HARVARD UNIVE RSIhy, LIBRARY OF THE MUSEUM OF COMPARATIVE ZOOLOGY. WAN. GIFT OF ALEXANDER AGASSIZ, Wasi, Qa, Vo. REPORT .FOR 1901 ON THE LANCASHIRE SEA-FISHERIES LABORATORY AT UNIVERSITY COLLEGE, LIVERPOOL. AND THE SHA-FISH HATCHERY AT PIEL. DRAWN UP BY Professor W. A. Herpuan, D.Sc., F.R.S., Hon. Director. of the Scientific Worl Assisted by Mr. Axprew Scorr and Mr. James JoHnstTone. WITH TWELVE PLATES. LIVERPOOL : PRINTED BY C. TINLING AND Co., 53, Victoria STREET. Vi901. Report on the Investiaations carried on during 1901 in connection with the lLancasaire Sra - FisHeriss Laporatory, at University College, Liverpool, and the Sea-Fisa Harcuery at Piel, near Barrow. Drawn up by Professor W. A. Herpman, F.R.S., Honorary Director of the Scientific Work ; assisted by Mr. ANDREW Scott, Resident Fisheries Assistant at Piel; Mr. Jamrs JOHNSTONE, B.Sc., Fisheries Assistant at Liverpool; and Mr. Frank J. Cone, of University College, Liverpool. With Eleven Plates. CONTENTS. 1. Introduction and General Account of the Work - - 1 2. Sea-Fish Hatching at Piel - - = - - - 14 8. Note on the Physical and Chemical characters of our Sea Waters - - = - - - - = - - 20 4. Memoir on the Common Plaice - - = - Appendix INTRODUCTION AND GENERAL ACCOUNT OF THE Work. Tue work of the past year has been chiefly :— (1) The hatching operations and other similar work car- ried out at Piel by Mr. Andrew Scott ; (2) Laboratory investigations by Mr. Johnstone at Liver- pool, chiefly this year upon the Plaice ; (3) Some investigations upon the chemical and physical characters of the sea-water of our district ; (4) The work of the Circulating Fisheries Exhibition ; and (5) The Practical Laboratory Classes for Fishermen. Some of these matters which can be treated shortly I shall remark upon here, the others will be discussed more fully in the separate sections that follow. We have aimed at having in each of these Annual Reports, in addition to a short account of the work of the 2 year, some more detailed contribution of permanent value to Fisheries Science. Thus, once we had the work on the Chemistry and Pathology of Oysters and other shell-fish, and their connection with disease in man; once we had the account of oyster culture on the West Coast of France ; Mr. Johnstone in one report dealt with the reproduction of the common mussel, and in another he gave us a very full account of the cockle ; and last year the report contained a detailed description of certain very important fish parasites. This year we have what I suppose is the most complete account of a single fish that has yet been produced. It is a memoir on the common Plaice (Plewronectes platessa), by Mr. F. J. Cole, of University College, and Mr. James John- stone. Mr. Cole and Mr. Johnstone have had this work in hand for the last two years, and the pages and plates that make up the greater part of this report represent an enor- mous amount of labour both in the laboratory and the study. The Plaice is one of our most important British fishes; it is | one of those local and sedentary forms in regard to which our apprehensions may well be excited in view of the marked increase of fishing power in recent years. It was one of the fish to which the attention of the Parliamen- tary Committees of 1893 and 1900 was. specially given, and in which all the Countries of Northern Europe are at present interested because of the scheme for an International Investigation of the North Sea. Under these circumstances any contribution to our knowledge of the Plaice must be especially welcome, and all knowledge it must be remem- bered is of value, and helps us to understand the nature and life of the fish in its manifold relations. For those readers, however, who do not feel interested in the details of structure, I may add that the Introduction, the discussion of the nature and origin of the asymmetry, and the two 3 sections of the Economic Appendix, will be found the most readable and instructive parts. It is only fair to state that the eleven beautiful plates that illustrate the Structure and Life-history of the Plaice have been presented to us, as the cost of their production has been defrayed by funds from an outside source. Mr. Scott’s account of the Sea-fish Hatching work at Piel will be found on p.14. During the past year the work has been done upon the Flounder, and over 13 millions of young have been hatched and distributed in suitable waters. Next year we hope to deal largely with the Plaice, and a supply of spawners, obtained by our steamer through the courtesy of the Fishery Board for Scotland from Luce Bay, has already been laid in. I desire to emphasise what I have pointed out before, that sea-fish hatcheries ought not to be regarded as merely for the purpose of hatching young fish and then setting them free in the sea. The Hatching and Kearing of fish is the end to have in view, and scientific men who have charge of Fish Hatcheries will not be content till they have succeeded in rearing into young fish, at a reasonable cost, a sufficiently large proportion of the fry which they can now hatch from the eggs by the million. Professor G. O. Sars first showed how the eggs of an edible fish (the Cod) could be hatched in small numbers as a laboratory experiment. Capt. Dannevig in Norway and the U.S. Fish Commission in America have devised the apparatus and technique by which it has become possible, with very slight mortality, to hatch out such eggs on an industrial scale by hundreds of millions. The next advance must be inrearing. It may be very useful to turn out large numbers of fry, but it is not sufficient as an ultimate aim; what we want to do ultimately is to hatch and rear fish. We must experiment 4 in the direction of how best to keep young fish alive at a moderate cost, till they attain a fair size. At present practical difficulties in feeding, and possibly in connection with the movements of the water, block the way, but Mr. H. Dannevig at the Hatchery of the Fishery Board for Scotland has had some success with the Plaice, and MM. Fabre-Domergue and Biétrix at Concarneau with the Sole, and we can scarcely doubt that further investi- gation and experience will show us the best methods to pursue. It is at institutions like ours, where scientific work is combined with the hatching, that experiments in feeding and aeration can be carried out which will eventually lead us to the successful rearing of the young fish that we now hatch and distribute as fry. The rearing of young Soles at Concarneau from the egg to a size of 35 mm., with a loss of only 50 per cent., is a striking and encouraging fact. The laboratory at Piel has been occupied by several scientific workers during the year. In addition to Mr. Seott, who has worked there throughout the year, Mr. Johnstone has paid several visits, and Mr. Cole, from University College, Liverpool, spent the greater part of September at Piel working at the anatomy of the Plaice. Dr. H. Lyster Jameson, from the Municipal Technical College at Derby, worked in the laboratory for short periods during the summer, carrying out some investigations con- nected with the formation of pearls in marine shell-fish. Mr. H. C. Chadwick, Curator of the Port Erin Biological Station, spent a couple of weeks in March studying the methods of sea-fish hatching. Amongst the numerous visitors who came to see the ex- hibition and the work going on in the establishment during the year were the following :— 5 Mr. Walter E. Archer, Chief Inspector of Fisheries to the Board of Trade. Rev. R. B. Billinge, Furness Rural District Technical Instruction Committee. Rev. Dr. Hayman, Aldingham near Baicliff. Rey. T. Fowler, Flookburgh. Mr. F. J. Ramsden, Furness Railway Company. Mr. A. Aslett, - Mr. F. Stileman, " The Mayor and Members of a Taputstion from the Barrow Free Public Library Committee. Col. Turner and Members of the Stockport Corporation. Mr. Fell (twice). Mr. Ascroft (twice). Mr. Houldsworth. Professor Herdman (twice). ” The Barrow Teachers, John Graham, B.Sc., P. H. Smith, B.Se., and J. E. Turner, B.A., who worked in the laboratory last year, were unable, through pressure of other duties, to follow up their studies in the laboratory, but hope to be able to resume them during the coming year. The travelling Fisheries Exhibition, which was fitted up in 1897, has during the last four years been on view in public institutions in the following Lancashire towns :— Liverpool, Salford (1898), Preston (1898-99), Bolton (1899), St. Helens (1900), Piel (1900-01), University College, Liverpool (1899-1900), and finally Barrow (1901) where it is at present. This exhibition was transferred from St. Helens to Piel in November, 1900. Various repairs to the jars and speci- mens had to be made, and all the labels attached to the former had to be renewed, owing to leakage of spirit from 6 some of the jars which had been damaged in transit. The exhibit was thrown open to the inspection of the public early in 1901, and, with the exception of the periods when the classes for fishermen were being held, remained on view until the beginning of September. During the time it was at Piel the Barrow Town Council completed negotiations for its removal to their town. The exhibit was removed to Barrow early in September, and set up in the reference room of the Free Public Library, where it will remain for the usual six months. The exhibit during its stay at Piel was visited by between 3,000 and 4,000 people, amongst whom were parties of school children with their teachers from the Barrow Schools. Everyone appeared much interested in the various contents of the cases, and the majority went away with more correct ideas and impressions of the work of the Sea Fisheries Committee. The exhibif was of great service during the time the classes for fishermen were held. The specimens of the fishes, the food of fishes, and the prepara: tions of shellfish, were removed from the cases and arranged on the shelves of the laboratory, and referred to from time to time to illustrate many points which were discussed during the teaching of the men. During the summer I gave evidence before the Royal Commission on Sewage Disposal, as to the effects of sewage and other materials in effluents upon fish and shellfish; and Mr. Scott has been able from time to time to make certain experiments for me in the tanks at Piel upon this question, which is of great practical importance in con- nection with some of our estuarine and shore fisheries. In regard to the Practical Classes for Fishermen which had been started in Liverpool during the previous year, three Courses of Instruction, the Third, Fourth and Fifth, ¥ i have been successfully carried on this year at Piel, and Messrs. Scott and Johnstone, who were mainly concerned in the work report to me as follows : “The Technical Instruction Committee of the County Council having given a further grant of money to enable Fishermen to obtain instruction in the life histories, &c., of the Economie Marine Animals, from our Scientific Department, arrangements were made for carrying on the Practical Classes at Piel, where a supply of living animals _ could be obtained easily, and where, during spring, the hatching of the eggs of Sea Fish could be shown. “Three Classes, each attended by ten men, were held during the year, two in the Fish Hatching Season, and one in the Summer. A Fourth Class, to follow immediately after the third, was also proposed, but this was abandoned owing to the difficulty the men had in coming at that time of the year. The following are the dates when the Classes were held, and the names of the men who attended them :— ‘March 18th to 29th—John Wright, Southport: William Rimmer, Southport; Robert Wilson, Morecambe; David Willacy, Morecambe; Richard Woodhouse, Morecambe; Daniel Hadwin, Flookbureh; Peter Butler, Flookburgh ; Samuel Bayliff, Baycliff: John Shaw, Bayeliff; Robert Porter, Baycliff. “April 15th to 27th—William J. Robinson, Formby ; Edward Rigby, Southport; Thomas Wright, Southport ; J. Johnson, Banks; J. Wearing, Banks; Samuel Colley, Sen., Fleetwood; Thomas Leadbetter, Fleetwood: Daniel Bell, Morecambe; James Dobson, Morecambe: James Swain, Morecambe. “July ist to July 12th—Thomas Hardman, Lytham ; John Robert