de- lineated by collections at daily intervals, but even in the somewhat irregular and at times chaotic data here presented, the evidence seems conclusive that the seasonal production of the dominant species and groups of planktonts tends to fall into coincident a ae ~ a 295 recurrent pulses, which, in turn, are the cause of the similar and often coincident volumetric fluctuations. Attention should be directed to the fact that without any im- portant exceptions this recurrent movement pervades all the organisms of the plankton which are eulimnetic,—such as Scenedes- mus, Melosira, Trachelomonas, Codonella, Syncheta, Daphnia, and Cyclops,—and often those which at certain seasons become tempo- rary planktonts, such as Dzfflugia and Hydra, but not with any regularity the tycholimnetic organisms, such as bdelloid rotifers or nematodes. It affects the more highly organized Kotifera and Entomostraca with slower growth, longer life, and consequent greater cumulative function as well as the alge, diatoms, and flagellates, where rapid multiplication, brief existence, and non- cumulative (in the individual) function prevail. The large share which the young (eggs and immature stages) play in the pulses of Rotifera and Entomostraca will be seen in Table I., and repeated attention has been called to this in the discussion of species. The prevalence of breeding females and of eggs or young during the rise of the pulse, and of eggless, moribund, or dead individuals or their skeletons during the decline, is a common phenomenon in all well- defined pulses. No species of plankton organisms appears to escape the operation of this recurrent movement in production. The proportion of individuals surviving from one pulse to the next is subject to great variation, being often least when the ampli- tude of the pulses is greatest,and largest when the pulses culminate at sight amplitudes. As a result of periods of minimum develop- ment, it follows that the possible length of life of most plankton organisms, even of the Rotifera and Entomostraca, in the plankton must fall within rather narrow limits of a few days or a fortnight at the most. Since the contrasts between minimum and maximum numbers are relatively greater among the chlorophyll-bearing organisms, it follows that the survival proportion is less in these groups. The duration and amplitude of the plankton pulses will vary within certain limits according to the method of delineation. The volumetric minima and maxima present the total product in cubic centimeters, and the pulses thus marked cat have been described in Part I. 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Pieces e sees eos poem soum0qUg | ee can he cae a en ree dich ae eee ene era FIOY oni cy < | i ca 5 61 ST e Oe eo ae eT OUdOdtiseAl oT en. PZ he 97 pee oh tp a eee ao eure ae el oT TAS RORGE LT ST “AON 97 SZ “PO on Nata aaa ae " -maokydosopy9 Bey WNUINIX BI Bey UWINUIIXe Jy ralictg [9 UWNUWIXe]y Pe eee aay eee soos" = Sosind LZ AON . 67 “PO 67 “ydag Terese sess * woour [INF Jo sozeqd % LomOCe Cis 0. O° OCerCGn Gl. 8-0) ZAON SG Wo ae HEXO) srr tts sss SUOrposTTOO Jo seyeq 300 nant or more typical species. In the case of the total plankton some obscurity results at times from the inclusion of unusual proportions of an adventitious population with flood waters. The selection of particular organisms as representative is also subject to some error, since seasonal changes in temperature and other more subtile causes often deflect or suppress their development. The totals of the Chlorophycee, Bacillariacee, and chlorophyll-bearing Mastt- gophora, and of the Rotifera and Entomostraca (P1. I.-IV.) probably give as complete and accurate a delineation of the recurrent pulses as the statistical data afford, since they include relatively few adventitious organisms, cover the entire year, and swamp more or less completely individual and temporary divergences of particular species. The delineation of the pulses by statistical data is obviously more significant than the volumetric method, since it more clearly presents the results of the reproductive processes which lie at the foundation of the phenomenon of recurrent pulses; and this method is also free from the unavoidable error arising from the presence of silt in the collections. The interval between collections introduces an error of consid- erable moment in any effort to determine with accuracy the duration of individual pulses, that is, the length of time between their minima or maxima. Daily collections would render this feasible, but with an interval of a week or more, not only the duration, but in some cases the probable separation of the pulses and location of their maxima, is to some undetermined degree obscured. The duration of the pulses of the an e groups of plankton organ- isms shown graphically on Plates I.-IV.,in the case of all chlorophyll- bearing organisms considered as a whole, is in 29 out of 36 instances between 21 and 35 days, less than 21 in 2 cases, and more than 35 in 5, reaching extreme limits of 14 and 49 days. They average 30.25 days between minima and 29.97 between maxima. The rotiferan data in the same months may be divided into 36 periods, in 33 of which pulses are traceable. The duration of pulses between minima les between 21 and 35 days in23 of the 36 instances, falls below 21 in 5, and is above 35 in 8. The extreme limits are 14 and 49 days. In the case of the Entomostraca, where also the pulses are obscure in a few of the intervals, we find that 22 of the 36 are between 21 and 35 days between minima, 5 are below 21, and 9 are above 35. we oe @ 301 The extreme limits are 12 and 49 days, and the average duration is 29.9 days. From the data here presented it is evident that the pulses are in the main from 3 to 5 weeks in duration, averaging approximately 29 + days—a little less than one calendar month. The amplitude of the pulses is affected profoundly by seasonal and local influences, such as the factors of temperature and chemical constituents of the water, and the hydrographic conditions. These have been discussed in connection with the volumetric data in Part I. and in the discussion of species in the first part of the present paper. Rising, or even uniform, temperatures, hydrographic stability, decaying vegetation or access of sewage or other fertilizing constitu- ents, all serve to increase the amplitude of the pulses. Declining temperatures, dilution or suspension of access of fertilizers, competi- tion of gross vegetation, access of flood waters and increase in current, all tend, in the main, to depress the amplitude of the pulses. The duration of the pulses is not, however, thereby essentially modified, though a tendency to override subsequent pulses and partially, rarely wholly, to submerge them is at times of major pulses often apparent in the data. The cause and significance of the phenomenon of recurrent pulses is not clearly and unmistakably evident, owing, on the one hand, to the irregularity of the data, and, on the other, to the great complex- ity of the problem, especially in the fluctuations and varying combinations of environmental factors. The plankton method itself is subject to great errors, but these are largely distributed, and careful examination, especially of the matter of dilution and computation, has failed to reveal any probable or even possible source 7m the method to which these recurrent pulses can be traced. It is not impossible that the rhythm here noted is merely a chance outcome of the statistical method and without biological significance; that it is wholly accidental, the resultant of the con- flicting and varying factors of the environment and not predomi- nantly or continuously initiated by any one factor. On the other hand, its nature, as we have described it, is such that we are led to look for some factor in the environment with which this rhythm of repetition in growth of the plankton organism might be correlated, or to some internal or inherent factor within the organisms constituting 302 the plankton, or to the interaction of environmental and internal factors. That there is a periodicity in the reproductive processes of organisms, of both plants and animals, is generally apparent. We see it in the flowering and fruiting seasons of the phanerogams, and in the breeding seasons of many invertebrates, of mollusks and insects, and of the vertebrates generally,—of fishes, amphibians, reptiles, birds, and most mammals. Fluctuations in environmental conditions, notably in food and temperature, influence these re- productive processes. The phenomenon of rise and decline of the microscopic population in laboratory aquaria is likewise an illustra- tion of the periodicity of organisms, but usually within a briefer interval than that of the organisms above mentioned. The studies of Maupas (’88) and Calkins (’02) have shown that even in the seemingly uniform conditions of the laboratory, the reproduction of the ciliate Protozoa is essentially periodic. On a priort grounds it seems highly improbable that in the case of the organisms of the plankton, internal factors should determine the coincidence of the periods of growth and reproduction in several hundred species. While it is not impossible, or indeed improbable, that these species of the plankton if bred in pure cultures or unzjform environment would still exhibit a periodic reproduction, it seems highly