Wignall, Lytham; Richard Wright, Fleetwood: 8 William Fairclough, Fleetwood ; Robert Butler, Flookburgh; John Inman, Flookburgh ; Edward Robinson, Flookburgh; - John Hadwin, Bardsea; John Hartley, Bardsea; Thomas Sumpton, Bardsea. ‘“‘The classes were carried on by Mr. Johnstone, and the work was on the same lines as in the laboratory classes _ held at Liverpool (University College) in 1900*, each man, as before, examining everything for himself. As much of the material dealt with was supplied alive, the interest. of the men was greatly increased by being able to watch the movements of the animals, in many cases through the microscope. A further important point in having the classes at Piel was the advantage of being able to practically demonstrate to the men how to save and fertilize the ripe eggs of fish, caught in the trawl, and to trace the develop- ment of the embryo from the moment the egg was fertilised until the young fish hatched out as a free-swimming larva. The study of the developing fish formed part of each day’s work in the case of the men who attended the first two classes. This was not possible at the third class owing to the fish spawning season being over by the end of May. ‘Two lantern demonstrations were given to each class —one at the end of each week—and these were practically a review of the work done on previous days. A special lantern demonstration was given by Professor Herdman when he visited the first class, and this was open to the local residents, and was largely taken advantage of. ‘At the conclusion of each course the senior member of the class, spontaneously, on behalf of himself and the other members of his class, expressed their indebtedness to the Technical Instruction Committee of the County Council, andthe Sea Fisheries Committee. Some of the * Vide Fish. Lab. Report for 1900. i) men have also written long after returning to their homes, expressing the pleasure they had had in the fortnight’s work at Piel and their thanks for the practical instruction they had been given and for the plain way in which things were put before them. ‘There can be no doubt that these classes are of practical value to the fishermen. All those who have been with us freely admitted that many of the views they held regarding the spawning, development, and rate of growth of fish and other economic marine animals were erroneous. We are convinced that it is only by allowing each man to study the animals for himself, make dissections and examine material with the microscope that lasting good can be obtained. A course of instruction such as is given to the fishermen would be of great help to the Bailiffs when collecting specimens and tow-nettings for the Laboratories. “ Various mémbers of the Committee visited the classes from time to time to see the progress of the work, including Mr. Fell, Mr. Ascroft, Mr. Houldsworth, Mr. Dawson and Professor Herdman, all of whom addressed the men on the objects and work of the classes.” We had hoped this year to have hada report from Mr. R. L. Ascroft upon the tow-net gatherings taken throughout the district. The scheme for the periodic collection of gatherings from the surface of the sea at certain fixed places was started in 1900, and during that year about 150 samples of material were examined by Mr. Ascroft and the results have been tabulated. During 1901 the work has been carried on, and about the same number of gatherings have been made, chiefly by Captain Wignall from the steamer “ John Fell,’’ by Mr. Eccles in Liverpool Bay, and by Mr. Wright in Barrow Channel and Morecambe Bay. Many of these have been examined, but Mr. Ascroft’s recent 10 illness has unfortunately stopped—we hope only for a time —this and other good work in connection with fisheries investigations in which he was engaged. The work that has been done of recent years by Scandi- navian Hydrographers, and the attention that has been directed to the matter by the International Conferences at Stockholm and at Christiania, and the appointment by the Board of Trade of an Ichthyological Research Committee, all emphasize still further the point that I dealt with in some detail in my last report, viz.:—the need for more exact and detailed knowledge of our coastal waters and their inhabitants. Such knowledge, both scientific and statistical, can only be obtained by some such scheme as I outlined last year, and by the use of a special steamer to supple- ment the information that can be derived from commercial trawlers. I have thought it important in the meantime to have some samples of water from different parts of our district examined as to their physical and chemical characters by the most recent hydrographical methods and by a competent chemist (1) with the object of noting what variations exist in the Irish sea, and still more (2) with a view of testing the methods as to their relative importance and practicability for future schemes of work at sea. Mr. Alfred Holt, Junr., B.A., (Cantab.), has kindly under- taken this work, and during the last three months has been examining samples of sea-water in my laboratory. I am glad to have from him the report which is printed at p. 20. At the Eleventh Annual Meeting of Representatives of Fisheries Authorities at the Board of Trade in June, 1901, the President, the Right Honourable Gerald Balfour, M.P., made some interesting observations bearing upon fisheries research which must carry weight, and, it is to be hoped, will receive the attention which they deserve. Speaking 11 of the lamented death of Sir Courtenay Boyle, he referred to communications between them on the subject of the best means of “improving and following up that scientific research into the life and habits of fish, the practical importance of which is coming to be more and more recognised.”’ Then, in referring to the Report of the Select Committee on the last Immature Fish Bill, he said, ‘‘ It came to the conclusion that it would not be expedient to press forward the Bill in the absence of further information. It fully recognised the danger we were running of having our seas depleted of fish. It further recognised that one cause of such depletion was the capture and destruction of small, immature fish.” And then he went on to point out the recommendations of that Committee, which were briefly (1) the international regulation of the North Sea, and (2) the effective pursuit of scientific investigation, and said ‘In the face of the conclusions arrived at by the Select Committee and of those recommendations, I think it is absolutely essential now that we should proceed upon the lines indicated by them before attempting any further legislation.”” He then referred to the action of the Govern- ment in international negotiations, with the view of arriv- ing at some agreement as to protected areas, which has not yet resulted in any definite conclusions; and proceeded as follows :— ‘Next with regard to scientific investigation of the life and habits of fishes. The Board of Trade are at the present time arranging for a Departmental Committee, of which Sir Herbert Maxwell has undertaken to be Chairman, with the following reference which I will read out. ‘To inquire and report as to the best means by which the State or Local Authorities can assist scientific research, as appled to problems affecting the fisheries of Great 12 Britain and Ireland, and in particular whether the object in view will be best attained by the creation of one central body or department acting for England, Scotland and Ireland, or by means of separate departments or agencies in each of the three countries.’ You will see, gentlemen, that that is a very wide reference, and I trust that the deliberations of this Committee, which is now in course of appointment, will end in some fruitful result.” So far the President of the Board of Trade, and I think all who are interested in the advancement of knowledge and the promotion of fisheries research will agree as to the im- portance of the announcement: but as I have had the honour of being appointed a member of Sir Herbert Max- well’s Committee on Ichthyological Research, it would be highly improper for me to make any comments upon the work that will be necessary in order to carry out Mr. Balfour's suggestions, or upon the results that are likely to follow. But quite apart from the Board of Trade Com- mittee, it is important that I should urge upon Lancashire my conviction that our local waters of the Irish Sea ought to be investigated under the auspices of our local Committee. Whether or not a great national or international scheme of investigation be entered upon, it is most desirable that Lancashire, which has obtained credit for an advanced and enlightened policy in the past, should recognise its obliga- tions—moral if not legal—and should carry out an adequate programme of work at sea on similar lines to that of the Scottish Fishery Board to the North of us, and to that of the Inish Board on our West. ‘The Fisheries Branch of the Irish Department of Agriculture has now an organised scientific department, with a well known marine biologist, Mr. Ernest Holt, as scientific adviser, and an efficient steamer, ‘‘ The Helga,” measuring 150 feet in length, which a 13 is now working from Dublin as a centre in the Western part of our own area. If this could be supplemented by a Lancashire steamer devoted wholly to statistical and scien- tifie work, the two working on a common programme, there would be a fair prospect that this the most definitely cir- cumscribed of the Rritish seas * would be adequately in- vestigated. It is only now a question of expense. Sufficient preliminary investigations have been made, we know exactly what we want to do, and the Irish steamer is now at work. All that is required is an additional steamer for scientific work in the Lancashire District and funds to carry out the scientific programme. In previous reports I have shown the suitability of the Irish Sea for such work, and I am interested to see that Dr. Johan Hjort, in a recent publicationt expresses a somewhat similar opinion in regard to some of the local sea-areas on the Norwegian Coast as compared with the North Sea. He says :—‘‘ We consider that the conditions affecting those small localities on our Coast are exceptionally synoptic, and far easier to orasp than those of the exceptionally complicated and vast territory of the North Sea, in which the Plaice lives.” Finally, | should, perhaps, explain here (1) that my approaching departure for Ceylon, to carry out an investigation on the Pear] Oyster fishery for the Govern- ment, has necessitated the issue of the present report a few weeks earlier than usual, and (2) that although all the manuscript and the first proofs have passed through my hands, I have had to leave Mr. Johnstone to read the pages for the press. W. A. Herpman. University CoLLeGe, LIvERPOOL. December, 1901. * The Irish Sea contains about 10,000 square miles, and is about one- twentieth part of the size of the North Sea. +Report on Norwegian Fishery and Marine Investigations, Vol. 1., p. 152; Kristiania, 1900. 