improbable that so diverse an assemblage of algze, diatoms, flagellates, protozoans, rotifers, and entomostracans as is found in the Illinois River, would exhibit in laboratory cultures under uniform conditions any such coincidence in the location and duration of their pulses as is found in the waters of the stream. Whatever the internal factors involved in the growth and reproduction of plankton organisms may be, it is patent that we must look for some environmental factor or factors lying at the foundation of the coincidence of seasons of growth and reproduction of plankton organisms, which results in the phenomenon of recurrent pulses in species, groups, and volumetric plankton. We may simplify the problem somewhat by recognizing at the outset the importance of nutrition in supplying the basis for the periodic growth of any organism. The rotifers and entomostracans, at least the limnetic types, depend in large measure, either directly or indirectly, upon the synthetic planktonts, such as the alge, dia- toms, and flagellates, for their food. Since the pulses of these animal i i 303 forms (ci. PlatesglIT. and IV. with I. and II.) coincide with or follow shortly after those of the synthetic planktonts on which they feed, we may conclude that the cause of the periodic movement of these animal groups lies in the periodic fluctuations of their food supply. In the causes which control this periodic growth of the chlorophyll-bearing organisms will be found the solution of the general periodic phenomenon in plankton. This rhythm is primarily one of growth and reproduction, and its solution must be sought in the forms of matter and energy which affect these processes. The nutrition of the chlorophyll-bearing organisms is drawn from matter in the river water. The analyses contained in Part I., Table X., and graphically presented on Plates XLIII. to XLV. trace the seasonal fluctuations in the nitrates—one of the important constituents of plant food. Neither in the seasonal curves of this or other forms of nitrogen delineated in the plates is there any such rhythm of occurrence, though, as has been pointed out in the discussion of the chemical conditions, there are instances of apparent correlation of plankton and nitrate pulses. They occur at irregular intervals, and do not form a continuous series. That there might be a rhythm in the utilized nitrates (the analysis repre- sents only the unused residuum) is of course possible, or that it might occur in some other constituent of the food not determined in the analysis is not impossible, but we have no evidence of its existence. The chlorine in our river waters is a fair index of the amount of sewage or pollution by animal wastes. It is subject to considerable fluctuations, resulting in part from dilution by floods or concentra- tion in low waters, and there are other pulses not traceable to hydrographic conditions, which perhaps result from industrial wastes. These fluctuations in some instances coincide with those of the phytoplankton in question, but the instances are few and the correlation is incomplete. Upon investigation I find that sewage pumpage at Bridgeport, which discharged the sewage of Chicago River into the Illinois and Michigan Canal and thence into the Illinois River, was practically continuous, and could not produce the rhythm in question. The sewage of Peoria has a much more immediate effect upon the chemical conditions in the river at Havana than has that of Chicago. The sewers of this city, I am informed by Mr. H. E. Beasley, City Engineer, are flushed as 304 follows: ‘The method used is that of flushing with a hose, a crew of men being kept constantly at work, taking them about a period of three weeks to cover the entire system. The water is allowed to run through a fire-hose at each point for a period of about ten minutes.”’ This system was in use during the years of our opera- tions, and it offers no occasion for the periodic pulses in growth of the organisms in question. Investigation of the discharges of dis- tillery and cattle-yard wastes into the stream has not revealed any periodic fertilization of the river waters from these sources. The available data thus fail to exhibit any periodic rhythm in food matters in solution and suspension in the river water with which these pulses of chlorophyll-bearing organisms might be correlated. Frequent reference has been made in previous pages to the appearance of pulses upon the decline of floods. Flood waters bring into the river, as shown by the chemical analyses, large quantities of silt and organic wastes in suspension and solution. They inundate great tracts of fertile territory rich in vegetation, and thus add to the available sources of food for the phytoplankton. Decline of the flood affords time for decay and solution of some of the food matters, and time also for breeding, and its run-off adds to the volume of the plankton in channel waters. A comparison of the hydrographs of the years in question (Part I., Pl. X.—XIII.) with these recurrent pulses (Pl. I.) will show that many if not most of the pulses appear on declining flood waters, and that many of the larger ones follow the major floods. Closer analysis, however, shows that there are sometimes two pulses of chlorophyll-bearing organisms on the decline of a single flood, and that they may also occur upon rising flood or even in its entire absence. Floods unquestionably affect the amplitude of the pulses, and to some extent modify their location. They seem inadequate, however, to explain their recurrence and their tendency toward a uniform interval. Minima between pulses also recur on declining floods. Energy as well as matter is necessary for the growth of the phytoplankton, and its source is primarily the radiant energy of the sun. A plot of the tri-daily air temperatures at Havana -for 1894-1896 (Part I., p. 478, Fig. C) inclusive, exhibits many irregu- larities, a few of which partake of the nature of recurrent pulses at approximately monthly intervals, but they are too fewand tooirregu- lar to be the basis of the recurrent growth of the phytoplankton. ——— ; , x, » 2 ; ; 305 The importance of light for the photosynthesis of chlorophyll- bearing plants is unquestioned. The liberation of oxygen by the plant declines as the light fades, and is at its lowest ebb in darkness. The access of light to the phytoplankton is limited by several factors of the environment, principally by silt, which increases the turbidity, and by clouds, which interfere with the penetration of the sun’s rays. The fluctuations of the silt are chiefly.the result of floods, and, as above stated, the floods do not exhibit a rhythmic pulse which can be correlated with that of the phytoplankton; much less do the periods of rising water which are most silt-laden. The cloudiness of the sky varies greatly at different seasons of the year, being predominant at times in the autumn or winter months. It is sub- ject to pulse-like occurrences of variable duration, but an examina- tion of the records for central Illinois for the years under discussion does not disclose any periodic rhythm which can be correlated continuously with that revealed in the statistical records of the growth of the phytoplankton. Another factor of the environment which modifies the quantity of light which impinges upon the chlorophyll-bearing organisms of the plankton is the light from the moon. The amount of light, both absolute and relative, derived from this source is not great. According to the calculations of Zdllner, the light from the sun is 618,000 times as bright as that from the full moon. In the pre- sent connection it is only important to know whether the moon- light contains an amount of solar energy sufficient to appreciably affect the photosynthesis of the phytoplankton. The amount of such energy utilized in photosynthesis is relatively a small propor- tion of the total, so that there is a possibility that moonlight may contribute to the process to an appreciable extent. This matter was investigated by Knauthe (’98), who determined the fluctuations in the gaseous contents of the waters of carp ponds rich in Euglena. While this author does not report upon the plank- ton of the ponds investigated, it seems quite probable that carp ponds rich in Euglena would present conditions very similar to those found in the Illinois River, which has a remarkably well-developed Euglena water-bloom, and abounds also in carp. The following table presents the results of his work bearing upon the point in discussion. "u10}}0q 9y3 Uodn wsTe jo YYMOIS Bye ii 3 s aIqelepisuoo vB pedojeAsp pey pue “XT Si yi os ee Suen oqem yonepuedg ott: wt ‘d 00:7 ‘X— -ZZ 9OUIS AIO}VIOGR] UT POO}S py 19}e \\ CpaT Bae av 431 a 3 -UOOUI UI MOPUIM 4V JojyeM Ionepueds [***°" ut ‘d QO: TT ‘x-¢ ae } WOOL HIVP UT JoyeM Jonepueds SUC OOM oxo ee St] ne -UOOUI UI MOPUIM ZY | JoyeM JoUTYyUOUTUTeS “sour -d 96:6" X—¢ £20 $70 | WOOL Yep UT | 10oJeM JoUTyJUOUTUIeSG BS ouciups Al (OKshG ee F270 z oF 0 FSH 7 yysjuoow Aq volyeurUN]]! sInoy Fp sayy | SG j -uoour ‘zea pur [NS | aovying |-*** urd o¢:0T ‘XI-F a S ‘uasti yoA JOU UOOT{ ee j yep ‘iespo pue [IWS eoejing |°**°*we-d 00:6 ‘XI-? i “qyspuNs url ‘peroqyyun ‘paorid pure ‘puajsny yuepunq aul[eyye | SO'Z ; aie daaees pratense ' -@ YJIM IAL] IOVJANS WLOI] UdYeZ 197k \\ COS } Se aun e aeanes eos We alae 00S Xe: ‘LOJOO UdeIS Yrep AIOA B JO SBM 19}VM 9} YNq ‘pasvaioep Yonut ZZ 0 ZOuT THs | MOU 919M BOPJINS 9Y} UO WIG Yoryy & po GGAW “AO II ‘ulel ouy “IY = 1aqtV QOeFING | ut “2 00:6 “XI-S -WIIOF []I}s o10Joq ‘siy $o YOM Duan y 460 1L°0 } Apnojo ‘T]14S SOCTING |e aes we O¢:S ‘KXI-S L6°0 1Z°0 ; Aas } yseo19A0 ANS ‘TS SOC IING Tees UW ‘8 OFZ ‘XI-F “OO UID) O 2 WD SYIVUIY JoyjJVoM pure YS] 901n0G S68T I9}VM - WD OOF UT ‘(ayynpu yy 4aj{D)—aALVM AGNOg dO SINALNOD Ssnoadsvy 307 The amount of oxygen present in the water in the dark, or on @ark niclits, is reported as 0.20, 0.25, and 0.27 cm.*® per 100 cm.* of water. In bright sunlight in the laboratory, and with the unusual abundance of Euglena due to the collection of the water sample from the region of the water-bloom, it rises to 2.05 cm.* In the case of the Spandauer samples it rises from 0.25 in the dark to 1.15 (an increase of 0.90 cm.*) after “long” exposure to bright sunlight in the laboratory. The oxygen in this water at 11:00 p. m., after exposure to moonlight, amounted to 0.45, or 0.20 cm.* more than was found in control water kept in the dark. In this instance the apparent increase due to moonlight is ?/, of that due to sunlight. In the case of the moonlight the analysis was made at 11:00 p. m., after not more than three hours’ exposure. The moon was not at its greatest efficiency, since full moon occurred four days prior to the date of analysis. In the case of the sample exposed to the sunlight the analysis was made at 4:00 p. m., after “langer intens Sonnen- schein.”’ It would seem probable that the effectiveness of moon- light in comparison with sunlight in photosynthesis by the phyto- plankton here indicated (2 to 9) is below the possible maximum and also above that of the average, since it was obtained when the moon was but four days past its maximum effectiveness. If we accept Knauthe’s data as sufficient to establish the effec- tiveness of moonlight in increasing photosynthesis, and thus the growth of the phytoplankton, we find in it a recurrent factor of the environment to whose influence we may seek to attribute the rhythm of growth of the chlorophyll-bearing organisms. On Plates I. and II. I have plotted the seasonal distribution of the totals of the Chlorophycee, of the Bacillariacee, and of the Mastigophora from July, 1897, to April, 1899, and have indicated the times of full moon throughout this period by marks at the bot- tom of the diagram. The diagram shows clearly the occurrence of these recurrent pulses, their approximation in the three groups of chlorophyll-bearing organisms upon the same or adjacent dates, and the occurrence of their maxima in some cases at the time of full moon or within an interval of ten days thereafter. In the table which follows, I have given the data bearing on the pulses of the total of all chlorophyll-bearing organisms from July, 1895, to October, 1896, and from July, 1897, to March, 1899, inclu- sive, 36 months in all, stating the location of the pulse as determined (21) ez ‘sny eet: pe Aqnf 98 + sz ounf Tf 0G. 97 AV Cte LZ [dy 9°1T+ 67 JPN Veet 82 “Gq vy 9T+ og “uef eG Ge Te 90d T+ T 900q 5° Ar T ‘AON Vs + € "PO a se € “ydag OMe Fe p sny Die 9 Ainf 6°6 + APARIS JO I9aqua9 wooul [[N} jo esslosqy jo ayeq woout [[N}F Jo Avp wrosyz uorzerasqg 7 Ve Sala 91 ‘ydag Sz OF a LS 8 ‘sny 1Z 67 Tens (eae gt Aqn{ Le 92 OD ¢ + TT ounf za 92 mn Ee t+ ST Av a4 e¢ a, SOX oS bz dy se 9¢ nee Al (= LT Jew Ir LG SAep ¢ ae b -99q 9¢ ep AED) a oS Og 99d 61 61 Or Ole IT ‘00q 9¢ Se neat ba § “AON SZ 8Z 8 eT IT “390 62 ee One c= ZT “ydas Ge Ke LD One Ic sny | rE Te ee | sAep 7] 0 0S 0) eZ WINUIEXP JT SuIUuUIsIg ; iseoreres mouse WNUTxX eur jooeq = sAep Ul [BAIOqUT ‘Oe “ydag 04 TZ “‘Bny Reet eras "1c ‘sny 07 ¢z A[nf eas Severe Seca ez Anf 07 4z ounf eee 7 oul Ope Sun fi Ac ame: 1 ounf 0} 6z Judy '6¢ Indy 03 7z ‘seq SES 7 we Cc ade Mik aOR oN SZ “Gey 0} gy ‘uel 968T Diats hare means ET ‘uel 07 ¢z ‘o0q Bar RN rare SZ “99d 01 OZ “AON Beanie ele "0% “AON 0} ¢7 300 RRS Asbo @ Moy A (6 = pabbte suerte 0z ‘3deg 03 67 ‘Sny Soriyh toner ‘6c ‘Sny 01 6z An[ Be eee 6 AN 07 9 Af S68T asjnd Jo uoryeoo7T “aTOAD UVNNT OL SWSINVOUAGO ONTYVAE-TIAHAOUOTHD 40 SHSTING ao NOILV Tay 308 cera Sa as Neaee ae ale ce neocons ae eI bZ 99g POG ESL Valier Pl Je 1Z 12 DO ORES TRE cau EN NE Lomb Tp EIN 9¢ “uel Ole Gists » 9¢ ‘Sar 1Z 2h (ag CC OP Unser We LEI OV 1 Care |. LZ 99d ilar A anit ats OT ‘uef 87 I Ail ee eae ere Te ‘uel 0 ¢ “uel 6681 LZ ‘AON Le oie » OF C+ et “90d 1Z Geely Sr eae ¢ ‘uel 0} 67 ‘AON 67 “PO GOW: a UG G = 7Z “AON 9s eGo Dd chee era "6% “AON 01 SZ “290 6c 4dag oe le ai ij LZ ydas 1Z cy Se iG aaa "SZ 490 0} 02 “3das Te “sny ey no ee 9 “ydag 82 82 eh ee ke 0z “ydag 0} ¢7 “ny T ‘sny 6 +f ame es 6 ‘Ssny 1Z OC angele eee “eg ‘Sny 0} 97 A[nf ¢ Aqnf Ose wenOD ae 6t Ane Se | Wiges Ma ages causa 9¢ Ainf 0} zt Af me F ounf SUA tar EOD Die pT ounf Se GP, Ail sce ee: zt AInf 04 $7 Av 9 Av ge + ae ¢ — or Ae tI oad aang Coca esse re Av 01 ¢ Avy 9 [dy Oalger reOG Tes 97 Indy se Rigid diylicie! ee ree ee ¢ AvW oO} ¢ Judy 8 Iv > Ort TA [= ZZ “IPN ce Cer clip ae, eae “"g tndy 07 se] 9 “qe Ong a: i Cb ST “qa se Be ee gellar eae "oes sT “rey 07 6g ‘uel L uel a0 ee [De Ty ‘uel 82 Se oe eres ai: Sz ‘uel 0 1z 00q 868T Fs +a: +4205 Ee “gag 61.08 “AON PROG sod rors olen ON PORE MECN AMOR Coal tay Lee) aR ISS 7) do Oy PZ Sny 310 in most cases by the delimiting minima, the interval between maxima and that between minima, the date of the maximum, the deviation of the beginning and of the maximum of each pulse from the day of full moon, the deviation of the abscissa of the center of gravity of the polygon formed by the plot of each pulse, and the date of full moon. Deviations prior to the day of full moon are preceded by the minus sign. The average duration between minima is 30.25 days and that between maxima is 29.97 days; the average location of the initial rise of the pulse is 5.1 days prior to full moon; and the average lags of the dates of maxima and abscissa of center of gravity of the polygon of occurrences are 11 and 10.45 days, respectively. The probable error of the location of the abscissa of a single pulse is + 7.5 days, and of the average deviation of the abscissa only + 1.25 days. The table on pages 296-299 shows the lag of the maximum individual pulses of Chlorophycee, Bactillariacee, chlorophyll- bearing Mastigophora, Rotifera, and Entomostraca. The average lag after the day of full moon for each of the groups, in the order nameéd, is 13:7, 14.8, 14.3, 13.45. and=.14.3 “daycyene- spectively, with a grand average of 14.1 days for the 175 pulses listed. Of these pulses, 135, or 76 per cent., culminate prior to the third week after the date of full moon, and 94, or 52 per cent., in the fortnight between 7 and 21 days after full moon. The averages and percentages given in this paragraph vary but slightly from the demands of chance in favor of a hypothesis that the pulses tend to culminate in a particular part of the lunar month, though the data of the total chlorophyll-bearing organisms given above, es- pecially the deviation of the abscissa of center of gravity of the polygon of their occurrences, point in the direction of a lunar factor. There is no douht of the fact of recurrent pulses and of their distribution at intervals whose average approximates that of the lunar month, though their correlation with any particular part of the month is in no way constant and much less apparent. It would not be strange that the duration interval, or that the position of maxima and minima, should be subject to disturbance, to accelera- tion and delay, even to obliteration, in the fluviatile environment with its multitudinous factors,-—flood and drouth, summer and winter, clear and turbid waters, bright skies and overcast, the rise and fall of nitrates and other substances in solution or suspension, 311 the fluctuating access of sewage and industrial wastes, the continuous current, the ever-shifting population and the never ceasing struggle for existence and continuance on the part of the interrelated organ- isms of the plankton and of the shores and bottom. The wonder is that any single factor of the environment, however constant, could make any orderly impression in this chaotic situation. This fact that the average interval of the pulses of the phyto- plankton is so nearly the lunar interval would seem to indicate some causal nexus between the two phenomena. An attempt to correlate the plankton pulse with any particular part of the lunar month is, however, less conclusive. The interval of collection, one week, is so great that the course of the pulse can be traced only approximately, since its beginning, maximum, and end can only, from our data, be located at one of these intervals, and more or less distortion results therefrom. Again, the large error in the plankton method may be responsible for some of the fluctuations in the data. Still more potent, probably, are the various factors of the environment of the plankton which combine with the lunar illumination to produce resultants which divert the pulse more or less from the course which the undisturbed lunar factor would cause it to take. Evidence in favor of this view appears in the fact that the greatest disturbances in the rhythmic sequence of the pulses are wont to occur in winter months, when floods, ice, and cloudy weather tend most to interfere with the full action of the lunar factor, while the correlation of full moon and phyto- plankton pulse is most intimate in the stable conditions of summer. _This is seen in the fact that the average of the average monthly lags for all of the May—August pulses is 11.9 days, and for the remaining eight months, 18.2 days. The subject here presented is one which lends itself readily to field and laboratory experiment, and it is to be hoped that the sug- gestions of a correlation between the plankton pulses and lunar cycle here made, will be put to the test of further quantitative and statistical, as well as experimental, tests in controlled environments where the disturbing factors of the fluviatile environment are elimi- nated. GENERAL CONSIDERATIONS ON SEASONAL CHANGES.