14 THe Fish Hatcuery at PIE. By Anprew Scort. In the operations carried on in the hatching season of 1901 only the eggs of the white fluke or flounder (Pleuronectes flesus) were dealt with. The flounder is a fish which is fairly plentiful in the in- shore waters of our coast, and with the Plaice forms the greater bulk of the fishes taken in the stake nets, especially in the northern part of the Lancashire area. It is therefore of considerable money value to the stake net fishermen, as well as to the small sailing trawlers that fish in the channels of the various estuaries in the district. Although not held in high esteem as an article of food by the fishermen them- selves, the white fluke finds a ready sale in the inland towns. The fishermen look upon the white fluke with a certain amount of disrespect, and have applied various uncompli- mentary names to thisfish. This is owing tothe questionable grounds which it frequents at particular times of the year. It is said to be more abundant, especially during the summer months, in areas affected by the discharge of sewage from large towns, than in clean sea water. This is true to a certain extent. but the fish does not frequent sewer outlets merely for the sake of any food that may be brought down. It is, more than anything, because of the low specific gravity of the contaminated area, due to the great quantity of fresh water which finds its way along sewers, that the flounder frequents such localities. The flounder is essentially a brackish water fish, and is known to ascend far up rivers in summer. In some parts of the country it is known as the fresh water fluke, and is not uncommon in Jakes which have an unobstructed connection with the sea. 15 On the approach of cold weather the fish migrate towards the sea. The majority of the sexually mature forms in time make their way out to sea tospawn. The movements of the immature flounders are greatly influenced by the conditions of the weather. When there is little or no frost they remain in the shallows of the estuaries. When frost of any severity sets in they quickly disappear into the deeper channels. It mild winters it is probable that even some of the sexually mature fish remain in the deeper parts of the channels and spawn there in the spring It is nota rare thing to find nearly ripe fish and occasionally partly spent ones in Barrow Channel in February, when the weather has been favourable. The food found in the stomachs of flounders varies con- siderably. Sometimes it is mollusca such as young mussels; at other times we find only crustacea, Mysis and Corophium, and occasionally marine worms. The incubation of the eggs of the Flounder has formed the principal part of our hatching work hitherto, for two reasons: (1) Mature Fish are easily collected in Barrow Channel during the latter part of the year; and (2) little difficulty is experienced in transferring them to our tanks and in keeping them in captivity. In future, however, we propose to devote more attention to the incubation of Plaice eggs, and have already secured a supply of mature fish for next (1902) season. Mature Plaice are not plentiful in the Lancashire waters, and after a week’s search, by the steamer, in the middle of November, only five were captured. Luce Bay, in the South of Scotland, was then suggested. This area was well known in former days for its large Plaice, and was fished with success by Fleetwood Sailing Trawlers, before 16 it was closed against trawling by the Fishing Board for - Scotland. Permission was courteously given by that Board for our Fisheries Steamer to trawl in the Bay. It was late in the year for such in-shore waters, when we made our visit, yet a good number of mature Plaice were secured in a single day’ trawling. The weather was favourable for the passage to Piel. After a run of nearly nine hours the fish were landed in good condition and placed in our tanks. The fact that we have to go to closed waters for our spawning fish, is a good proof that protected grounds, in which all trawling is prohibited, are undoubtedly a benefit to the fisheries. They are the means of preserving the maturing fish, and so of ensuring that at least some fish will be left to spawn. With no protection, and the improved methods for catching fish that are now employed, the adult fish, especially of sedentary species like the Plaice, would soon be as scarce as they now are in the Irish Sea between Lancashire and the Isle of Man, where they were formerly plentiful. During the hatching season of 1901 we had upwards of 250 flounders in the tanks. The ratio of sexes, as far as could be judged by size, was three females to two males. The males of flat fishes are, as a rule, smaller than the females; but there is no certain guide from external appear- ances unless the fish are ready to shed the reproductive elements. The females are then recognisable by the very swollen abdomen. Consequently there are usually a num- ber of fish amongst the stock that do not reproduce owing to immaturity. These cannot be detected when the fish are collected. The fish were collected in Barrow Channel by Mr. Wright, as in former years, and kept in tanks till the spawning season was over, when they were set free. 17 Throughout the whole period the fish were in the tanks they were fed on lug worm (Arenicola), which is plentiful in the vicinity of the hatchery. Mussels with the shells removed were tried at first, but were not eaten by the fish, and were discontinued after a few days. The first fertilised eggs were collected on February 28th. From that date onwards the numbers gradually increased, until the maximum was reached in April. After that the numbers rapidly decreased, and the spawning was over by May 10th. During the spawning period nearly fifteen and a half millions of eggs were collected and placed in the boxes for incubation. T and a half millions of fry, which were set free about the hese eggs produced over thirteen centre of Morecambe Bay. The period of incubation varied from eleven days at the beginning to six days at the end of the season—a reduction of time entirely due to the in- creasing temperature of the sea water, which is shown by the table of temperatures and specific gravities given below. It will be noticed from the table that the specific gravity during the hatching was satisfactory, and never fell below 1°026 till after the middle of May. ‘The loss of eggs during incubation from all causes averaged about 11°5 per cent., practically the same as last year. A number of attempts were made to rear flounder larve, but these experiments failed owing to the difficulty of getting minute natural food in the waters of the channel. The following tables show the numbers of eggs collected and fry set free, and also the specific gravity* and tempera- ture of the water during the spawning season. *The figures given are simply the uncorrected readings taken with the Kiel areometers. ; B Eggs Collected. Fry set free. Feb. 28 235,500 210,900 March 13 Mareh 4 —571,000 432,000 ee ee ole. OWS 230,800 7) eee 11 471,300 416,300 i. j, | 14. 392,700 349,000 April 1 18 471,000 428,000 1 20 358,000 318,600 1 25 595,500 529,900 9 28 614,000 545,000 9 20. VS88300 702,400 9 April 1 985,000 876,400 16 3 828,000 736,700 16 6 1,000,000 887,000 16 9 900,000 800,000 22 12 = 928,000 825,000 22 5, 15 1,000,000 888,000 26 18 800,000 711,800 > ae ,, 22 1,000,000 887,500 May 1 26 800,000 711,900 53°) Se », 80 840,000 747,400 10 May 3 600,000 533,600 » SEG 6 500,000 445,000 4° Ee 3 OF (OG000 444,800 ie 15,480,300 13,658,000 All the Fry were set free by Mr. Wright, the Chief Bailiff, from his boat, between Walney and Morecambe Bay Ship. 19 TABLE showing Temperature and Specific Gravity of the sea water pumped into the store tanks each day during the spawning season. Date. | Temperature ee Date. | Temperature Geavity Feb. 4 4:2°C 1-0264 Mar.29 3°6° C 1-0268 5 3°8° ,, 1-0262 30 42°" 5 1-0264 6 B07 53 1-0262 31 Di Ome, 1:0268 ff BOS oe 1:0264 April 1 Dames 1-0266 8 4-2°.,, 1-0264 2 Brae s, 1-0266 9 4° 82055 1-0264 3 SOns 1-0264 10 4°8° ,, 10264 4 DiGi: 1-0262 11 42° ,, 10264 5 Brae 35 1-0262 12 3°82, 1-0264 6 Dede iy 1-0264 13 36° ,, 10264 7 G08; 10264 14 ArOrase | 4 20264 8 62555 1-0264 15 Bi Ome. 10264 9 6°4° ,, 1-0262 16 BOR 55 1-0264 10 667.5 1-0262 17 BZ. 55 1-0264 tS a he eee 1-0262 18 Bele Igy 1-0264 IDs. FPS, 1-0262 19 SOE Op 1-0266 13 Pee oe 214 1-0260 20 ee 1:0266 14 Ge ane 1-0260 21 Aon 1-0266 15 G4e55 1-0260 22 4°4° ,, 1-0266 16 Sollee 1-0260 23 4°6° ,, 1:0264 17 702 55 1:0260 24 DOr, 1-0268 18 TROP 5 1-0260 25 54° ,, 1:0268 19 Ay 1-0260 26 i ae 1-0267 20 (Cass 1-0260 27 DiOws 1:0267 21 8:07, 1-0260 28 56° ,, 1-0267 22 She cone 1-0260 Mar. 1 prea es 1-0268 23 854555 1-0260 2 Dae a 1-0270 24 Shee 1-0260 3 Oe ss 1:0268 25 Oss 1-0260 4 Os 1:0268 26 T0255 1-0260 5 Ones 55 1-0264 27 8-4° ,, 1-0262 6 Oi Aan 1-0264 28 ei bas 1-0262 Ul Oran 5 1-0262 29 Siar oss 1:0262 8 BAS Ap 1-0262 30 S 2s 1-0262 9 OEaess 1-0262 May 1 972°. ., 1-0260 10 56° ,, 1-0262 2 ee 1-0260 11 C:O55 1-0262 3 See lop 1-0260 12 6:42 "5, 1-0262 4 Orage: 1-0260 13 66"; 1-0262 5 SSI 1-0260 14 64° ,, 1-0262 6 OBE, 1-0260 15 G:6u5, 1:0262 7 OSes 1-0260 16 D°6° 55 1-0262 8 38>, 1-0260 17 Exar Fe 10262 9 10:07 5, 1:0260 18 0) pp 1:0262 10 10°4° ,, 1-0260 19 DOr, 1-0262 11 10°6° ,, 1-0260 20 4:4° ,, 1-0262 12 10585 3 1-0260 21 4:8° ,, 1-0262 13 TEO°F, 1-0260 22 HEP 1:0264 14 a pie 1-0260 23 oD: Ons 1:0256 15 ue he aa 1-0260 24 Sat Sass 1-0266 16 ula ee 1-0260 25 aes 1-0266 17 UNS Se 1:0260 26 a-Si, 1:0268 18 TBA OP ee 1:0260 27 4:4° ,, 1-0268 19 1 1:0258 28 4-2°,, 1:0268 20 THe DETERMINATION OF SOME PHYSICAL AND CHEMICAL CHARACTERS OF SOME SAMPLES OF WATER FROM THE IrIsH SEA. By Aurrep Hout, Jun., B.A. (CAnvas.) Having been requested by Prof. Herdman to examine for him some of the physical and chemical characters of the water of the Irish Sea, with a view to selecting the most practical and effective methods for their rapid determina- tion, I obtained a number of samples from different places, and estimated, as far as time allowed, the specific gravity, colour, chlorine, total salinity, alkalinity, carbonic acid gas in solution, and the lime. The ratios between these were then determined, in order to see if they were sufficiently constant to enable one to calculate from an accurate determination of one character the values of the others. At the same time it was hoped that some knowledge of the movements of the water of the Irish Sea area might be obtained from examination of the values obtained. This will be discussed at the end of this paper. Though every eare has been taken to obtain accuracy, the fact that I had not always at my disposal a sufficiently accurate balance forced me to use in the titrations known volumes of solu- tions instead of known weights of them, which undoubtedly may cause error. Further, for the same reason, some pro- cesses had to be done volumetrically instead of gravimetric- ally, which though easier to perform do not produce quite such good results. 21 Very little hydrographical work seems to have been done in the Irish Sea, though the Clyde sea-area immediately to the north of it has received much attention ; in fact almost the only analyses of the water seem to be those done by Thorpe and Morton in 1870 (J. C. 8. xxiv. p. 506), so there is little past work to comment on. The working here described has been done mainly by the methods employed by Dittmar, Knudsen, Jacobson, &c., and the working out of many of the results has been performed with the assistance of Knudsen’s Hydrographical Tables. The values obtained are given in considerable detail, which it is hoped may be of some value as showing the degree of accuracy obtained by different methods. 1.—CoLLECTION OF THE SPECIMENS. These were obtained mainly from the Biological Stations and from our fisheries steamer, but not on any regular ex- pedition. For this reason I have not yet been able to obtain specimens from several desirable places, nor to obtain in all cases the temperature of the water at the time of collection ; further, the specimens are all of surface water, as a Mill’s water bottle which was ordered did not arrive early enough to be of use. I hope to use it in a further investigation on a future occasion. The samples consisted each of about a litre and a half of water, and were kept in glass bottles with ground glass stoppers till used in the laboratory. This method has the disadvantage of slightly increasing the alkalinity owing to the action of the water on the glass, but this increase is probably very slight since it was seldom that the water was kept more than two or three days before being used for the determination of this character. 99 9.