* It follows from the facts set forth in the preceding discussion that in general each month of the year, characterized by a certain range of hydrographic, thermal, and chemical conditions, and of illumi- nation, has a plankton characterized as follows :— 1. There is a certain range of component species, some of which are occasional stragglers and others more or less uniformly present. 2. There is a certain range of numbers of individuals, varying with the species and profoundly affected by fluctuations in the environmental factors, which change the proportions of the various species from year to year. These proportions vary also from month to month and constitute one of the main elements in the seasonal changes of the plankton. 3. Transitions from month to month are most profound at seasons of greatest environmental change, as, for example, at the times of vernal increase and autumnal decline in temperatures. 4. Seasonal changes in the plankton follow the environmental changes and not the calendar. Autumnal plankton is found when autumnal temperatures arrive. 5. In the main, but two types of plankton are found in the Illinois River—the summer, and the winter assemblage. The vernal and autumnal types are only transitions between the two when organisms from both are present. The winter plankton is charac- terized by a small number of species peculiar to that season, and a number of perennial forms; the summer, by a larger number of summer organisms with the perennial types. LAKE VERSUS RIVER PLANKTON. Is the plankton of streams (potamoplankton) different from that of lakes (limnoplankton) and ponds (heleoplankton)? This terminology, introduced by Zacharias (’98 and ’98a), seems to imply a distinction which lies not only in the differences in the configura- * The detailed discussion of seasonal changes in the plankton is deferred to a later paper. Sul, Sls) tion of the basin and in the matter of movement in the water, but also in the constitution of the plankton itself. The examination of the plankton of the Illinois River, and of its backwaters and tributaries; has shown that the plankton of the channel is not im- mediately derived from the tributaries, but comes in large part from the impounding backwaters, and at low-water stages is almost exclusively indigenous in the channel itself. Upon the basis of the data from the Illinois River the potamoplankton is distinguished from the other types named by the following characters -— 1. Itas a polymixic plankton. This is due to the mingling of planktons from all sources in the drainage basin, especially from tributary backwaters, and the consequent seeding of the channel waters with a great range and variety of organisms. In all of our col- lections in channel waters monotonic planktons can scarely be said to be present. The nearest approach to such conditions occurred at low-water stages, when channel waters are most fully isolated. 2. It is subject to extreme fluctuations in quantity and con- stitution. This naturally follows from the manifold factors of the fluviatile environment and the directness with which they impinge upon the plankton. Changes in volume, contact of shore and bottom, access of heat and light, and changes in chemical con- stituents are frequently both more extensive and more widely effective in the stream than they are in the other types of aquatic environment. In consequence, the plankton of the stream is sub- ject to more catastrophic changes than that of the lake. 3. The potamoplankton is not characterized by any species peculiar to it, nor by any precise assemblages of eulimnetic organ- isms. It may be distinguished, in a general way only, by the greater proportion of littoral or benthal forms which are nnaghee with the more typical planktonts. Zoological Laboratory, University of California, May 10, 1904, 314 AP NGS ID, 1B) 1k ORGANISMS PER CuBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) | \| H S sais 3 : ait = eats) 23 a #8 = Be ts ss = = SES os S = S 1898 Ss 8 ss | $8 oa = g BS wet oS y bo a | ss& Stee Ee gs ae 88 ss Be ies 293 oY} SS Se Ss 2 © Sey eae G Sb * S* ao | : acer ies aaes 399,600,000 7,477,200 0 0 7,200,000} 277,200| 14,400,200 SOS ae, 105,000,000} 3,046,800 0 0} 3,000;000) 46,800 9,000,200 Se eae ee 10,800,000) 14,511,837 0} 387. 14,400/000/ 111,456] 108,000,386 Bebew a0 Sn. 575,000,000 5,440,000 0 0 5,400,000} 39,600 9,000, 500 EO eB hres 275,400,000 400 O 0) 0 400 9,014,800 tS See as. 602,200,000 14,800 0, 400) 0} 14,400 3,600,400 Loner D pane ee 43,200,000 7,200,000 oO 0 0| 9,477 3,159 Mar, Ton =: 0 0 om 0. 0 0 400 oh eae a Peet 79,200,000 2,700,400 0 0| 2,700,000 400 3,600,400 PFET woes 40,500,000} 9 }000,600 0 0 — 9;000}000 600| 22,502,800 Ny Berka ae 21,600,000} 9,000,000 0 0 9}000;000 0 600 COUT reat oy | 18;900,000} 6 ;300;200 0 0) 6,300,000 200 2,704,200 pre. Ste. A 14,400,000 1,800,100 0 0) 1,800,000 100 2,700,300 Ad 21,600,000) 2/701,100 07 0 2'700,000) 1,100 1,980,100 mead OF manne | 21,600,000} 18/002/400 0| 0, 18,000;000) 2,400) 55,800,800 Be ADOF aaa | 10,800,000} 190;835,200 0 0, 190;800;000) 35,200) 135,906,400 Miiyin €3'ace 42,000,000} 174,000,000 O| 0 168,000,000, 19,200| 212,406,400 Ee Wt On eae 7,200,000} 216/057,600 0. 0 216;,000;000| 57,600| 194,531,200 cS SW pe saoee 14,400,000| 50;443;200 0 0} 50,400,000) 43,200} 64,901,200 NEM an, Bee 7,200,000| 10;}800;000 0) 0} 9,000,000 0} 125,602,200 mL ee ae 10,800,000} 14}400,000 200, 0} 43;200;000 200} 27,001,600 tmiee eee 18,000,000} 21,600,000 0. 0| 14,400,000 0} 21,616,000 a aes 18,000,000| 19)800;000 0 | 0, 3,600,000 0} 46,801,000 MD thee TE: 43,200,000} 43/200,000 0 0} 18;000;000 0| 21,658,400 a 38 auc 25,200,000} 18,000,000 0 0 18,000,000 0| 34,260,800 . | | aly. See 50,400,000} 21,600,040 0. 0} 21,600,000 40 9,049,200 SHO} ee 10,800,000| 28}800;060 0) 0, 28,800,000 60} 10,851,200 RE NAON sce ees 43;200,000| 162;000,400| 400. 0 162}000;000 0| 277,340,400 4 96 Eee 7,200,000} 34/200,000 0| 0| 34,200,000] 3,200] 31,651,600 Auge 2 aaa 21,600,000} 81,003,200 oO 0, 79,200,000 0) 45,304,800 ce MON Ae ae 57,600,000 |1,700,600, 000 oO 0 | 1,697,000 ;000 0| 370,948:400 Cai gantoae: 21,600,000) '212;400,800 0, 800, 208,800,000 0| 68,468,800 ie ee 25,200,000} 100/817,600| 1,600 2,400 100,800,000| 13,600] 108,200,000 et 1) eee 14,400}000| 288/121,600| ‘800) ‘800, 288,000,000} 27,200) 189,334,400 Séptlonmrnee 18,000,000] 118,800,800) 800. 0 111,600,000) 46,400) 87,489,600 ai cpr ts 28,000,000 | 3785013,500 0. 0, 378,000;000) 13,500) 54,042,500 NON Ae 50,400,000| 72/008;000 0 0} 723000;000) 8,000) 57,684,500 CSIR ie ed 68,400,000} 111,614,400 0) 0} 108}000;000) 14,400} 70,526,400 Oct eA ee 34,200,000) 23,400,000 0 0 14,400,000} 10,500) 27,024,000 Se Aen 21,600,000} 28,803,500 0 0} 28/800;000} 3,500) 14,420,000 ce Gat a 86,400,000} 3,600,500 0 0 3,600,000 500, 15,312,500 C25) ee 1,800,000} 52,201,560 0 60, 52,200,000} 1,500} 28,833,000 Nova= 125... 93,600,000} 21,601,000} 500) 0 21,600,000 500| 14,408,000 ee eon ed 176,400,000} 10/800;000 O 0) 7,200,000 0 3,604,000 ae Creer 190;800,000} 10}400,000 0 0, 10}400,000 0| 34,205,000 Lp) ee 124) 400,000 7,200 ;000 0 0 7,200;000 0| 16,206,000 BE Orr te 57,600,000 7,200,000 0 0 7,200,000 0 7,205,000 Dees (6.005: 136,800,000} 14,400,000 0 0 14,400,000 0| 13,500,000 SF Sean 468 ,000}000| 54,000,000 0 0, 54,000,000 0| 84,600,500 DO as 640,800,000} 59,400,000 0 0 59}400,000 0} 58,500,000 oa y epee 497,200| 37,800,000 0 0 37,800,000 0| 27,000,200 | } | Average...... 55,428,792| 85,909,984 83 93/ 83,059,615| 15,431) 53,175,104 ies ERS Re igh le aa Me a ng * a a a ee S15) TABLE I—continued. ORGANISMS PER CuBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 1898 * * 2 S4 = = S x a Ss es S8 23 ‘S* es =§ B38 Ss RS 5% 8 SS && SEN Qs Ss ws 2.8 aS aS = 5 £3 os Ss8 S38 Sf Bo es ss ss SS 3 iso QS x ae 1S Gs © iY aX 0 0 0 0 0 0 0 0 om 0 3,000,000 0 100 0 0 0 | 0 0 0 387 0 0 0 0 0 0 300 0 0 0 | 0 0 0 0 0 0. 0 0 0 0 400 0 0 0 0 0 0 0 0 0 0 0 0 0 400 0 0 0 0 0 0 0 0 0 0 0 0 600 1,800 0 0 0 0 0 0 800 0 0 0 0 0 600 200 o| 100| 0 0 0 0 200 0) 100 0 0 0 0 0 0 0) 0 0 0 400 400 0 0 0 0/1,800,000! 3,200 1,600 0) 3,200 0 0!7,200,000! 3,200 0 0| 0 0| 57,600,000 |7;200;000| 6,400 4,800 1,800,000 0 0 0 0| 4,800 57600 0 0 0 0 0 600 1,600 0 0 0 oe , 400 ,000 0} 1,000 600 0 0 0 pate 0} 3,200 12,800 0 0 1,800,000 0 0| 32,000 39,400 0 0 O| 4g 0 0 800 56,000 0 0 3,600,000] © ° 0 0| 6,400 55/200 0 0 0 FO 0| 3,600 44,800 1,800,000 0 0 0 0| 1,200 49/200 10800,000 0 0| 61,200,000 0| 4/000] 136,000 5’ 400,000 0 9,000,000 1'800,000/1,800,000| 4,000} 247,600 3,600,000 0 0 9,000,000 0| 4,800! 295,200 5? 400,000 0} 10,800,000) 158/400;000 0| 2/800! 145,600 5’ 400,000 0 3,600,000} 18,000;000 0} 1,600 66,400 10/800/000 0 900,000} 14/4007000 0| 5.600] 194,400 21/600;000 0 1,800,000} 21/600/000/7,200,000) 8,000} 326,400 10,800,000 0 0 0 0| 12,000) 177,600 7'200;000 0 1,800,000 7,200,000 0 500 42,000 7°200;000 0 0 0 0| 8,500 76,000 ny) 0 0 1,800,000/1,800,000| 65,600) 259,200 1,800,000 0 0 0 0! 5,500 18,500 1'800;000| 500 0 3,600,000 0! 8/000 11/500 900;000 0 0 17800/000 0} 33500 9/000 5,400,000 0 0 3/600,000 0} 18,500 14/500 1,800,000 0 0 0 0} 5,000 3,000 0 0 0 0 0} 1,000 3/000 0 0 0 3,600,000 0} 3/000 3,000 0 0 0 0 0| 4/000 2/000 0 0) 0 0 0 500 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 199,038 75 640,384 7,153,846| 519,231| 4,510 44,372 316 TABLE I—continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) x 3 = ” ” ay BS Sy Se = SS =s = S% 5 Sx S5 aS a> 2 is} S35 1898 35 LVS 3 Ss 5 Bs a5 Ss 88 33 SS gs SS s&s ss 85. 35 8 ws H 4 A 4 H a ian ell Sees 0 0 0 0 14,400,000 0 oe DM Ae, 0 0 0 0 6,000,000 0 ic See 0 0 0 3,600,000 7,200,000 0 Rebs 303 ase 0 0 0 0 9,000,000 0 ae Sie Sarees 0 0 0 0 9,000,000 0 wae lie sesntane nt 0 0 0 3,600,000 43 ,200 ,000 0 ee A tactiess 0 0 0 0 0 0 Mary (ae es ce: 0 0 0 0 0 0 os Siac there 900,000 0 0 900,000 1,800,000 0 tO caterers 5,400,000 0 0 0 17,100,000 0 ee AD Mowatt 16,200,000 0 0 0 7,200,000 0 © ee parce 0 0 0 | 0 2,700,000 0 Aiprs= fine sar 0 0 0 | 900 ,000 1,800,000 0 oe 2a ae 0 om 0 1,800,000 0 0 aed Oi ea ae 7,200,000 0 0) 1,800,000 46,800,000 0 Se DOR ae ses: 0 0 900,000} 13,500,000!) 108,000,000 0 WER? Bo ooo oe 24,000,000; 1,800,000 0 23,400,000 |} 150,000,000 0 ete LOR ae tars 7,200,000 0 1,800,000) 70,200,000 50,400,000 0 sae “ily Pek enemas 7,200,000 0 0 34,200,000 21,600,000 0 Or ce eee 3,600 , 000 0 0 5,400,000 3,600,000 0 rie Wola 3,600,000 0 0 3,600,000 14,400,000 0) Jame: i ss c.as 9,000,000 0 900,000 1,800,000 9,000 , 000 0 Po OLA Sf icle 21,600,000 0 900,000 0 21,600,000 0 ty U2 eet. 0 0 0 7,200,000 10,800,000 0 SR Sinatra 1,800,000 0 0 10,800,000 10,800,000 0 Ifuikys “Sino soae 1,800,000} 1,800,000 0 1,800,000 3,600,000 0 ees Ae 1,800,000 900,000 0 4,500,000 1,800,000 0 ee Oye ha 75,600,000} 3,600,000 10,800,000 79,200,000 25,200,000 0 So DOs a 5,400,000 0 1,800,000 900,000 1,800,000 0 WNicten Don gags 7,200,000 0 0 9,000,000 12,600,000) 1,800,000 ie ORES stage 57,600,000 | 19,800,000 36,000,000 39,600,000 28,800,000} 1,800,000 Fina nai lec teste 7,300,000, 7,200,000 1,800,000 12,600,000 7,200,000 |} 3,600,000 de DOT re 3 ees 1,800,000 0 900,000 17,100,000 46 ,800 ,000 900 , 000 Boke SO) et ras 18,000,000, 3,600,000 2,700,000 54,000,000 46,800,000 | 7,200,000 Septs) Ome cer 900,000 0 8,100,000 12,600,000 50,400,000 | 3,600,000 Some SIE, cp 0 0 3,600,000 16,200,000 10,800,000} 1,800,000 ice AOR RS ata 7,200,000 0 0 5,400,000 28,800,000 0 ee BON ee eee 10,800,000 0 3,600,000 12,600,000 28,800,000 0 Oct... 4a sano 5,400,000 900,000 900,000 7,200,000 9,000,000} 1,800,000 ree A ert eee 0 0 | 0 5,400,000 3,600,000 0 set SL QEN a Rr 1,800,000 (0) 0 4,500,000 5,400,000 900,000 pete Doc oe 0 0 1,800,000 10,800,000 7,200,000 0 INGWASe ei 0 0 0 1,800,000 10,800,000 0 ee Sieve tc 0 0 7 0 1,800,000 0 0 aati bes) Sar ey eae 3,600,000 0 1,800,000 0 21,600,000} 3,600,000 OPEL emeae 3,600,000 0 0 1,800,000 10,800,000 0 ie LD es Econets 0 (0) 0 0 7,200,000 0 Decy 65.255. 0 0 0 0 12,600,000 0 Se Wel Olevecs meet 0 0 0 900,000 82,800,000 0 mete 2 Ol rns 0 900,000 0 0 57,600,000 0 ca AY [ips ee ae 0 0 0 0 27,000,000 0 Average...... 61,230,769 778,846 1,505,769 9,276,923 21,450,000 519,235 on, . ee — eae ee AD AN 18} 1; 18, Sully) I—continued. ORGANISMS PER CuBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) g | % 8 | = * ~ 4 88 £8 ‘s ~ = ps Sse S s a a: 23 38 Pe, | 38 S2 > 8 S38 25 ws 5a SS gs ES gs os & x S q & 3 | eral sala Deere 239,580 146,280 | 0 0 0 10,000 SOs eae eee 49 ,003 ,100 1,200 3,000,000 | 12,000,000 0 0 oe Oe eee 3,774,901 10,620 0} 0 0 29,025 | | Rebi) 34-1522. 9,268,530 5,500 | 0 | 120,000 3,180 11,250 2 Bice a aces 7,464,880 17,200 0 60,000 0 0 Ome. 29,266,000 12 ,000 7,200,000 200 , 000 0 0 Col 0) eee eet 21,911,653 0 0} 14,400,000 0 78,975 Wie Phe eee 11,850,400 0 0 3,600,000 | 0 0 a Se Asa ats 9,080,800 0 0 0 0 0 eee lS). 28 254) 24,342,400 3,200 8,100,000; 4,500,000 3,000 130,000 Co 0 a eee 42,589,120 5,920 16,200,000 0 0 161,600 Seen ZO, eo cel 18,693 ,300 17 ,000 10,800,000 0 0 72,500 | Aare OTe k}.5) axes | 8,760,120 42,320 900,000 | 60,000 0 15,600 sede Diego sretas 36,990,300 170,500) 22,500,000 | 0 19,920 40,000 ml a2 794,044,320 24,059,000! 725,400,000 0 19,920 40,000 BME? One 88 rags: 3,453,778,080 891,648,000 2,880,000,000 1,800,000 374,080 200,000 WES Sia eee 2,583 ,832,560 197,683,200} 891,000,000 18,000,000 924,800 390,000 Pe NOS ysec5. 298 3,865 257,360 | 27,175,680 |2,668,000,000 | 10,800,000 | 14,469,120 255,960,000 Sy US eee 1,795,608,400 19,699,200 1,260,000,000 | 14,400,000 388,800 4,110,400 ites DATS ) 318 TABLE I—continued. ORGANISMS PER CuBIc METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) : 3 RS SBS 1898 RS aS Ss ae SS g Se ae es So aS Re SES S&e S8 8 Ss 58 = = = z YH % ais eter. 0 0 0 0 0 14,400 See Dham 1,000! 24,000,000 2,200 3,000,000 900 800 eS eee 9,090 0 49 ,536 0 3,870 42,183 Io Soacccc 2,204 0 3,180 5,400,000 300 5,400 as ee eee 3,200 120,000 6,000 3,600,000 880 1,200 SS BSS) ers aa 2,800, 14,400,000 48 ,000 0 4,000 400 SGD Bea sewate nc 0 0 120,042 0 6,318 3,159 Micire Sam Leis orci 0 7,200,000 0 90,000 400 400 “ Severance 0 3,600,000 0 1,800,000 800 400 SLO eee 8,640 8,100,000 5,200 900,000 800 2,800 ee AS eet Oe 60,800 10,800,000 800 1,800,000 0 14,000 29% eae c 30,240 2,700,000 1,000 900,000 400 8,600 IN aa Sts ONS c 1,620 900,000 1,800 4,500,000 0 1,400 ge, PLDs. castes 3,960 1,620,000 0 4,500,000 300 2,100 Spe LOR Ean cate 2,800 7,200,000 3,600 3,600,000 800 6,800 See eZOrtiesendcc 595,840 0 72,960 | 1,800,000 800 614,400 Wikia. Suse as 230,400 9,000,000 552,960 6,000,000 6,200|} 2,016,000 Se mOn fas 3,421,440 0 3,164,160| 21,600,000 1,600} 9,043,200 apa tilly ee earner 259,200 0 1,241,200 64,800,000 0 3,801,600 DANE hn 109,040) 10,800,000 126,720 9,000,000 200 86,400 cages) Gee 293,360 1,008 ,000 101,760 5,400,000 40 14,400 Pune Vos Ae. 26,028,800, 103,320,000 998 , 400 | 1,800,000 0 28,800 BU nenvavc 0} 128,560,000 488 ,320 3,600,000 800 57,600 PE a ene s 32,114,880) 232,200,000 470,400) 14,400,000 1,600 127,200 Se RD Oh ner, ve tae S320 44,100,000 72,960 2,700,000 800 20,800 uly U5 te 3,628,800 70,200,000 | 34,560 7,200,000 1,600 3,200 feos Foe tae 1,811,520} 41,040,000 86,400 2,700,000 1,600 3,600 SOs Rad ae 947,520) 115,200,000 5,600 54,000,000 1,600 1,600 EMD 6 ogee 133,920) 20,200,000 1,000 3,600,000 800 400 [are Daa coe 316,240} 50,400,000 | 12,800} 12,600,000 6,400 4,000 * Oe eke 1,484,000 27,720,000 | 800 10,800,000 2,000 800 LommtOnea ne 1,250,656 0 6,400 7,200,000 1,600 800 ORC feet ed 366,400) 50,475,000 12,800 7,200,000 3,200 2,400 ieee (Sate ete 5,028,800} 104,490,000 0 3,600,000 0 6,400 Sep iain 1,122,000} 56,250,000 0 5,400,000 0 800 Soa iene aera 1,200,000 64,800,000 7,000 16,200,000 500 1,500 a) eae 2,227,000} 33,480,000 30,000 1,800,000 500 5,000 ays) (rare ae 5,499,840) 146,520,000 94,080 9,000,000 4,800 17,600 Octane 805,800) 40,500,000 18,900 9,900,000 0 2,000 wr)» ee cen 840,000 37,800,000 ; 55,350 0 0 3,500 ey ee 436,650) 27,360,000 348,000| 12,600,000 0 8,000 So PDO te, heres 736,000 56,700,000 214,500 5,400,000 1,000 2,000 Novae eens 83,200 3,600,000 70,550} 12,600,000 0 12,500 e Be ae 98,700 10,800,000 25120) 19,800,000 0 19,000 oe MSee ee 7,000 22,680,000 57,400 21,600,000 3,000 6,000 LORD OSS me 60,000] 194,400,000 0! 23,400,000 0 4,000 ADO eee mae 2,000 13,500,000 9,500 18,000,000 0 3,500 DH Ba5denc 0 5,400,000 0 6,300,000 0 1,500 Sept eerie 0 0 0 6,300,000 0 2,600 ee) eran 0 4,500,000 0 4,500,000 0 4,400 ee Dt arava 0 0 0 2,700,000 0 1,600 Average...... 1,181,125] 34,762,365 148 ,626 8,569,038 1,612 308 ,330 DAB LE, S19 I— continued. ORGANISMS PER CUBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries init are based on filter-paper collections.) 1898 8 = 3 Bs = = s 3 a % Ss Ss Ss 3.8 § Se 1 Ss SS) oS s oS iS) ~ tt AN Se Ls x xs =335 28 £8 Bs aS Rs SS ae 8 e} ae] ~s SS ss io) a = 8 Si 8 H = 40,000 0 0. 0 0 0| 123,518,320 0 80.| 0) 0 80 0} 36,316,000 0 ) 0 0 0 0 43.464, 482 0 100 0 0 100 0} 21,691,300 60,000 80 0) 0 80 0 6/096; 160 3,600,000 400 | 0 0 400 0 19,093,280 0 0 0 0 0 0| 44:060,478 900,000 0 0| 0 0 0| 11,727,360 3,600,000 40 0 0 40 0 2'516,240 2'480;000 1,000 400 0 600 200, 221368;600 13/620;000 | 600 200 0 400 | 0| 29,817,200 4'200/000 1,000 0) 400 600 0 7,169,620 2,340,000 | 440 | 40. 100 200 0 15,052,540 4/500/000 | 300 200 100 0 0| 29,011,320 23/580,000 | 1,200 0 800 400 0} 39:856;000 82/800,000 3,200 0 0} 3,200 0| 94,337,920 240,000,000 3,200 3,200 0 0 0 | 1,081,381,200 813,600,000, 1,800,000 0 0 0 0| '222;233,400 367,200,000 3,000 800 1,600 800 0} 252,834,800 1/800/000 | 7°00 1,000 200} 6,000 O41 4212175.,320 37,800,000 | 62/200 400 0} 1,800 Qa| *31,584,920 17,100,000 | 1,200 800 0 400 0} 27,679,000 21,600,000 | 2000 200 0| 1,800 0| 49,614,800 79 200,000 1/100 800 | 0 300 0| 230;167,200 5/400;000 800 0 0 0 0) 191,626,440 | 1,800,000 800 0 0 0 400| 78,477,400 5' 400,000 | 0 0 0 0 0} 49:852,520 39 /600,000 1,200 400 800 0 0| 295,478,560 900/000 60 60 0 0 0| 121,362,600 0 40 | 40 0 0 0| 112,224,400 0} 80 80 0 ) 0| 566,013,480 5,400,000 120 | 60 0 60 0| 166,746,460 0 0 0 0 0 0} 129:617,660 6,300,000 240,200 120 0 80 0} 95:553/600 3,600,000 2,400 2,400 0 0 0} 137,009,680 7/200,000 1/060 500 500 60 0| 505995,120 16/ 200,000 120/620 500 2,500 120 0} 65;106,000 1,800,000 6.800 200 6,400 200 0| 46,830,100 42,300,000 241,000 0 1,000 500 500| 49,825,580 1/800;000 1/160 80. 