—DETERMINATION OF THE SPECIFIC GRAVITY. The specific gravity of the water at 17°5° C. (0 17°5) was determined by means of areometers made by Stegel at Kiel, a salinometer of Hicks, and in a few cases by a hydrometer of Negretti and Zambra. Several observations were made with the Kiel areometers at different temperatures, and these values were then cor- rected for temperature and the glass of the instrument by means of Knudsen’s Hydrographical Tables. The temperature of the water at the time of the observa- tions was observed by means of a thermometer made by Stegel of Kiel, and graduated to 0°2° C. . By means of these determinations at different tempera- tures it was hoped that some of the errors due to a single observation would be avoided. The Negretti and Zambra hydrometer was a small pocket instrument, and it is remark- able how good are the values obtained by it, but at the same time it must be noted that all values obtained with it are about 0°0014 too low. This presumably is due to the scale not being accurately placed in the stem. The values of the Kiel areometers are given to five decimal places, but the last place is obtained in the correction above mentioned, and is not read on the instrument except in a very few cases. In the annexed table the values obtained with these in- struments are given, and also, for comparison, that calculated from the chlorine value. In the case of the areometer read- ings the maximum and minimum are given so as to show the error between the readings at different temperatures. The density of the water at 0° C. referred to distilled water at 4° C. was calculated from the above figures, and is given in the tables at the end of the paper. 28 Seeciric Gravity TABLE. LOCALITY. Areometers. Sp. Gr, at 17°5 according to Kiel 1. Landing Stage (High WWAEDGT!) IN cs cunescsnececieces 2. Landing Stage (Low Watery) ss.deesssiesseweess 3. New Brighton 4. Crosby Channel (1 how HOOG)) sas selon cues eect ee: . 1 Mile N. of Bar Ship | (Low Water) Cn 6. Blackpool “1 . Piel (Barrow Channel) 8. Port Erin (High Water), 9. Port Erin (Low Water) 10. Fleshwick(High Water 11. Douglas Bay (Low Wiationy Wh Sassccenerseevase 12. 15 Miles S.E. of WOUPIAS <0. scanemses nese. 13. 30 Miles S.E. of WMOWO aS ho ios seco ae ceae se 14. 45 Miles S.E. of WD OUCTAG see. Sener ees 15. Near N.W. Light Ship 16. N.W. Light Ship ....... 17. 2 Miles W. of N.W. | LUTE oT IS) oH eee onesacdecceee 18. 24 Miles N.W., + W. of Walney Light ...... | 19. 10 Miles N.W. by N. 3 N. from Point of Ayre Light Ship, 1.0.M. ... (1.02312 (| 1.02313 (1.01629 (1.01631 (1.02476 (| 1.02478 (1.02334 ( 1.02337 (1.02527 {1.02538 (1.02388 \ 1.02390 (1.02516 {1.02517 (1.02584 | 1.02585 (1.02588 {| 1.02582 (1.02594 (1.02594 | (1.02568 | 1.02572 (1.02601 (1.02603 {1.02606 (| 1.02610 (1.02587 (1.02591 (1.02441 (1.02447 | (1.02543 | 1.02544 (1.02546 ( 1.02546 (1.02549 (1.02551 | (1.02575 | | 1.02576 esse] a3 a Seles 0-00001 | 1-0232 | 0-00002 | 0163 | 0-00002 | 1-0247 0-00003 | 1:0234 | | | 000011 | 1-0254 -0-00002 | 1-0241 | 0-00001 | 1-0252 | 000001 | 10260 | 0-00006 | 1-0259 | | nil 1:0259 | | 0-00004 | 1-0257 | 0-00002 | 1-0261 0-00004 | 1-0259 | o-00004 | 1-0259 | §0-00006 10244 | eae 1-0253 | | nil. ~ 10352 | a ae 1-025 | /0-00001 | 10258 | Sp. Gr. at 17°5 according to Hydro. meter of Negretti and Zambra. 1:0218 1:0150 1:0229 | 10219 1-0239 | 1:0226 | 1:0237 1:0245 1:0245 1:0245 10242 1-0245 1:0245 1:0245 1-0230 | calculated from Chlorine Values Sp. Gr. at 175 | 102315 1:01632 1:02466 1:02333 102529 1-02387 | 1-02520 | 1-02584 | 1:02587 102593 1:02571 1-02602 1:02608 1-02589 1:02441 | 1:02546 | 102546 1:02549 | 1-02576 —————————— a a ___ el 24 3.—CoLouR oF THE WATER. This is an unimportant character, but it was found to vary so much that it seemed useful to note it. The colour was observed by looking down a column of water about 20 inches high on to a brightly illuminated white surface. ‘The words used to denote the colour are, it is hoped, not too vague. Colour. 1. Landing Stage (high water) - greenish grey. 2. Landing Stage (low water) - greenish orange. 3. New Brighton - - - greenish brown. 4. Crosby Channel (1 hour flood) - - - - pale olive green. 5. 1 mile N. of Bar ship (low water) - - - - yellowish green. 6. Blackpool = - - - - greenish grey. 7. Piel (Barrow Channel) - - greenish grey. 8. Port Erm (high water) - - bluish green. 9. Port Erin (low water) - - pale grey. 10. Fleshwick (high water) - - bluish grey. 11. Douglas Bay (low water) - greenish grey. 12. 15 miles 8.E. of Douglas —_-_ greenish blue. 13. 30 miles S.E. of Douglas - pale bluish green. 14. 45 miles 8.E. of Douglas —-_ greenish grey. 15. Near N.W. light ship - - pale olive green. 16. N.W. light ship - - - bright yellow green. 17. 2 miles W. of N.W. light ship - - : - grey green. 18. 24 miles N.W. + W. of Wal- ney light - - - 19. 10 miles N.W. by N. 3 N. from Point of Ayre light house, ereenish grey I.0.M. - . - - grey green 25 4.—SusPpENDED MatTTER. This was not estimated owing to the difficulty of filtering large volumes of water, and of weighing small amounts of ash. It was very abundant in the Mersey water at low tide, but not nearly so abundant at high tide. There was quite an appreciable amount of suspended material in all samples off the Lancashire coast, but in the open sea, and round the Isle of Man, the water is practically clear. For the determination of the specific gravity and other characters, the sediment was allowed to settle and the clear water decanted off. 5.—EstTiMATION OF THE CHLORINE. This was done with the greatest possible accuracy, both by Volhard’s method and also by titration with standard silver nitrate solution, using potassium chromate as indicator. The exact process by each method is given and also the results, for the sake of comparison. (a.)—PREPARATION OF SOLUTIONS. The silver nitrate solution was prepared by dissolving the pure fused salt in sufficient water to make about. a decinormal solution. ‘his could then be diluted to any desired extent. It was standardised by titration against the sodium chloride solution (which see), using potassium chromate as indicator, and also by Volhard’s method It was restandardised about once a week, in order to be certain as to its exact strength from time to time. The sodinm chloride solution was prepared by dis- solving an accurately determined weight of pure, dry (b. 26 sodium chloride in a known volume of distilled water. The sodium chloride was obtained from the ordinary crude salt by precipitation from a strong aqueous solution by means of hydrochloric acid. The precipi- tate was filtered, dried, and finally fused. The actual strength of the salt solution prepared was nearly 3p, and this could then be diluted as required. The ammonium sulphocyanide solution was prepared by dissolving the pure salt in water in such proportions as to make it approximately decinormal. Its actual strength was not determined as it was titrated against the silver nitrate solution only to find the ratio between their strengths. Saturated solutions of the two indicators—potassium chromate and iron alum—were employed, one drop being sufficient for each titration. )—VonHarp’s MeruHop. A sample of sea water (about 10°0 ce.) was titrated as a preliminary, in order to find about the amount of silver solution required. The accurate determinations were then done in the following way :— 10cc. of the sea water was measured from a pipette into a beaker, and mixed with a little distilled water. About 2ce. more silver solution was then added than the preliminary examination had shown to be neces- sary. The mixture was thoroughly shaken and allowed to settle, and the nearly clear supernatent liquid was filtered. The residue in the beaker was twice washed with distilled water, and the washings were mixed with the filtrate after they themselves had also been filtered. All the excess of silver was now found to be in the 27 filtrate. It is to be regretted that the filtering could not have been done through a Gooch filter, as then the precipitated silver haloid could have been collected and weighed, and so would have formed an additional check on the values obtained by titrating the filtrate. This filtrate was poured into a porcelain dish, a drop of iron alum solution added, and titrated drop by drop with the sulpho-cyanide solution till a red colour appeared. To this was then added enough silver nitrate solution to just destroy the colour, and this process of alternately titrating with the sulpho-cyanide and the silver solution was frequently repeated in a zigzag manner till a great number of determinations of the end point were obtained, the mean of which was considered accurate. (c.—Usine Porasstum CHromatE as INDICATOR. As in the previous case a sample of about 10cce. of the water was first titrated to find about how much silver solution was required. In the accurate determinations 10cce. of the sea water were mixed with some distilled water in a beaker, and then about O’dce. less silver solution than was required for complete precipitation was added. The mixture was well mixed and a drop of the chromate solution added, and then the silver solution was run in drop by drop till a permanent change of colour was obtained. The end point was obtained repeatedly by zigzag titrations with the salt and silver nitrate solutions. A mean of the values obtained was taken as correct. Considerable errors oceur in both methods, but more especially in this, owing to the difficulty of obtaining a standard of colour which represents the end of the reaction. 28 In order to get as much accuracy as possible, a sample solution coloured by chromate was used, and the end point was considered to have_been reached as soon as the tint of the solution being titrated differed permanently from this sample. CHLORINE TABLE. Bes 3 n 5 Sac. | Sa, | ogs £422 | 283 | 82. sas) S49 | wae . eees | Oba. ieee ieicycies Be 38m a0 ae She Le Pes erave i peek es ae Eee ee > 1. Landing Stage (High water).................. 16°84 16°79 16°76 2. Landing Stage (Low water)...............06 11°82 11°81 11°80 3 | IN Si/ dB Oh Ov aie sobechbsbocduneesuatinedbanooedsn 17-94 17°87 17°95 4. Crosby Channel (1 hour flood)............... 17°03 16°91 16°92 5. 1 mile N. of Bar Ship (Low water)......... 18°38 18°33 18°32 G. an Oo a in mgs. per litre. ALKALINITY 53-30 50°90 50°88 53°64 576 50°88 54°60 54°86 19°70 49.54 ALKALINITY a h Ro = | s Be aS S ic) 2 5| 2 es S as 54:8 DO7O! | execss 47°6 1804 | .-.... 56°7 ‘1813 RESAG 4°05 "T6L2 | xcs. poy el} “GSU |i earacs 49-7 "1545 | eens 49-6 LSS en ae trestas 522 "1580 | «sense 504. “T5TL | cence 49-6 “1625 0.57 j5°2 “1650 | <«+-.. 53-4 "1G26 |... 54:0 MGBB TY) sass aya hu) GTO | seeeee 52:0 “1509 | «+. 48°9 L5TT | cceeee 51°2 “1492 058 48.4 1469 O57 418°3 at 175. Ratio of CHLORINE obtained to 16: ate 18: 40 62 Bu “08 OL 20 Sp. Gr. at 17°5. Per centage of fresh water. Od nil, ial niet) APPENDIX.» Pie NIBNOLES, No. VII. PLEURONECTHS. (THE Paice.) BY F. J. Coun, Jesus College, Oxford, Lecturer in the Victoria University, Demonstrator of Zoology, University College, Liverpool ; AND J. JOHNSTONE, B.Sc., Lond., Fisheries Assistant, University College, Liverpool. CONTENTS. INTRODUCTION. PAGE PAGE A. EXTERNAL CHARACTERS ... .4 | EH. NERVOUS SYSTEM ..........0 103 Asymmetry of Plaice 8 and 184 1. Brain and Spinal Cord... 103 18)... (Sieeob ora tHON' cpesdsnsoqseode oaoeocdeL 11 2. Cranial Nerves ............ 110 Skull and visceral arches... 11 Component Theory ...... 111 Vertebral column ,........... 40 V.-VIL. Gomplex......... 121 WTedian HIS) rewe+sssoescecrn ss 49 NesProfaundus Vi...co--ss- 125 Limb girdles and paired xX Gomplex v.<.tscceseaes 138 isbeG} So sceeosoce¢ condena0 Tac noLooee 58 3. Spinal Nerves ............ 145 C. Bopy Cavity AND VISCERA 63 4. Sympathetic ............... 156 1. Coelomic Spaces ......... G30 REE OENSH ORGANS cocseseccoccmonses 163 2. Alimentary canal and 1. Lateral Sense Organs ... 163 GalGhaGlS cc’ cng aouoooDOriSeneee 63 PL -ASTCRE\ cconobapubobodencocnada: 171 3. Ductless glands............ 73 Boy HVS) Ressctcesesecescs Sseiees 174 4. Renal organs............++. 77 Discussion of Asymmetry 8 D. Broop VascunaRr SysTEM... 84 and 184 Structure of Gills ............ 87 OV ALDI 8 Ceeecooaascapnadncodcoag6ee 190 Pseudobranch.............s000 96 | G. REPRODUCTIVE ORGANS...... 193 APPENDIX—ECONOMIC. A. Lirm History aNnD HaBits.— Distribution and Spawning D | B. Puatice FISHERIES .— Regulations and Remedial Measures wercerreceeeree sfiione 219 INTRODUCTION. Tue subject of the present Memoir belongs to the old group of fishes known as the “ Teleostei,” but the dispersal of the “ Ganoid”’ fishes has necessitated a new classifica- tion, with the result that the familiar word “ Teleost ” may in the future be “ missed from its accustomed hill” in all classifications of fishes. The Plaice (Pleuronectes platessa) is the most familiar example in British seas of a group having an almost universal geographical distribution. It is the typical member of this group, which has long been known as the Pleuronectide, a family of fishes belonging to the sub-order Anacanthini. In recent times this family has itself received sub-ordinal rank, and has been termed the Heterosomata, being divided into two families (the Pleuronectide and Soleide) and six sub-families, containing a large number of genera and species. The principal diagnostic characters of the group are the torsion which the anterior region of the skull undergoes during development, and the modification and use of the left side as the under side of the body. ‘The lateral compression of the body is paralleled among other Teleostean fishes, but the [apparent] presence of both eyes on the right or left side of the body is a unique feature. The nearest relatives of the Pleuronectide among the Teleosts are the Gadide, and, curiously enough, these two families afford the major portion of the fishes used as food by man. The striking differences in general body form and habits between the Plaice and Cod (typical examples of the two groups) form a marked instance of how external differences may coincide with deep seated morphological similarity. ‘The Plaice is a fish which is-sluggish in its movements, and has a very limited range of migration. 3 It lives, too, permanently on the sea bottom, often buried in the sand, feeds almost exclusively on bivalve molluscs, and in body form departs widely from that usual in fishes. On the other hand, the Cod is an active fish, which may migrate over wide sea areas, and although it affects the bottom, it may be found in any vertical zone of the sea. Further, it is voracious and even cannibalistic, and, although it feeds mostly on Crustacea, many marine groups of animals contribute to its food, whilst it has the typical piscine form. Nevertheless, we shall show that the morphological differences between the two fishes, apart from the question of symmetry, are comparatively few and unimportant. The following parasites of the Plaice have come under our own observation :—(1) Unidentified Sporozoan cysts imbedded in the wall of the gut, and reducing it to a thin membrane; (2) unidentified Myxosporidian cysts within the cartilage of the auditory capsule, and causing a con- siderable hypertrophy of the same; (3) the Cyclops stage of Lernea, attached to the gill filaments; (4) Chondra- canthus cornutus, inside the gill cover; (5) Lepeophtherus pectoralis, on the skin under the pectoral and pelvic fins ; and (6) Bomolochus solew in the nasal chamber. There are of course others, but these we have seen. Only seven genera and fourteen species of Pleuronectide are known to inhabit British seas. The members of the genus Plewronectes are P. platessa (plaice), P. limanda (dab), P. flesus (flounder), P. cynoglossus (witch), and P. microcephalus (lemon sole). All these species are known in the Irish Sea area. The Plaice is probably the most abundant, and the order of the species in the above list gives the relative abundance of the others. All are important edible fishes, and are the objects of an active fishing industry. A.—EXTERNAL CHARACTERS. The Plaice is not a large fish compared with many edible fishes, and the largest of which we find a record was 33 inches long, 21 inches high, and weighed 15lbs.t An average large plaice, however, would have an extreme measurement of 24 inches long and 14 high. The com- pression of the body is not from above downwards, as in the skate, but from side to side, so that when the fish is lying on the sea bottom its left side is downwards and the right only is exposed. For the sake of convenience, and for obvious reasons, we shall follow Traquair in referring to the right and left sides as the “ocular” and “ eyeless ” sides respectively. The eyeless side of the Plaice is colourless and flat, whilst the ocular side is pigmented and convex. The colour varies very greatly according to the nature of the sea bottom, and may be anything from grey to dark brown. The characteristic orange red spots (ocelli) form a row of about 6 on the dorsal fin, 15 or so on the body, one on the caudal fin, and another row of about 6 on the anal fin. Specimens with the eyeless side more or less coloured, and also reversed examples, are occasionally met with. The nature of the colouration has been investigated by Pouchet,f and by Cunningham and MacMunn.* We follow the latter memoir. If a superficial section be made of the fresh unprepared skin of the ocular side, two struc- tures only are apparent. These are the colour cells or chromatophores, situated largely in the dermis, but also found in the epidermis, and the opaque somewhat iridescent reflecting bodies or iridocytes. One layer of + Thirteenth Annual Report for 1898 of Inspectors of Sea Fisheries (England and Wales), 1899, p. 10. { Jour. l’anat. phys., 1877, No. 1. * Phil. Trans., 1893, B, p. 765. 5 chromatophores and iridocytes occurs outside the scales, and there is another layer of both on the inner surface of the skin and between it and the muscles. In the skin of the eyeless side no chromatophores whatever are normally present, and also only the external layer of iridocytes is found. Internally, however, there is a ‘ thin perfectly opaque layer of material giving a dead-white reflection. Examined with the microscope, this layer presents only a minutely granular structure, and is everywhere uniform and continuous.” On account of its capacity of reflecting light in such a way as to produce a silvery appearance it is called the argenteum. It is doubtful whether the chromatophores, iridocytes or argenteum are cellular structures. It may be mentioned that Cunningham and MacMunn were able by experiment to induce a coloura- tion of the under side of the flounder. The dorsal fin commences vertically above the left eye, a short distance behind the left posterior nostril. It extends back to the root of the tail, and is highest about two-thirds of its length from the snout. The number of fin rays varies considerably,t and in six specimens selected at random ranged from 66 to 74. The anal fin commences very far forward, immediately behind the so-called “ anal spine,’ and stretches as far back as the dorsal fin. It is highest at about its anterior third. In the same six examples above the fin rays varied from 52 to 57. The caudal fin belongs to the masked heterocercal or homocercal type, and has usually about 20 fin rays. The pectoral fin is situated immediately below the posterior angle of the operculum. It has usually the same number of fin rays on both sides (about 10), but on the eyeless side one is small and may be overlooked. The pelvic fin is jugular + See especially Cunningham, Jour. M. Biol, Assoc. N. S., vol. iv., 1897. 6 in position, slightly in front of the pectoral, and imme- diately in front of the anus. As far as we have seen its fin rays never vary in number, and we have always found six on both sides. This is very striking when we consider the variability in the number of fin rays in all the other fins. The opercular fold or gill cover of the Plaice is large, and the branchial cavity opens behind by a wide aperture. This aperture is bounded on the inner side by the clavicle and on the outer side by the loose branchiostegal mem- brane supported by the branchiostegal rays. On lifting the opercular fold the gill-like pseudobranch is easily seen in a slight recess on the inner side of the operculum immediately over the dorsal extremities of the gill arches. Ventrally the opercular folds are separated by a conical fleshy mass containing the “ inter-clavicle,” and known as the isthmus. The lateral line of systematists commences on the tail, and courses straight forward at about the middle of the side of the body for the greater part of its length. It curves slightly upwards over the pectoral fin, and there- after becomes buried in the bones of the head. Further portions, however, of the lateral line system are visible on the surface, notably the right infraorbital canal under the right eye, and a portion of the supratemporal canal under the dorsal fin. The scales are mostly cycloid, but according to Cunningham (op. cz.) the so-called “ciliated” or “spinulated scale” is found only in mature males, and may form a conspicuous local peculiarity. The scales of this character that we have seen had three or four blunt processes on their posterior border. Regarding the apertures, the mouth is terminal and markedly asymmetrical. If the mouth of a plaice be 7 opened, it will be seen that the whole jaw apparatus swerves towards the eyeless side. The mechanism by which this is effected is described elsewhere. The expla- nation of this asymmetry is that the fish must seek its food on the sea bottom, and the torsion of the jaws towards the under side is hence a physiological convenience, if not a necessity. The same consideration explains why the teeth, which are blunt and flattened, and not pointed like those of the cod, are situated almost entirely on the eyeless side. Within the mouth will be seen the maxillary and mandibular breathing valves, or “‘ internal lips,” to pre- vent the regurgitation of water through the mouth on the fall of the gill cover.t The anus is situated very far forward in front of the ‘anal spine,” and is a large median opening elongated from before backwards. The urinary papilla of the female is distinctly on the ocular side, a little distance above and behind the anus. The oviducal aperture is large and lies immediately behind the anus. It is very obvious in the spawning season, but at other times of the year we have failed to find it, so that it is either occluded then, or very minute. In the imma- ture fish we believe it does not exist. In the male the papilla is in the same position, but is here a urinogenital papilla. We have failed to find any external distinction between the two sexes, but the male is smaller than the female when it first becomes mature. The pair of anterior and posterior nostrils on each side are on the ocular side situated between and in front of the two eyes, and on the eyeless side in front and to the left of the left eye. Between the two eyes and passing backwards there is a prominent ridge formed almost entirely by the right frontal, and in a line continued back from this ridge are ¢ Dahlgren, Zool. Bull., 1898. 8 seen five {uberosities or tubercles. These may vary con- siderably both in prominence and in number. Sometimes only four are present, and also any one or all may be in duplicate. As a rule, however, five occur, situated as follows:—1l and 2 on the right frontal, 3 on the right sphenotic, 4 on the right pterotic, and 5 on the zight post- temporal (see figs. 1 and 21). Tur ASYMMETRY OF THE Puatce.t The most striking feature in the external appearance of the plaice, and also the most interesting in its anatomy, is the apparent presence of both eyes on the upper, right, or ocular side. But this is not the only respect in which the head of the Plaice has undergone torsion. The jaw apparatus is also very asymmetrical, and in a different direction, for whilst the eyes are twisted towards the ocular side, the jaws incline towards the eyeless side. Now it must be obvious at the outset that the asymmetry of the jaws has been superimposed on that of the eyes, and is in fact a special adaptation to an already asym- metrical fish, living on the sea bottom, and lying on its left side. We may therefore leave this asymmetry to be described in its proper place, and confine ourselves to that of the eyes. The asymmetry of the Pleuronectide was first cor- rectly explained by Traquair in 1865. The question is beset with numerous difficulties, in the form of many secondary modifications tending to mask the true course of the original torsion. Traquair, however, in his now + We have no space to refer to the extensive literature on the asymmetry of the Pleuronectide, especially as the work of Traquair covers most of the facts. We should like, however, to mention an interesting paper by Holt on an abnormal sole (P.Z.S., 1894, p. 482). 