1/000 80 0 15/982/080 13/500,000 160 80 0 80 0 19/122/540 27.000;000 500 500 0 0 0 6,776,060 5,400,000 9,500 2,500 500} 6,500 500| 26,343,120 16,200;000 2.000 1,000 0} 1,000 0 15,566,060 37,800,000 1,100 1/000 0} 1,000 0| 36,542,100 23/ 400,000 200 0 0) 2,000 0} 24,435,040 30/600,000 a) 500 0 20 0| 74,444,400 8,100,000 520 0 0 0 0| 57,242,080 27/000;000 0 0 0 0 0| 149'284/900 5/ 400,000 0 0 0 0 0| 116,833,160 10/800;000 200 200 0 0 0| 68,456,620 39,639,231 48,456 348 305 556 31| 102,220,941 divetis) Ue 1e, 320 I—continued. ORGANISMS PER CuBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) = = 3 os S$ S 38 £8 a8 ss 1898 ny os Qs Es oa os $ 82 SS os os. 8 = aD Siew 38 SopHw as $8 m5 QS sias os 2.8 Sy 38 SOEs a isa) S Q Q Aebs ila oo do 122,484,100 0 0 0 0 ee 36,086,000 0 0 0 0 nD Don ie ees 43,208,127 0 0 0 0 (Reb Wo eco 20,321,700 0 0 0 0 a Sic setae 5,461,600 0 0 0 0 eo WSs sionees 18,039,600 0 0 0 0 Dee UH eS 43 ,400, 000 0 0 0 0 Wiese, Wil eeianod 9,940,000 0 0 0 0 se Biss veve.ste 2,009 , 200 (6) 0 0 0) OIG 5 Se aac 22,035,600 0 0 0 0 OE ONES ae 29 ,539 ,000 0 10) 17,800 0 2 OM ris yeu 6,864,800 0 0 0 0 LANDES, ssSiete faye ae 14,809,800 0 0 0 0 Sg WhO: Srenereyers 27 ,662 ,900 0 0 , 0 0 cau Ova rarer 38,507,900 0 60,000 8,000 35,040 Se 2 Oey cake 86,614,400 0 10,800,000 1,806,400 598 , 400 IWER - “Sinigaicied 1,063 ,924,800 0 7,200,000 2,764,800 0 ad yO) oas ieione 203,922,800 0 1,800,000 16,153,600 4,432,000 et ly apenas 231,154,200 0 0 43,200 0 fe Ne DAS Swaine 120,175,000 0 1,800,000 3,600 0 SS loteeeene 29,293,200 0 0 0 0 JENS Woo gous 19,855,400 460,800 0 0 14,400 SE Sr Wate ee 43,112,400] 3,801,600 0 0 ils Sanaa 218,131,200 432,000 0 3,200 12,000 558 NED BEER tote 185,098,240 86,400 0 172,800 atts W7Sieponsees 42 ,053 ,200 72,000 0 0 0 SRA DES a arate 45,923,600 14,400 0 0 0 Sie HS ra, 294,724,520 0 0 0 (0) Gt w2 Ode io.er 120,850,000 0 0 0 0 Sieh me eA oromatarerr 107,710,800 0 0 0 0 Bc eaotetoe 496 ,927 ,200 0 0 0 0 EF PAG arectroncone 166,452,800 0 0 0 (0) Pat) DSN ctn kore 128,830,460 0 0 0 0 S23 Osa he ar 95,423,200 0 0 0 0 Dept Oa 76,982,440 0 0 0 0 daa Ota acces 49,515,000 7,500 0 0 0 St Oe een ean 63,144,000 0 0 0 0 re Ds chene far 45,854,000 218,400 0 0 0 Oyen, Ns one 48 ,193 ,000 251,000 1,800,000 0 0 coe ue aoe 15,129,540 486,000 0 0 0 Seu tlSnaeneeAs 17,367,000 | 25,000 0 0 0 So DO hye ecters 5,416,500 13,500 0 0 | 0 INOVs elites 25,325,500 2,000 0 0 a Sieveis ears 14,564,000 0 3,600,000 25,000 0 oe agi yee Orca 36,011,000 (0) 0 0 0 SAE DOE 23,494,000 0 0 0 Se DOV ene 73,719,000 0 0 38,500 0 Wes 16). sve: 56,400,500 0 0 | 0 0 ee id an ee eae 148,740,000 0 1,800,000 0 0 Se BD OST ae 116,344,800 0 0 247,200 6,000 vise UsMy enn 67,965,800 0 0 69,600 0 Average...... 95,852,602 112,896 | 555,000 407,602 101,358 divergens Dinobryon sertularia var. oo O0000 COCO coo 8,000 1,555/200 2,104,100 39,648,000 1,584,000 18,000 56,000 16,000 0 47,040 e000 SCOCOCOO 82000 CO0O0 C0000 COCO 866,083 | . ; SA TABLE I—continued. ORGANISMS PER CuBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 3 3 8 =: S is} 7) ee) th 1898 as 8 Ss 3 Sy aS SES Suk 5 & 38 ae ASS 8s S83 3s &§ ¥§ 28 EIS g a g g g iy ~ Miartivg Wlice 1s 5.72 0 0 0 0 0 0. oS Tae ee 0 0 0 0 100 0 ene er ante * 0 0 0 0 0 (6) OGIO. eG eenne 0 0 0 0 0 0 A Site aaers 0 0 0 0 0 0 So eerie’ 0 0 0 0 0 (6) eID ED Is toe 0 0 0 0 0 0 Mar i eeeeeO nee 0 0 0 0 0 0 ie Oiercracaunts 0 0 0 0 (0) 0 MISS a eeee 0 3,600 0 40,000 0 40,000 SOOO kh 0 800 0 0 0 0 pene) ene 2. 0 2,600 0 0 (0) 0 Apr AIS eae 0 2,800 0 0 0 0 a ee ee 0 1,800 100 0 (0) 0 im LO eae 9,960 36,000 0 0 100 0 eed Gite pondect 1,830,400 240,000 800 0 0 0 IMaryee Sag «sac 4,883,200 240,000 0 0 3,200 0 ede Om ee 24,608 , 000 48 ,800 0 0 0 0 vam Listas Scho 28,800 32,800 0 90,000 10) 180,000 pee POA esha 0 1,000 0 0 10) (6) OT liege Re 0 400 0 0 0 0 ALIMVEF G7iscuas sn (0) 9,600 0 0 0 0 Seam PAYS oie 5 10) 60,000 0 900,000 1,600 0 Os 2 ieee ae te (0) 30,400 0 0 2,400 10) SD Sie trses 0 4,000 0 0 0 1,800,000 spel Ske sets 0 B 400 400 0 800 (9) hoe ee ae 0 800 400 0 1,200 0 SAE LO) ere 0| & 7,600 0 0 400 3,600,000 es eA2O2 es ee 0 4,000 0 18) 2,400 3,600,000 EN eES VEE es 0 8,000 800 120,000 3,200 1,800,000 os re etcts 0 400 800 (0) 1,200 3,600,000 pa lOmin Ac 0 800 800 120,000 10) 3,600,000 er DO ts 0 3,200 1,600 0 6,400 120,000 or le eau rea 0 2,400 800 0 3,200 4,500,000 WED tH IG & 38 & 88 ss of 3 is} Mfanselite set ..: 440,500 100 0 0 0 0 0 0 El Fhe ee 32,800 100 200 0 0 100 0 0 CG ii ee 66,338 387 387 387 1,161] 20,898 774| 1,935 inoe eee 122,900 0 0 0 0 1,300 0 0 MS at, 4,880 0 0 0 0 3,200 0 0 CONES pai 34,880 800 0 800 800 0 0 80 SOO a a 141,524 632 DE SPTP 12,636 9,477 3,159 0 0 Mare dideccn i 11,200 400 0 400 400 400 0 0 Soe ae 11,720 400 0 400 1,200 0 400 40 URS IDG site 7,600 600 0 0 400 0 0 0 ES ee 4,800 400 0 0 0 0 0 0 em Ors. ada 61,400 400 0 200 0 0 0 0 iit. See 700 100 100 100 0 0 0 0 RID er: 3,520 300 200 0 20 100 0 0 os (Oa eae 7,300 400 0 0 0 400| 400 0 COS Ginn 6,720 0 0 0 0 0 0 0 MaveusSin ech «2 26,000 0 0 0 400 0 0 0 ae tO a. 49/800 0 0 0 1,600 0 0 0 a TS eee 23,800 2,400 0 0 800 0 0 0 Goya 9,320 600 0 0 200 0] 400 80 ceo (Goa 8,920 400 0 200 0 0| 400 200 umes 3 > QS os a = Ge is lS a = eal iS Jess, Meso ecie 100 0 0 0 0 0 0 100 pie aeellete, ciretees 100 0 0 40 40 0 1) 0 SUES eaea ae Leto 0) 0 0 0 0 0 1,548 lntelel © Sic einer. 0 0 0 0 0 0 0 100 a Suomen acee 0 0 0 0 10) 0 0 0 momen Ce ue ei. 800 0 0 0 0 0 0 2,400 Bm Da tars. svete ‘ 632 0 0 0 0 0 0 3,791 WEIS HLS Gigenao 480 0 0 80 80 0 0 480 ie Sit ore sie 160 0 0 0 0 0 0 840 amet LO etraiste tc: 1,000 0 0 160 160 0 0 400 ee 2,920 0 0 2,000 2,000 400 0 440 oe BLO evens) ots 420 0 0 1,800 1,800 1,200 0 20 AMDT) pObener-be «ce 20 0) 0 700 700 400 0 120 SO Sih Nene ery eae 240 0 0 140 140 60 0 200 gt ALIGN os ese 5,200 0 0 400 400 0 0 0 Saree Olas. it A 324,280 0 0) 6,400 6,400 0 0 0 Maye = Sith. ac 661,200 0 0 0 0 0) 0 41,600 set gills c.:.: 118,400 0 0 0 0 0 9,600 pep meal dbase, Bre cits 101,700 0 0 0 0 0 0 800 Pee ee Ee miosis 1,040 0 0 0 0 0 200 80 pry Gide aps. 2420 0 0 0 (6) 0 0 0 400 JficiaX “Hiomem es 0 0 0 0 0 0 0, 0 ae heen een 400 0 0 0 0 0 0 400 oe AVA hee ee 100 800 0 0 800 0 0 0 TE) YON erected 100 100 0 0 100 0 0 100 Aficio 40 0 0 0 0 0 0 40 hina WAS ee 400 (0) 0 0) 0 0 0 1,200 Sat AOS Se id 400 40 200 0 240 0 0 400 TD Ohsncterte. 800 0 400 0 400 0 0 1,200 ATER ED et 1,200 0 0 0 0 0 0 0 Ss Oregons 8,400 0 800 0 800 6) 0 0 pap BLIGH Re Piso: 5,600 (0) 800 0 800 0 0. 800 os Te 14,400 0 2,400 0 2,400 800 0 4,800 Sans Olafavtays rae 12,000 0 0 0 0 0 (0) Sept. (6 a. 0%. 22,400 0 5,600 0 5,600 0 0 800 a LE en 3,000 0 500 0 500 0 (0) 0 BR? Oscnveccercc%: 5,500 (0) 0 0 0 0 0 500 OH eee ae 32,000 0 0 0 0 0 0 1,600 Oct Ly oeeeted 500 0 0 0 0 0 0 500 es ae Sees (0) 0 (0) 0 0 0 0 500 pe EL Sierene cbc 500 0 (6) 0 0 0 (0) 540 RR OLDS racer: ts 4,500 0 0 0 0 0 0 500 INowsary Wrens ciate (0) 0 0 0 (0) 0 0 2,000 ee Bis eran a 120 0 0 (0) 0 0 0 2,000 ie LORS Sessa 1,200 0 0 0 0 0 0 1,000 We 1S 12) are One 2,400 0. 0 0 0 0 0 2 ,000 pie Deyo ate | 1,500 0 0 0 0 0 0 500 IDEs Si Geioniaae 860 0 0 100 100 0 0 0 Sot ised ste aie 10,600 0 0 600 600 0 0 0 page DO Re cane 1,800 0 0 | 40 40 0 0 0 aH he eae 100 0 0) 240 | 240 | 80 | 0 1,621 | | | Average...... 25,974 18 206 244 468 56 4 1,685 334 TABLE I—continued. ORGANISMS PER CuBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) (Syoyiey Teyleieic) (efeve) | 8 3 RS 3 Polyartha platyptera Con Ss 8 as) ST Bi 8 &§ Male eggs 1898 38 82 | = Se | Ses Ba BS Q8 os e8 8.8 § SES TS 33 => SS | SSR 2s ; Ss 8 Ss 8 Ss ss S Ss © q Free Carried Janeen ae: 0 0 0 0 0 1,000 0 a) eee 0 0 0 100 0 1,200 0 Seg WO Spee ae 0 0 0 0 0 11,997 0 Os Because 0) 0 0 0 24 3,200 0 ‘cies ares 0 0 0 0 240 2,000 0 Cy er oe 0. 0 0 0 80 1,600 0 CO) Ate tae 0) 0 0 0 0 6,318 0 Man, lc casas. 400 | 0 0 0 800 3,200 0 SO 5 Bice 0) 0 0 0} 1,200 5,200 0 SSAC ee 0 0 0 0} 6,400 22,200 0 eee 0 400 0 0} 10,800 37,600 0 1,600 LO DIGGS an 0 0 0 0 200 40,400 0 2,600 prado Aas 0 0 100 0 300 42,800 1,300 2,800 iad yee ae 0 100 100 0 0 26,700 900 1,900 2 Oe 0} 400; 400 0 0 148,200 8,800 1,600 OY as re 0 0 0 0 0 696,000 150,400 53,800 May 3...... 0 0 0 0) 0 582,400 0 19,200 GS SHO eee 0 0 0 0 0 137,600 4,800 0 SLAC eWeek 0} 300 0 0 0 195,200 12,000 2,400 CU Are ee a ) 0} 200/ 800 0 52,200 0 0 HOD i eee 0) 0} 200] 400 0 52,400 200 0 Wee Poocsac 0 0 0 0 0 304,000 1,600 0 eV oe eae 0 0 0 0 0 432,800 0 0 LO [aera 19,200} g00 0 0 0 241,600 0 0 See SI ie te 11,200} 800 0 0 0 56,800 0 a) icity BSooscee 2,000 0 0 0 0 6,400 0 0 OEE acet 1,600 0 0 0 0 21,600 0 0 See mings 7,200 0} 800 0 0 89,200 0 0 he) Oe 4,000 0} 800 0 0 86,400 0 0 INGE, Bo once 15,200 0 0 0 0 288 ,000 0 0 OS ae 4,000 0) 0 0 0 55,200 0 0 COS Etta BE 5,600 0 0 0 0 84,800 0 0 CU Oke beet se. 5,600 0 0 0 0 96,000 0 0 cl Oe 300 0 0 0 0 51,200 0 0 Sepes Osea: 0 0 0 0 0 4,000 0 0 TaD fe ee 2,500 0 0 0 0 31,000 0 0 sD Oe ae 1,500 0 0 500 0 72,500 0 0 OUD Pei aee 4,800 0 0 0 0 238,400 0 1,600 Oct eee 1,000 0 0 0 0 24,500 0 0 lal a aes 0 0 0 0 0 47,500 5,000 500 Ca ane te 0 0 0 0 0 27,000 2,000 1,500 eS eae 0 0 0 0 0 37,500 0 2,000 Won iloceore 60 0 0 0 0 500 0 0 Cy Sat ee 0 0 0 0 0 1,000 0 0 SONG ie see ae 0 0 0 0 0 2,000 0 0 cca? Ne Ge 400 0 0 0 0 6,000 0 0 SUMED OMe SS 0 0 0 0 0 1,000 0 0 Dec pore ae 0 0 0 0 0 6,020 0 160 ae 0 0 0 a) 0 42,100 100 100 BN Ns shan 0] 200 0 0 40 63,400 0 200 So) 57h Man te 0 0 0 ) 0 19,200 0 0 Average...... 