9 classic memoir,* the facts and conclusions of which we can fully confirm, was enabled by an exhaustive examina- tion of the skull and lateral line system to map out the exact course followed by the head before it reached its present remarkable form. The first difficulty in the solution of the problem is the position of the anterior extremity of the dorsal fin. If this occupies the mid-dorsal line of the head, then it is obvious that the left eye must have actually passed through the substance of the head to reach the ocular side. This supposition, absurd as it may seem to us now, was in fact believed by such an observer as Steenstrup. But the anterior extremity of the dorsal fin is not situated in the mid-dorsal line of the head. Its skeletal support (fig. 17) and nervous supply (fig. 27.) prove conclusively, (1) that morphologically it does not belong to the head at all, and (2) that it has secondarily passed forwards over the cranium from behind. Further, an examination of the connection between the dorsal fin skeleton and the skull (fig. 17) shows us that the fin extends forwards in a straight line over the cranium without being affected in any way by the torsion of the head. (Cp. the course of the fin indicated in fig. 1.) It is therefore certain that the forward extension of the fin took place after the torsion was complete. Hence it does not occupy the median line, but follows what Traquair calls a ‘ pseudomesial ”’ course, and, being a purely secondary character, may be eliminated from the discussion. The second difficulty is the mischievous assumption that the left eye has travelled over the top of the head to the right side. The fact is that the left eye is not on the right side at all. Its presence there is purely illusory. What has happened is that the whole of the *Trans. Linn. Soc., vol. xxvy., p. 263, 1865, 10 cranium zn the region of the orbit has rotated on its longi- tudinal axis to the right side, until the two eyes, instead of occupying a horizontal plane, have assumed a vertical one, and the left eye is dorsal to the right. Then the dorsal fin grew forwards over the roof of the cranium, but naturally cannot define the morphological right and left sides of the orbital region. Thus the ocular side com- prises not only the right side but a portion of the left, and the true morphological median line hes between the two eyes and not above them. ‘The relation of the eyes to the skull is, notwithstanding the rotation of the orbital region of the latter, exactly the same as in a symmetrical fish, and the only differences of importance are the atrophy of the anterior portion of the left frontal, and the purely secondary junction under the left eye of the left prefrontal and frontal (fig. 1). Now whilst the above is a satisfactory answer to the question how, it does not help us with the question why. It is to be presumed that the first stimulus to asymmetry was an increasing tendency of the fish to lie on the sea bottom on one side of its body. Cunningham then in- vokes the principle of the inheritance of acquired charac- ters, and believes that the torsion itself was produced by the action of the eye muscles. We have considered this point of view very carefully both per se, as a theory, and also as a supposed explanation of the facts, with the result that we cannot subscribe to Cunningham’s conclusions. It is to us simply inconceivable how any action of the eye muscles, as they are found in fishes, could have pro- duced the existing torsion of the head, and this quite apart from the question whether such results, if produced, would have been inherited. This, however, will be further referred to in the section on the eye. In the meantime we prefer to believe that the asymmetry of the 11 Pleuronectidx has been produced by the action of natural selection, 2.e., by the accumulated effects of congenital variations. B.—THE SKELETON. This may be divided, as usual, into an axial and an appendicular skeleton. | We shall describe the former first, but the precise order must to a certain extent depend on convenience rather than upon strict logical sequence. We therefore begin with the cranium itself, afterwards proceeding to the remaining constituents of the skull, then to the vertebral column and unpaired fins, and finally to the limb girdles and paired fins. 1.—Craniumt (Figs. 1 to 4). Owing to the difficulty of making an independent preparation of the chondrocranium it is, in the following description, described in the pieces into which it is divided when the cranium is disarticulated. Seen from behind (Pl. II., fig. 4) the ceciput is markedly asymmetrical, and a line traversing median structures would be convex towards the ocular side. This is observable also in the occipital condyle and in the paroccipital condyles (0.C., P.C.), and of the latter, the eyeless one, as may be assumed from the description of the atlas, is larger than the ocular. In those two extensions of the auditory capsule, the epiotic and parotic processes + Cp. especially, Traqwair, Trans. Linn, Soc., xxv., p. 263. Space forbids a discussion of the literature in the text, but the following papers should be consulted :—Schmid-Monnard, Jena. Zeits., xxxix.; T. J. Parker, Trans. Zool. Soc., xii., p. 5; Sagemehl, Morph. Jahrb., ix.,p.177; x., p.1; Xvli., p. 489; Shufeldt, Report U. S. F. C., 1883, p. 747; Allis, Jour. Morph., xii., p. 487; xiv., p. 425; Zool. Bull.,i., p. 1; Anat. Anz.,xvi., p. 49; xvii., p. 433; Cole, Trams. Linn. Soc., ser. ii. vii., p. 115; W. K. Parker, Phil. Trans., vol. 173, pp. 189 and 443: vol. 163, p.95; McMurrich, Proc, Canadian Inst., N. S., ii., p. 270; Brooks, Sci. Proc. R. Soc., Dubline INS S:, tv., Ds 166; 12 (fig. 4, Hp.P., Pa.P.), there is a marked difference from the cod, the former being more prominent than the latter, instead of vice versa. In a dorsal view (fig. 1) the asymmetry is most pro- nounced in front of the parietal region. In the cod the frontals completely meet (and indeed fuse) in the mid- dorsal line. In the Plaice, on the other hand, there is a very wide separation of the frontals anteriorly, so as to form a large secondary frontal fontanelle or left orbit. The asymmetry is, however, more evident on the ventral surface (fig. 2). This is due to the fact that in front of the alisphenoids there is no side wall to the cranium, which, therefore, here consists ef the paras- phenoid only. In front of the prootic the parasphenoid turns sharply towards the eyeless side to such an extent that the head of the vomer was, in the specimen figured, deflected by about a centimetre from the middle line. As the parasphenoid is the most prominent feature on the base of the cranium, the appearance of torsion in this region is, in a full-sized fish, most striking. Seen from the side the cranium on the eyeless side falls more into one plane than on the ocular, but this is obviously due to the inclination of the parasphenoid and vomer to that side (cp. fig. 2). The interior of the brain case is extremely irregular. Owing to the lateral walls meeting ventrally at a some-, what acute angle a false floor for the brain becomes neces- sary, and this consists of two distinct parts. In front there is a rather narrow transverse bridge connecting the two alisphenoids, a strong sutural union being effected in the middle line. Behind there is a similar but much broader bridge joining the two prootics, the two processes meeting as before in a median suture. The true floor of the cranium is formed in the former of these cases by the 18 parasphenoid and in the latter partly by the prootic and partly by the parasphenoid. In both cases there is an obvious ‘space between the false and true floors, and this space is the eye muscle canal. At the region of this posterior bridge the side walls of the cranium are greatly strengthened internally (and the cranial cavity hence reduced) by a stout ridge of bone borne on the prootic, sphenotic and supraoccipital. From the middle of this ridge there extends backwards another process which be- ‘comes larger and more complex as it passes backwards. This is formed mostly by the sphenotic, supraoccipital, pterotic, epiotic and exoccipital, and consists of both bone and cartilage. It is here that the cranial wall is thickest. The foramen magnum does not open at once into the cranial cavity, but into a bony canal formed by the basioccipital and exoccipitals (fig. 4). Basioccipital (2.0., figs. 2, 3, 4)—A stout bone, partly cartilaginous in front, and bearing the single con- cave occipital condyle for the centrum of the atlas. Above it forms a small portion of the floor of the foramen magnum. Mid-ventrally it exhibits a deep depression into which fits the posterior extremity of the parasphenoid. The basioccipital is bounded above by the exoccipitals, and laterally by the prootics, opisthotics and exoccipitals. Exoccipital (Hwv.0., figs. 2, 3, 4)—Forms most of the occipital foramen or foramen magnum (fig. 4, /’.d/.). It is not completely ossified, and above its cartilage forms part of the cross-shaped wedge of cartilage appearing on the surface of the occiput (fig. 4). Each exoccipital bears a very prominent ridge and concave facet lined with car- tilage for the corresponding process on the atlas. The asymmetry of these paroccipital condyles (P.C.) has been elsewhere noticed. The exoccipital is bounded above by the epiotic, laterally by the pterotic and opisthotic, below 14 by the basi-occipital and opisthotic, and internally by its fellow of the opposite side. It bears a conspicuous foramen with a long canal (f.vg., figs. 2, 3) for the exit of the vagus nerve, and at least one other for the first spinal nerve. Supraoccipital (S.O., figs. 1, 4)—A large asymmetri- cal bone hardly appearing on the occiput, from which it is excluded by the epiotics, and having only a feeble occipital spine (Oc.S.), so well marked in the Cod. This spine and ridge is continued forwards to the left anterior corner of the bone, where it forms a furrow developed in connection with the interspinous bones or axonosts of that part of the dorsal fin situated over the head, as elsewhere described. In front the supraoccipital is thin and laminate, so that the roof of the cranial cavity is here very slender, but behind the cerebral surface cf the bone is supported by three strong ridges of bone and cartilage. The supraoccipital is bounded in front by the frontals, laterally by the parietals, and behind by the epiotics. The basi-, ex- and supraoccipitals together form the occipital segment of the cranium. The Auditory Capsule of the Plaice is formed by the following five bones, as in the Cod: — Sphenotic (Sp.0., figs. 1, 2, 3)—Does not contain much cartilage. Externally on the dorsal surface a strong process is sent out and supported by a ridge of bone coming up from below. The sphenotic forms the external and upper half of the deep cup for the ball of the hyomandibular (cp. fig. 5), the prootic half of the same being more or less separated from it by a strip of the chondrocranium (cp. the two sides in fig. 2, Hm.F.*). The cerebral surface of the sphenotic has two large cavities separated by a thick wall of bone and cartilage. The sphenotics are not quite symmetrical—that of the ocular : 15 side being the larger and more densely calcified. They are bounded by the frontal, alisphenoid, prootic, pterotic and parietal. Prootic or Petrosal (Pr.0., figs. 2, 3)—A stout bone containing a quantity of cartilage. It is perforated by the large canal or foramen jugulare (f.jug.), which transmits the internal jugular vein, the ophthalmic artery and the truncus hyomandibularis nervi facialis. It also forms the postero-lateral wall of the trigemino-facial foramen (f.tr.fa) and the external wall of the carotid foramen (f.car.), transmitting the internal carotid artery. Further it forms the internal and lower half of the hyomandibular cup (Hm.F’.'), and its part in forming a false floor to the cranial cavity by processes of bone and cartilage has been already mentioned. The prootic has two conical depres- sions on its cerebral surface, the ventral one being much the larger. It is bounded by the parasphenoid, alisphenoid, sphenotic, pterotic and basioccipital. Epiotic (Hp.0., figs. 1, 5, 4)—A dense structure largely cartilaginous, but having a thin outer shell of bone, prolonged into the somewhat prominent epiotic process (H'p.P., fig. 4). The cerebral surface bears two or three deep conical pits with a thin bony lining, cne being much larger than the others. The epiotie provides the remainder of the cartilage for the occipital cross already mentioned. It is bounded by the supraoccipital, parietal, pterotic and exoccipital. Pterotic (Pt.0., figs. 1, 2, 5)-—Forms the greater part of the parotic process (Pa.P., fig. 4). It is more densely caleified than the epiotic, and its cerebral surface bears three deep conical pits, one being partially subdivided into two. Laterally it bears an imperfect oval bony facet for the posterior condyle of the hyomandibular (/m./.?, figs. 2, 3, and ep. fig. 5). The left facet is appreciably 16 larger than the right (cp. description of the hyomandibu- lar), in contradistinction to the hyomandibular cup which is smaller on this side. The pterotic forms the anterior boundary of the glosso-pharyngeal foramen as shown in fig. 2 (f.gl.). It is bounded by the parietal, sphenotic, prootic, opisthotic and exoccipital. Opisthotic or Intercalar (Op.0., figs. 2, 3, 4).—Forms the remainder of the parotic process. It is by far the smallest of the otic bones, consists of a thin flat plate of irregular shape, and contains no cartilage whatever. Its development should therefore be studied. It forms the posterior boundary of the glosso-pharyngeal foramen (f.9/., figs. 2, 3), and is bounded by the basioccipital, pterotic and exoccipital. There can be no question in forms such as the Cod and Plaice that the ear bones described as pterotic and sphenotic are something more than what they seem, ~., they have a compound and not a simple origin. Added to the so-called cartilage bone in each case is a dermal element, originally developed around that part of the sensory canal system associated with these bones, and now more or less completely fused on to them. In some Fishes (such as the sphenotic of Amia) the two portions remain distinct throughout life, and in others the line of fusion may be plainly seen, with, however, the bones remaining separate as an occasional abnormality (such as the pterotic of the Cod). But as a rule the two portions fuse com- pletely, so as to be indistinguishable in the adult. Now in the one case the terms pterotic and squamosal have been applied indifferently to the compound of the adult. We may therefore, in those cases where the two parts of the compound remain separate in the adult, call the car- tilage pterotic, or ear bone, the true pterotic, and the dermal pterotic, or lateral line bone, the squamosal. In 17 the other case, however, in which the terms sphenotic and post-frontal are synonyms, we cannot adopt the same plan, since the term post-frontal cannot be correctly applied to a membrane bone ?n Fishes. We must hence distinguish between the cartilage true sphenotic, or ear bone, and the dermal sphenotic, or lateral line bone, without giving the latter a definite name. The subject would repay investi- gation. Parietal (Par., figs. 1, 5).—Flat conspicuous bones containing of course no cartilage. On the dorsal surface the inner portion is laminate, but the outer portion is much more densely calcified (cp. fig. 1). The boundary separating these two parts is where the skull begins to shelve down. The two parietals are markedly asymmetri- cal, as shown in fig. 1. The parietal is bounded by the supraoccipital, frontal, sphenotic, pterotic and epiotic. Alisphenoid (A/.S., figs. 2, 3)—Forms, as described above, a false floor to the cranial cavity, separating the latter from the eye muscle canal. The greater part of the dorsal portion of the alisphenoid consists of two thin plates of bone with a layer of cartilage between them. Behind, the alisphenoid forms the anterior boundary of the foramen for the fifth and seventh cranial nerves, and it is at this region that the bone is most densely calcified. It is bounded by the parasphenoid, prootic, sphenotic, and frontal. In front a portion of the border is free. Anterior to the parietal region the asymmetry of the skull is most emphasized, and its rotation in the direction of the hands of a watch is quite manifest. The bones of the two sides therefore differ more or less considerably. Right Frontal (/?./’r., figs. 1, 2, 3)—Very elongated from before backwards and narrowed from side to side. It is the anterior prolongation of the right frontal that forms the stout bar between the eyes so prominent in the E 18 undissected fish. The right frontal, like the left, is bounded by the frontal of the other side, supraoccipital, parietal, sphenotic, alisphenoid, the prefrontal of its side and median ethmoid, except that the left frontal does not reach the median ethmoid. Left Frontal (L.F'r., figs. 1, 2, 3).—Takes no part in forming the interorbital ridge. Compared with the right frontal it is broad from side to side and shorter from before backwards. As shown in figs. 1 and 2, it sends out on the right a strong transverse process which overlaps the dorsal surface of the right frontal. The forward process on the left of the left frontal lying over the left prefrontal, together with the posterior portion of the latter bone, are lying apparently in a very anomalous position, i.e., they are situated under the left eye instead of over it. This, however, has been produced by the frontal growing forwards, and the prefrontal growing backwards, after the torsion of the cranium was an accomplished fact. It is thus a precisely analogous case to the anomalous position of the anterior extremity of the dorsal fin. Prefrontal (/?. and L. P.F'r., figs. 1, 2, 3)—The left prefrontal is in every respect larger than the right—due apparently to the circumstance just mentioned. Both contain in front some cartilage which is continuous with what we have termed the ethmoid cartilage. Both also are perforated by a foramen transmitting the olfactory nerve to the olfactory lamin of the nasal organ, the left foramen being perceptibly smaller than the right—due to the lett olfactory nerve being so much smaller than the right. The left prefrontal fits by means of a long narrow backward process into a groove on the dorsal surface of the front end-of the parasphenoid—a process entirely absent on the right side. The articular surface for the lachrymal is smaller on the left side, and similarly the process 19 bearing it is also the smaller of the two (cp. fig. 1). Above the olfactory foramen the left prefrontal is prolonged upwards and backwards to assist the median ethmoid in forming the anterior boundary of the left orbit. This process is practically absent on the right side. The pre- frontal, which is called by other authors ectethmoid, lateral ethmoid, parethmoid, or paired ethmoid, is bounded by the lachrymal, vomer, mesethmoid, ethmoid cartilage, and frontal, the left one further by the parasphenoid. Mesethmoid (J/./., M.E.}, figs. 1, 5)—Presumably an ossification of the ethmoid cartilage. In front it bears a prominent beak (4/./.!), and above this is a depression, both of which are connected with the motion of the inter- maxillary cartilage. The marked inclination of the former to the eyeless side will be noted, thus civerting the motion of the jaws to that side. Behind, the mesethmoid is laminate, and takes a sharp turn upwards, forming by a graceful curve the anterior wall of the left orbit, the remainder of which is contributed by the left prefrontal. In front and on each side of the mesethmoid the ethmoid cartilage appears on the surface of the cranium, whilst behind and above on the right is an attachment for the right nasal. The mesethmoid is bounded by the vomer, ethmoid cartilage, prefrontals, right nasal and right frontal. Ethmoid Cartilage (Wth., figs. 1, 2, 5)—An asym- metrical unpaired cartilage quite separable in the macerated skull except for those portions embedded in the prefrontals. It consists of two parts, one a long basal horizontal rod tapering to a point behind and fitting into a deep groove on the upper surface of the parasphenoid alongside the caudal process of the left prefrontal, the other a vertical plate with a marked deflection to the ocular side and perforated by a large fenestra. The latter 20 transmits the origins of the two right oblique muscles of the eye. The ethmoid cartilage appears on the surface of the skull on each side of the mesethmoid immediately above the vomer. It is bounded by the parasphenoid, the prefrontals, mesethmoid and vomer. Vomer (Vo., figs. 1, 2, 35)—A median unpaired bone consisting of an anterior head and a posterior shaft taper- ing to a point. The latter is firmly fixed into a long tapering cavity in the base of the parasphenoid to such an extent that the extremity of the parasphenoid is brought very near the anterior end of the vomer. The cavity in the parasphenoid lodging the vomer is quite distinct from that immediately above it for the ethmoid cartilage and the left prefrontal. The head of the vomer is inarkedly asymmetrical, and has a laminate process on each side, the right of which is appreciably larger than the teft. In front the vertical face is inclined towards the eyeless side, thus further deflecting the motion of the intermaxillary cartilage, and hence the jaw apparatus, to that side. The vomer is bounded by the parasphenoid, prefrontals, ethmoid cartilage and mesethmoid. Parasphenoid (Pa.S., figs. 1, 2, 5)—A very Jong un- paired bone with a prominent keel. It is very asymmetri- eal, taking at the region of the alisphenoids a sharp turn towards the eyeless side. Behind it fits into a depression on the base of the basioccipital, and forms a portion of the floor of the cranial cavity in front of the latter bone, its dorsal surface being here deeply grooved. Its relations in front to the ethmoid cartilage, left prefrontal and vomer have been described above. The parasphenoid is bounded by the basioccipital, prootics, alisphenoids, left prefrontal, ethmoid cartilage and vomer. Nasal (/?.WVa., figs. 1, 2, 3)—Only the right nasal is present—the left having completely aborted with the 21 almost complete loss of the left supraorbital sensory canal, the anterior extremity of which it is its main function to protect.t The existence of the left nasal would cf course also be jeopardised by the motion of the intermaxillary cartilage over the beak of the mesethmoid, which, whilst not affecting the right nasal, would tend to reduce the left. It is necessary to assume some co-operative cause such as this, since the disappearance of a sensory canal does not necessarily involve the reduction of the true lateral line bones, or the Plaice would not possess a right !achrymal. The nasal of the Plaice is a small semilunar bone attached to the right side of the posterior vertical plate of the -mesethmoid. It supports the anterior extremity of the right and only supraorbital sensory canal, and bounds the right nasal sack internally. It is sometimes called the turbinal. Lachrymal (7?.Lc., L.Lc., figs. 1, 2, 3)—These have been modified from the first of the suborbital series or chain of lateral line ossicles supporting the infraorbital sensory canal, and may hence be called the first sub- orbitals. They have also been called the adnasal bones, on account of their relations to the nasal sack. In the Plaice they differ from the bones of the same name in most Teleosts (including the Cod) in being closely attached to the cranium. They differ in shape as shown in fig. 1, the left being more concentrated than the right (the latter best shown in fig. 3). The right lachrymal has no connection whatever with the right infraorbital sensory canal. Both lachrymals bound their nasal sack externally, + Traquair (loc. cit., p. 284) describes a “‘minute turbinal [nasal] ossicle”’ on the left side, supporting the ‘‘remnant of the main [supra- orbital] canal of the eyeless side.”” We have found the latter in our sections as Traquair describes it (see elsewhere), but not the rudimentary nasal. Dr. Traquair’s work, however, is so accurate, that he is doubtless correct in this also, 22, and are both attached to anterior projections from the prefrontals in the manner described above. The suborbital and supratemporal chains of lateral line ossicles are described with their respective sensory canals. 2.