1,674 | 67 | 50 37 | 388 | 86,674 | 3,598 1,768 359 TABLE I—conitinued. ORGANISMS PER CuBic METER. IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) D Polyarthra < % platyptera 8 = . SS 8 2 ws J Q Ss 1898 Winter eggs cs &q aS 8 3 Ss 3 = S38 & aS She 3.5 23 Ss RSS Sas =e 8 28 8 £8 | 8s | 88 | $8 SS s.45 Free |Carried © QO & & es ee a of & ey Jigna, als Geen 0 0 2,200 0 0 0 0 2,120 100 oe 74 ee eee 100 0 1,000 0 100 0 0 300 0 Boe 82S ie sr scsrs =a) 0 8,127 0 0 (0) 0 4,257 0 IAS SInoacod 0 0 1,700 0 0 0 72 1,000 0 of Sign at 0 0 2,800 0 0 0 0 1,200 0 Oe a Oeeoeereee 800 0 3,200 0 400 0 0 800 1,200 St ILD aires conus 0 0 6,318 0 0 0 632 0 0 Wik, ala omens 0 0 3,200 0 0 0 0 6,400 0 of Se saree 0 0 4,000 0 0 0 0 4,800 0 STS he 200 0 17/800 0 40 0 0 15/200 160 IE ay ERG 0 0 26,400 0 400 (0) 0 58,000 0 pet DOS costes 200 0 11,400 0 100 0 0 47,000 0 joie eS Seco ee 100 100 10,400 0 300 0 0 21,000 0 Bee LD sya sv ails 0 100 8,700 0 200 0 0 11,500 0 iy ie ree cece 1,600 0 104,200 0 0 0 400 368 ,000 0 pm e OR och «(ee 22,400 0 502 ,400 0 0 0 1,600 954,400 6,400 Wanye Ore slays .< 51,200 0 316,800 0 0 0 0 1,139,000 9,600 eel COS Eanes 11,200 0 72 ,000- 0| 3,200 0 0 233,600 6,400 me LN Fes) sere cepis 2,400 0 120,000 0 0 0 800 206 , 400 800 LE: cos lisa 400 0 28,800 200 0 0 200 60,480 0 eer venena a 400 0 10,600 40 200 0 600 61,600 0 Wise! Toe. eas 0 0 96,000 | 1,600 0 0} 3,200 48 ,000 0 hes | Ce eee 800 0 119,200 0 0 0 800 19 ,200 800 See i eee 800 0 154,400 0 0 0 | 112,000 795,200 3,200 2B ceva ie 800 0 16,000 100 0 0 22 ,400 0 icihy > -Syorap ons 400 0 7,200 0 40 0 800 22,800 400 | Seo Sinise sts 183 400 0 13,600 0 400 0 400 9,600 800 . vee a2 eee 800 0 24,000 0 0 0; 20,800 64,800 0 Oo Oana 0 0 53,600 0 0 0 400 8,000 0 ATI 2 eeu 31 0 0 295,200 0 800 800|} 12,000 170,400 0 me Qe iers cus 800 0 84,800 0 400 0 4,800 52,000 800 - ES MO var) -3::5; 0 0 108 ,000 0 800 0 1,600 18,400 60 Ce Ss ees 0 0 63,200 0 800 0 3,200 24,800 0 . GeO nea ercees 800 0) 47,200 0} 1,600 800 800 1,600 0 . Sept. 66.00: 0 0 8,800 0 0} 800 0 a) 0 pe tl Siewets. ces 1,000 0 20,000 0 0 0 1,000 14,000 500 een ED. Olivas sa sc3 1,000 0 | 103,000 | 0 0 0 4,500 27,000 500 C1 i ai 1/600 1,600 86,400 0 0 0| 30,400 265,600 100 Oecta aie aie a: 1,000 0 15,000 0} 1,000 0 500 5,000 0 Co a een 0 0 5,500 0 0 0 0 27,000 0 Coa (Saas 0 0 14,000 0 0 0 500 77,000 0 Be 2 Deere. ok. 0 0 17 ,000 0 0 0 0 824,500 0 I fopies, saleiaespeats 500 0 4,500 0 0 0 0 110,500 0 oe Bia tenes 1,000 0 2,000 0 0 0 0 97,000 60 Maa MO cotrs Paci 1,000 0 2,000 0 0 0 0 110,000 0 oR ier erent 0 0 6,000 0 0 0 0 38,000 0 Gy 1 et ae 0 0 3,000 0 0 0 500 39,000 0 WD eCiy BGisne ice 0 0 9,160 0 0 0 20 42,500 0 SOIR 2 terete 0 0 29300 0 0 0) 2,500 55.720 0 See 2 Olemers tie 0 0 52,000 0 0 0 0 59 ,200 0 ee DUN fare yeereicaske 0 0 11,000 0 0 0 400 17,640 0 IAVEKAge ne ae ne 1,994 | 34 52,560 | si7/ 207 | 46 3,950 120,391 611 TABLE I—continued. 336 ORGANISMS PER CuBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) S = 8 8 ns > ~ bp sf 3k m & 3 = 1898 m8 S38 53 s&s S 8 3 = = = 00 8.8 Os Cie a8 ora 28 [882] Ss 5a 26 HS ~ Ly S an S S ny a a a Tanee diet. see 500 0 0 0 700 0 100 ie ee aes 200 | 0 0 0 1,380 0 440 SO SD ghee: 6,579 0 0 0 4°788 0 462 Bete y eee 300 0 0 0 216 0 24 Ce eee A 400 0 0 0 320 0 0 ce sian oe 5,600 0 0 0 1,200 80 0 yy ae 0 0 0 0 3,285 0 0 Mase tite: 1,200 80 0 0 804 0 160 Geek Bh dip 800 0 0 0 3,080 40 80 Ca ee 3,960 0 0 0 12880 40 560 Gh potent at 4'000 | 40 40 0 19'440} 320 800 ODO BI 3,200 | 100 0 0 22,180 160 360 Rope Soe 1,100 300} 200 0 34,560| 200 360 eR aeseene 1/000 | 200; 100 0 28/060, 320 320 co {Oye ee 84/000 | 400 0 0 34/200, 200 400 Ca TGs gat 38/400 | 3,200 0 0 56,800| 800 1,920 Mey ni3 eo 278,400 | 9,600 0 0 204,800 0 2,800 40. bs. 91600 38/400 0 0 235.400| 400 5600 Semi ae 56/800 17,600 | 3,200 0 182/300| 1,600 8,500 CER ee 97400 600} 200 0 167,080} °400 24/080 cc O34 as ye 12,000 1,000 0 0 162/800} 440 51,480 Nate’. t7eice oe 11,200 200 0 0 438,800 | 1,600 136,000 Oe Ae at 131600 0 0 0 211,400| 400 29/200 oy eres 257600 800 0 100 83/100} 200 2/300 ON ae 51,200 800 | 0 500 45/600, 400 107100 Talges Soe oes 47,200 | 400 0 | 1,600 4,920} 440 640 CTR 1 aan 16/800 1,600, 400 1/200 1/620 60 360 10 ys 34/800 4°000 0 9'200 14/040 0 1,240 UDCA ee 4800 28000 | 1,600 99600 23/000 0 97960 Aups 242.0. 178,400 18,400 | 1,600 30,400 22,160 40 10,520 CS ra ara 20/000 4/400 | 1,200 4000 47120 0 1/080 Co Cee 10/460 | 3,200 0 22/400 4°500| 800 1/320 Ge I 8. iy ei 35/200 4000 | 0 17.600 17,340| 180 1/820 CO se 7/200 6,400 | 0 14/400 27/080 0 47040 Scents Gaver 0 0 0 0 9,080 0 720 ee ea Nae 9,500 1,000 0 5,000 . 24'720| 500 3,420 DOL eae 17,500 | 500 0 5,500 21/880 0 1/560 ar) ares 38/500 14,400 3,200 191200 99/300 0 1,100 Oct. Aovs.% 0 1,500} 500 2,000 33,880 0 1,320 foe dah OES 2,000 0 0 2000 | 34/060 | 0 27000 a gies ta 10,500 1,500} 500 500 25,640 0 2°120 Ce oe ee 78,000 500 | 0 0 26/020 0 1/020 Nowe ceo. 29,000 0 | 0 60 8,600 0 120 a 41/060 0. 0 0 15/080 0 0 MISE et 60/000 0 0 0 13/100 0 100 pele ig 66,000 0 0 0 13,920| 320 2,800 NOC ene 500 500| 500 0 6,180 0 80 Deer once 0 0 | 0 0 9,740 0 260 Cae eee 800 0 0 0 21,740 0 240 coh 2,600 0 0 0 2/440 | 0 400 | Ce Bae is 400 40 0 0 6800 0 280 | - Average...... 31,620 31475) 2 255 4,524 47,042} 1914 6,241 ———EIE Soh TABLE I—continued. ORGANISMS PER CuBIcC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) 4 8 5 E ee es Sey = 3 gs 1898 2 gs g8 zs 88 38 SES < 38 RS 3 S8 ss ss 2s 8 = x m ie) S) Q Q Q = Jane iden... 0 0 0 100 0 0 0 0 COED (ates 240 ) 0 200 a) a) 0 0 Ch a a 154 308 0 0 0 0 0 0 Bebi waiencet 0 24 0 0 0 0 0 0 a NS eee 0 0 0 0 0 0 0 0 COT eee 0 0 0 0 0 0 0 0 CS OO ee 0 0 0 0 0 0 0 0 Mar: da. .ce- 0 80 0 80 0 0 0 0 Cr) A rane 0 40 0 40 0 0 0 0 Ree 15 econ 0 120 0 440 0 0 0 0 Cs DN ve 0 280 0 480 0 0 0 0 Bo DYS) ae 0 20 20 240 20 0 0 0 Wor Sistas. 20 100 40 200 0 0 0 0 CIs) wee Tea 20 60 20 200 20 0 0 0 SEV G arava 200 100 0 0 0 ) 0 0 LS GY a oer 0 800 | 320 300 0 0) ) 0 Maio 3h. c20c 0 2,800 0 0 0 0) 0 0 Giles noge 0 3,600 400 600} 600 0 0 0 COT era ies 200 3,500 400 3,300} 300 0 0 0 CaF iis alee 480 5/920 2,960 7.880] 440 160 0 0 Seanad Rk. ob 40 33,920 8,720 5,040 1,000 720 40 0 NaineletiA Ska 200 62,800 55,800 600 | 3,400 11,600 0 0 Ci Sie ee 0 6,000 10,600 200 | 2/400 9,200 0 0 OO ie ee 100 1,500 400 0 100 0 0 200 LO oe 200 700 | 0 0 0 0 0 100 inciky “Sneoaes 0 200 0 40 0 0) 0 400 TPP Manes 0 180 0 0 0 om 60 120 bal NiO ea tae 0 0 | 160 0 0 40 1,040 CIED.G ts Oe ee 0 180 120 0 0 0} 8,580 1,080 IN Soa 0 40 0 0 0 0} 6,960 3,520 cam O pe 7. 0 0 0 0 0 0) 360 360 COM EG Ie hes 0 0 0 0 0 0 60 1,260 Win Dich ene 0 0 0 0 0 0} 1,020 900 HS Oise ak 0 0 40 0) 0 0| 2,520 1,440 Septh iOlns 0%: 0 0 40 0 0 0) 240 440 poe: free... 0 0 0 60 ) 0} 1,800 1,560 ee Oke oe 0 60 0. 60 0 oO. 960 480 ay Ce 0 0 100 | 0 0 0 400 500 Oct. 42 p05 5. 0 0 | 0. 40| 400 0) 880 120 Ce IRL ae 0 920 | 0. 0| 600 0 400 40 Ci eae 0 1,360 | om 80 80 0 560 0 OO NGS Oey ie 0 840 Oo. 120 60 0 0 0 Wow, lsscese 0 60 | on 60 0 0 0 0 CRG h eA. 0 0 | 0 | 0 0 0 0 0 OF TS eek 0. 100 | 0 0 0 0 0 0 Le ie ee 0 0 0 0) 0 0 ) 0 Ch GX) 5 Tenis 0 0 0 80 | 0 0 0 0 Mec. Ge .. 0 40 | 20 | 200 0 0 0 0 NTS eae 0 140 | 40 | 220 0 0 0 0 SER ON Be oP: 0 120 0 | 160 0 ) 0 0 Cy br ee ee 0 0 | 0 280 a) 0 0 0 Asam San 36 2,441 | 1,539 | 422| 181 417 479 261 eAS Blas, ORGANISMS PER CuBIc METER IN PLANKTON OF ILLINOIS RIVER IN 338 I—continued. 1898 (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) nm ~ $ 2 > gx pay TS} $ 8 is} H = uw ‘ g 5 ¢ = 2) SR | Se 1898 = 3 8 aS ae gS | £28 R28 aes BS (2 aS aS S38 83 Ss eS BS S15 83 Ss se SA Ses S5.8 a 6) o S S S o o Jane att. <6. 600 0 0 160 0 0 0 0 Ce 4s, Manne 940 40 0 160 0 0 0 40 Mi eet ges 4,326 77 0 308 0 0 0 308 Hebe ene. 192 | 0 0 48 0 0 0 0 An NS Ni secs | 320 0 0 160 0 0 0 0 ra sen ee 1,120 0 0. 80 0 0 80 0 oe ae te 3,285 0 0 0 0 0 0 0 Maras 10 facta: 644 0 0 0 0 0 0 0 SPA Lee bs 2,960 40 | 0 120 0 0 0 0 Ca Coe 12/280 40 0 400 0 0 80 120 Dip Og nS 18/320 200 0 80 0 0 0 80° ee Olan tee 21/660 100 20 60 0 0 20 0 Noor edt tees. 34,000 200 20 40 0 0 0 0 ie ipeee te A 27.420 1,120 0 0 0 0 0 40 ee tOR Saree 33,600 0 200 200 0 0 0 500 eT MDG ne 54/080 0 1,600| 2,880 0 0 0 4,160 Mayss.3h ten 202 ,000 400 0) 8,000 400 0 0 1,200 Tae a 229° 400 0 600) 5,200 0 0 600 2'200 Se ee se 172,200 800 200 600 0 0 0 3,300 Lay age 142/600 80 920 320 0 0} 1,080 11640 SON Eailis hes 110/880 0 200 0 80 0 400 640 pana Mina 301,200 0 400 0 0 0| 2,600 4,000 “rg” manatee 181/800 0 200 0 200 0 800 4’ 400 oie 80/600 0 0 0 0 0 0 400 Cs eee 35/100 0 0 0 0 0 0 200 aly ois aloe: 3,840 0 0 0 0 0 0 120 ie c/a a 1/200 0 0 0 0 0 0 180 Co eeepc 12/800 0 0 0 40 0 40 240 oa) IY ig syne 13040 0 0 0 120 0 120 300 Age Sao a oe 11,600 0 0 0 0 0 0 40 Soe ON ee 3/040 0 0 0 80 0 0 160 eG Sena 2/380 0 0 0 0 0 0 180 EPP er Coin 15/340 0 0 0 120 0 0 660 CASEY es 23/040 0 0 0 440 0 0 480 Sep sOtcce 8,360 0 40 0 80 0 0 0 ae cee Get 207800 0 0 0 120 0 0 240 GE SID cae 20/320 0 0 0 120 0 60 360 ae ea, eee 98/200 100 700 0 300 0 200 700 Oct: 4 Fak 32,560 0 120 0 320 0 200 400 Cae kee a 32/060 0 200 0 80 0 0 120 at i tees 23,520 0 280 0 0 0 40 40 Cay pete 25/000 0 60 0 60 0 120 240 Novwesil sae 8,480 0 120 0 0 0 0 60 cay Si ae 15/080 0 0 0 0 0 0 120 cea eee ee 13/000 200 0 200 0 0 0 0 ee ea 10/880 480 0 80 0 0 0 160 CoE OMe Raut 6,100 80 0 4 0 0 0 0 Dae Ons aac. 9,480 20 0 0 0 0 20 0 ei see ae! 21/500 0 0 40 0 40 0 0 Oye, ial 2/040 40 0 160 0 40 0 0 eae Ore te, th 6/520 0 0 120 0 40 0 0 |. — Average...... 40 ,609 78 113 373 49 2 124 539 (An asterisk at head of co AUB tas, ORGANISMS PER CuBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1908. 3359 filter-paper collections.) I—continued. lumn indicates that all entries in it are based on s Ro 38 = UO: = 3 g x3} 5 le ag cies tees tae eis o- = = . roe) a8 8 “8 a go gs 3 Sus q 38 Bois aS 5 eo) § 8 S Qo se Be a G no ee 120 240 40 0 0 0 0 Cody ee eae 200 500 0 0 0| 4,700 0 5 ene 770 2,709 0 0 0 7174 hte Bebo, Soca: 72 72 0 0 0 24 100 Bion. 80 80 0 0 0 80 0 Sere HiS| Pees 160 800 0 0 0 400 0 i ea 126 3,159 0 0 0 0 126 Mar) 4s%..<:. 4 640 0 0 0 0 4 Cee 800 2,000 0 0 0 400 0 RAISE Sos. 220 11/200 40 40 0 600 0 eo aaa 1,120 16/800 40 0 0 40 40 Cy ee ae 760 | 20/700 0 0 0 40 20 Ire ee 1,420 32,300 20 0 0 0 40 CO a he 1/100 25/100) 60 0 0 40 40 1) ae 57500 27200 0 0 0 100 100 he Wy ae 24'320 20/800 0 0 0 320 0 May ay oct. 19,600 182,400 | 0 0 0 | 0 0 Gee AOR ck xc 53,200 | 169600 | 0 0 0 0 200 Veh eae 27,900 166,400 0 0 900} 4,800 200 Cala ea 12/160 126,400 0 0 840 40 80 PERS, 5680 103,800 0 0 160 40 80 Thoin ey eee 23,800 270,400 0 0 0 0 200 apes 11/400 164.800 0 0 0 200 0 Cea) | aa 14/600 65,600 0 0 0 0 0 SoBe: 4/500 30,400 0 0 0 300 100 icine eee 1,320 2,400 0 0 0 0 40 Oe "1 te 780 180 0 60 0 0 180 ey dO coca 1,680 10,800 0 60 0 0 80 Us 0 ae 840 11/600 0 0 0 0 0 INeraee BOP ce ke 360 11,200 0 0 80 0 40 SEAL Si 400 | 2/400 0 0 0 0 0 Eva cieee bs cA. 600 1/600 0 0 0 60 60 8 a eae 3,360 11/200 | 0 0 60 60 180 Baas aig ca: 5,520 13/600 0 0 0 0 120 Sepia 6h-2 Ac 240 8,000 0 0 0 40 160 i kaa 4,320 16/000 60. 0 0 120 180 DON as. to: 0 19/000 0 60 0 120 60 Deine. 0 96,000 0 200 0 300 400 Get Ais 2,600 29 ,000 0 0 0 0 80 SORIA Ri, 3/920 277500 0 80 0 40 40 2S tae 5/580 17,500 40 40 0 40 40 2 naa 2/400 22/000) 120 0 0 120 60 Mower dick ae. 300 8,000 0 0 0 0 180 (Eph aes 960 14/000 0 0 0 120 120 ie Kean 400 12/000} 100 0 0 100 100 ROT ye: 160 10/000 0 0 0| 2,000 320 oD ON ees” | 4 6/000 12 0 0 0 120 Des nee 440 9,000 | 0 0 0 500 0 OE Ea eae 1,060 30,300 60 0 0 0 0 iE Ohise. el: 1/800 0 0 20 0 0 0 ED TAA nd 920 5,400 0 0 0 20 0 Average...... 4,780 36,707 11 10 39 318 76 larva oooo O00 (23) DLA B LE 340 I—concluded. ORGANISMS PER CuBIC METER IN PLANKTON OF ILLINOIS RIVER IN 1898. (An asterisk at head of column indicates that all entries in it are based on filter-paper collections.) n co 3 ms 8 n fo) ~ 3% & my & A fs) Se ss) a a. & 2 1898 g | §3 5 = 8 at z ='s See rAd — i a —=:0 =f as |>$s (282 GE REG ge as 1) in ee) oS royste= oN eye a Q = sal = [= i= sjarice il gon.) caf: 0 0 20 1,220); 544,201,080 1,042,720 545,243,800 Se Db cl apera s 2 0 80 40 7,720| 202,136,180 288,340 202,424,520 So) DO ie emade ere | 0 0 154 7,738 | 180,295,247 195,484 180,490,731 | Into. Se aheon 0 0 24 648 | 619,030,830 1,381,960 620,412,790 ie Sites oeerecs 0 0 80 2,160} 297,341,760 651,200 279,992,960 te US eae sere 07 0 | 0 6,000} 653,122,000 | 1,091,920 654,213,920 fis 2 Dick nagrrere 0! 0 0 3,200), LIS eS 14812 715,697 116,230,509 Maire ol rays a2: 0 800 160 1,124 21,790,800 1,809,688 23,600,488 zs ciBeccroine 0 40 | 40 3,360 96,590,840 542 ,680 97,133,520 pom LO) hagsgh ors 0| 40 40 3,280} 118,382,400 453,100 118,835,500 BP DD eyes ee. | om 1,640 0 1,600 | 127,930,360 484,760 128,451,120 Soe 2 OL eer merece! 0) 80 0 2,020 53,463 ,040 444 ,900 53,907 ,940 ire tS eee 0 900 100 | 1,860} 42,470,860 | 363,980| 42,834,834 at As it ae 0 220 20 980 90,934,320 | 1,432,400 102 ,366,720 vipa ba US) aaren a an 0) 400 0) 3,100 | 927,956,220 2,134,800} 930,091,020 bs S2O ns cee: (om 320 | 0 | 20,160 |3,872 ,537,280 16,092 ,840 | 3,848 ,630,120 WERGS SSis enyaoo | 0 0 | 0 10,800 3,200,166,960} 898,919,800 | 4,099 ,086,760 be BO wc ancn see 0 0 | 200 13,600 4,467,165,760) 42,826,200 | 4,509,991 ,960 pe. WUT Ae oncgews Ad) | 100 | 0 | 213,900 2,148 ,960,400 31,091,900 | 2,180,052 ,300 ie ee Aetna 0) 1,200 | 0 5,360} 190,671,160 4,969,580 195,640,740 caer i WP leone 120 100 | 6) 4,720) 252,704,250 2,772,200 255,476,450 Weta Sige eo 200 | 0) 0) 24,000 |} 259,129,000 9,695,000 268,824,000 ee TAR Stes one | om 0 0 4,400 |1,149,333,480| 10,959,400 | 1,160,292 ,880 ie POA cctesPratyss oes 300 | 0 | (0) 3,300} 641,056,900 15,159,600 656,216,500 Pte 2 Or eae ais | 200 | 0} 300 10,900 | 612,686,240 7,796,700 620,482,940 [Gill eee lecoe ae c 40 | 0 120 5,800 |} 257,668,840} 495,337,320 753,000,160 ip WZ vores 120 | 60 120 5,320 | 223,936,060 4,135,160 228,071,220 eS AG msg s,s 40. 80 | 0 2,680 |1,578,635,720 1,717,240 | 1,580,352 ,960 fa 2 Olen ees = 0 60 0 1,440 | 281,604,120 907 ,240 282,511,360 USS Dee Renstc.s 0 0) 0 | 3,440 | 369,169,240 5,833,440 375,002 ,680 43 Oat ace 0 0 | 0) 840 |3,084,000,880 69,874,760 | 3,153,875 ,640 Bel aes 0 0. 0 6,580 | 583,940,376 1/240,920| 585,181,296 CRG te | 60} 0 0 8'420| 544/041,260 1'519'140| 545'560'400 Oo Olerenens ps, 160 80 0 5,320 | 797,312,600 597 ,880 797,910,480 SeEpty . Operas | (6) 0 0 4,320}; 488,146,040 60,463 ,480 548 ,609 ,520 oe SRL SR oat 60 60 0 2,420! 676,773,060 | 1,712,720 678,485,780 soe 2 One ete 180 0 (0) 5,920 | 330,837,120) 32,466,660 363,303,780 See Se Ai enyene 6) 0 (0) 12,100} 511,099,300 3,201,200 514,300,500 (0 Yel ena: See eer 40 40 0 | 2,740 | 264,225,400 | 2,035,720 266,261,120 sates yi be eee 0 0 0 1,240} 126,398,510 1,495 ,880 127,894,390 Sime Sinead: 0) 0 400. 1,140) 182,891,160 1,967,020 184,858,180 spc OAS evar Seay ces 0 0 0 3,860 | 218,768,810 2,453,060 221,221,870 Nowe | laa: 0} 120 60 4,360 | 209,418,675 1,189,380 210,608,055 = Sree | 0 0 0 15,300 | 277,953,180 1,281,220 279,234,400 ee Uy rae 0 0 100 7,500 | 407,573,600 732,300 408 ,305 ,900 po DD emgrer Fe 400 0 0 25,680 | 466,411,780 1,109,040 467,520,820 Sth 20 5 ate suet 0 80 240 1,900 | 364,032,900 799,980 364,832,880 Decs 65 4. .2 = 0 0 180 680} 529,250,270 916,780 530,167,050 PA QU Sere 0 500 60 1,560 |1,715,442.415 1,757,440 | 1,717,199,855 PE ZO: ayciern 3 0 0 80 320} 848,243,820 682,440 848 ,926,260 SOD Tames, 0 0 80 340 |} 387,414,000 548,700 387,926,700 Average...-... 37 | 135 52 9,393 723,283,871) 34,226,468 756,548,801 “—PTY . —_—- = BIBLIOGRAPHY. 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PrArE Ie Seasonal distribution of synthetic groups of planktonts, Chlorophycee, Bacillariacee, and Mastigophora, from July 1, 1895, to October 1, 1896. Note changes of scale indicated at bottom of diagram. Numbers in column at left apply only to 1895. In this plate and in II. and IV., apices exceeding the limit of the diagram are dropped down between dotted lines to show location. Circles at bottom indicate location of day of full moon. Prare le The same as above, from July 1, 1897, to April 1, 1899. Note change in scale from previous plate. Piranesi: Seasonal distribution of total Rotifera and Crustacea from July 1, 1895, to October 1, 1896. The Crustacea included, belong almost exclusively to the Ento- mostraca. Apices exceeding the limits of the diagram are dropped down between dotted lines to show location. Totals include both adult and immature stages of the Entomostraca when detached from parent, and both free and attached eggs of __ the Rotijfera. PuaTte IV. The same as above, from July 1, 1897, to April 1, 1899. PiatTeE V. Seasonal distribution of Polyarthra platyptera. Total number of individuals, not including eggs, represented by ordinants, parts of which exceeding 200,000 are represented by diagonal lines instead of solid vertical lines. Thus parts of a seasonal plot which overlap those above it on the plate are represented by the diagonally-lined ordinants. (24) 355 i ¥ o---- +e: psoydobyysop) 00'000' = ry ve eee vioydobrso 000’ 00] = AS Seen SS oy AS ale 968] 200 000'e= “ coe ueny S681 ree a ALT ia ia i a E HE ess oat i Nady h 4 Bu AL. t 7 reall : [/ mall es ryt x ae lal : aL NaH | , f - , ; ‘ \ | ‘ fl 20000 f f q o00'02 v |i p H | : rt | f Hy H 1 AR ii lial a Lf EAA AT H H A ; — ‘a iE ANE | a he 1 aly UJ 4 =F + + —} iZ t ‘ + ital = 7 E aaa i \ ; 1 0 r r It 000°000'» | ’ +} i ial r i ; 000'Ob + f + t a a | | H M WE: 1 | a RAE | | ! 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AL = 360 000'002 40) $$39X] NJ SLNWNICUE)= TERE Raat HERS sahama a IE m0 to : a | t +—| — HY ail 4 EREEE CST EEE EP oi 0 el Lett It GTA dl ee | tl ae Hf [ | CPT if | ane ele TTL { ee 4 ee a | +t FATT TT Mobde TT TTT + + + —}— — + 4 fi} ——) -4 44+ r el iu | iia ISIE Je 2 1) iG. 76s) 2 eu s Diay 0} TAY PVH|OY JO waIyNgIyS1f7 ToUOSoac 000'0S 000 091 00006 1000 OS! 000 0° 000 0S) 000 0¢ 0000S! 000'0S 0000'S! ERRATA AND ADDENDA. Page 58, line 7, for ovalzs read ovata. Page 85, line 8, for longicaudus read longicauda, and just above Phacus pleuro- nectes read the following paragraph :— Phacus longicauda var. torta, n. var.—This variety, for which I propose the name torta because of the twisted body, is figured by Stein (’78, Taf. 20, Fig. 3). It occurred sparingly in midsummer from July to September, rarely in October, in 1896 and 1897. Page 91, line 18, after T. caudata Ehrb. read T. lagenella Stein. Pages}153, line 3 from bottom, 168, line 16, and 178, line 14, for ’98 read ’98a. 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