—TuHE Patato-Prerycorp ArcaDE (Fig. 5). Ocular Side. Hyomandibular ({7m.)—This bone articulates with the skull in two ways. First by a well marked ball and socket joint situated at the anterior extremity of its articular surface, and second by a !ong and less con- spicuous facet behind this. The socket is a deep depres- sion very obvious in dried skulls and situated between the sphenotic and prootic. From this depression the other facet passes upwards and backwards, and is placed mostly if not entirely on the pterotic. The head of the hyoman- dibular is cartilaginous in three places, at the two facets for the skull and at the projection articulating with the operculum. Parallel with the posterior edge, and at a short distance from it, is a stout bony ridge (the most strongly calcified part of the bone) which is closely attached to the pre-operculum. In front of this a shaft — of cartilage passes downwards and forwards, which bears a thin cartilaginous cap, and articulates with the inter- hyal in such a way as to admirably illustrate what is now a commonplace of vertebrate morphology, that the hyomandibular is the modified dorsal segment of the hyoid arch, which has in many forms lost its connection with the hyoid arch, and has acquired on the one hand a con- nection with the auditory capsule and on the other with the jaw suspensorial apparatus. The Plaice is therefore hyostylic. The remainder of the ventral edge of the 23 hyomandibular articulates with the meta-pterygoid. In front of the cartilaginous shaft it simply consists of a thin leafy plate—a part which is absent in the Sole according to Cunningham. Symplectic (Sv.).—Consists of a cartilaginous shaft with a semilunar plate of bone opposed to its anterior edge. Its head forms with the shaft of the hyomandibu- lar the cup for the upper end of the inter-hyal, and also bears a cartilaginous epiphysis. Ventrally its extremity lies under the quadrate. Its anterior bony margin is attached to the metapterygoid and its upper posterior edge to the pre-operctlum. Quadrate (Qu.).—A laminate bone bearing posteriorly a strongly calcified ridge and ledge for the pre-operculum. Dorsally it is prolonged into a spine situated in front of the pre-operculum and wedged in between that bone and the symplectic. In front it articulates with the pterygoid, and below its free extremity bears a stout knob with a saddle-shaped articulation for the articular. Meta-pterygoid (J/.Pt.)—A very thin leafy bone attached to the hyomandibular above, the symplectic behind, the quadrate below, and the meso-pterygoid in front below. Meso- or Ento-pterygoid (J/s.Pt.).—Also a thin leafy bone very strongly attached to the pterygoid below. Its lower border in front is more strongly calcified than the rest. Pterygoid or KEcto-pterygoid (Pt.)—A peculiarly shaped bone. It consists of a strongly calcified piece having the shape of an isosceles triangle, the apex point- ing downwards and forwards. When the jaws are shut it is opposed in front to the articular of the mandible. From the posterior angle of its base it sends forwards almost at right-angles a transparent bony rod which is tightly 24 wedged in, and connected by ligament with the meso- pterygoid and the palatine, as in Sebastolobus.t Palatine (Pa.).—A curved rod largely of bone, but partly of cartilage. Its posterior half is partly cartila- ginous and is closely connected with the meso-pterygoid, pterygoid, and the anterior angle on the base of the ptery- goid. Its anterior half is bony except at the extremity, which bears a cartilaginous cap attached by ligament to a dorso-posterior elevation on the maxilla. The palatine sends down opposite the anterior end of the meso-ptery- goid a rounded process which is strongly attached to the enlarged anterior extremity of the vomer, and apparently also to the ventral process of the pre-frontal as described by Brookst in the Haddock. In the natural disposition of the bones the palatine lies internal to the maxilla. Eyeless Side. Hyomandibular.—Much smaller and less densely calcified, and is altogether an obviously feebler bone, although the ball and socket articulation with the skull, whilst slightly smaller, is yet deeper and stronger. The cartilaginous cap for the inter-hyal is also present on this side. Symplectic.—Considerably shorter, but more robust, and has only a cartilaginous wedge at its upper extremity. Quadrate.—Slightly shorter but wider antero- posteriorly and more densely calcified, especially at its free ventral extremity. Its dorso-anterior margin is, however, cartilaginous where it articulates with the meta- pterygoid. Meta-pterygoid.—Somewhat shorter and narrower, { Starks, Proc. Californian Acad. Sci., ser. iii., vol. i., 1898. t Sci. Proce. R. Soc., Dublin, iv., 1884. 25 but the same thin laminate bone. Has only a slight articulation with the meso-pterygoid in front instead of the extensive one of the other side. Meso-pterygoid.—Not attached to the dorsal edge of the pterygoid but to its inner face. It hence occupies a different plane to that of the meta-pterygoid. It is further much smaller on this side. Pterygoid. —This is of a different shape on this side. The forward thin rod is here thick and in fact stouter than the remainder of the bone, which is reduced and apparently merged into the forward portion. The left pterygoid is both larger and stouter than the right—thus differing from the left palatine, as will be seen below. Palatine.—Greatly modified on this side. Its anterior extremity articulates directly with the upper end of the maxilla instead of by the intervention of a short ligament. In one specimen there was also a short hgament contain- ing a sesamoid connecting the same extremity with the anterior forward process of the pre-frontal. Behind the anterior end there is a strong dorsal articulation with the pre-frontal which is not found on the other side. The ligamentous connection with the vomer also exists on this side, but is apparently with the vomer only. The articu- lation with the pterygoid is also different, the posterior extremity of the palatine being forked and the pterygoid fitting into the split thus formed, the interstices being filled with cartilage. The left palatine is much shorter than the right, but is more robust (ep. the pterygoid). 3.—THE Jaw Apparatus (Fig. 5). Ocular Side. Articular (Ar.).—Has a saddle-shaped articular sur- face for the quadrate, behind which it sends up a promi- 26 nent ‘ post-glenoid”’ process. The bone is stoutest at this region, but becomes gradually lamelliform forwards, and its anterior margin is furcate—the lower limb fitting into the cavity of the dentary. The quadrate facet is cartilaginous. ‘The outer face of the articular is convex, the inner concave. Angular (An.)—A small but perfectly distinct bone situated at the postero-inferior angle of the articular. Meckel's Cartilage. —A long thin rod ef cartilage embedded in the articular behind, but lying quite freely for the greater part of its length. It is situated near and parallel to the ventral border of the inner or concave face of the articular and projects shghtly beyond the anterior extremity of the ventral limb of the latter, the free end not being ossified as a mentomeckelian as in Ama. The free portion of Meckel’s cartilage was 12mm. long in the speci- men now described, and in a very large fish it attains a diameter of about 2mm. Dentary (V.).—A thin bone, but well ossified at its dorsal and ventra! borders and strongly attached to the dentary of the other side. It is strongly and almost equally forked behind, and contains a large triangular cavity for the reception of the lower hmb ef the articular. Like the latter, its outer face is convex and the inner concave. Quite near the symphysis it on this side and in this specimen bore 3 teeth, opposite to which on the ven- tral border a prominent process was set down. In a very large specimen examined there were no teeth cn this side, their position being occupied by a roughened ridge. Maxilla (J/w.). club-shaped bone, the expanded lower extremity of the Takes no part in the gape. A stout handle or shaft of which overlaps the lower jaw externally at about the junction of the articular and dentary when the mouth is closed. The shaft narrows down before ex- 27 panding to form the head, which is capped with cartilage. The head has 4 articulations: (1) It is closely attached above to the large unpaired intermaxillary cartilaget (7.M.C.), to which on its other side the left maxilla is also attached ; (2) ventrally the maxilla is capped by a move- able piece of cartilage, much smaller and situated ventral to the inter-maxillary. This moveable or gliding car- tilaget works in the groove to the right of the beak-like mesethmoid prominence (cp. fig. 1), and also over the large convex facet on the right side of the head of the vomer. It is connected with the inter-maxillary and is excavated on both surfaces to receive the head of the maxilla and the vomerine facet. The latter or free exca- vation is so contrived as not to interfere with the move- ment of the cartilage above the vomer. The reader who consults a dried cranium of the Plaice when reading this description will understand that the movement permitted to the maxillary bones by these facets and gliding car- tilage is an oblique dorse-ventral one- in the direction of the eyeless side (cp. Traquair op. cit.). This explains the well-known fact that Plaice are able to pick up food lying on the sea bottom by twisting the mouth towards the lower or eyeless side. In other Pleuronectidee the twisting of the mouth is towards the ocular side ; (3) dorso-posteriorly the maxilla, as already pointed out, is connected by liga- ment with the anterior extremity of the palatine; (4) anteriorly and externally it is deeply excavated for the reception of the pre-maxilla. The anterior edge of the maxilla is close to and partly overlaps the pre-maxilla. Pre-Maxilla (?.1/7.)—Forms the dorsal part of the gape and consists of two arms. Its vertical arm forms the gape and bore 4 teeth in the specimen now described, { We follow Traquair in using this term for the cartilage in question. + Cp. Traquair, Trans. Linnean Soc., London, xxy., 1865. 28 which, however, contrary to those of the opposite side, were placed nearer the posterior edge of the pre-maxilla than the anterior. Dorsally this arm is closely connected with the left pre-maxilla. The posterior arm passes back- wards over the inter-maxillary, with which it is closely connected. At the junction of the two arms a process is sent backwards which is both capped with cartilage and is covered by a further loose moveable piece. This process fits into the excavation on the maxilla already described, and hence the pre-maxilla here overlaps the maxilla. Inter-Maxillary Cartilage (/.4/.C.).—This cartilage plays an important part in the movement of the jaws. It is asymmetrical, blunt in front but more pointed behind, and forms as it were a pivot on which the two maxillary bones on each side turn. It fits into the depression very obvious in the dried cranium above and to the !eft of the mesethmoid prominence, and glides up and down from this depression over the prominence itself. Its posterior surface is obliquely grooved, and in such a way that as it moves downwards it passes obliquely over the prominence towards the eyeless side—thus further assisting in the torsion of the jaws to that side. Eyeless Side. Articular, Angular and Meckel’s Cartilage —The two former are slightly larger than the right, and are also shghtly more densely calcified, but the differences are small. Meckel’s cartilage was in the specimen now described 2mm. longer on this side. Dentary.—Appreciably larger and more densely caleci- fied, and is strongly curved whilst the right is almost flat. The forking of the posterior margin is further very unequal, the lower limb being much the larger (ep. fig. 5). The depression at about the middle of the outer tace of the 29 articular immediately in front of the quadrate articulation for the M. adductor mandibule is much more marked on this side, where the muscle is naturally larger and, further, the distortion of the jaws to the eyeless side is assisted by a tendon from it inserted into the maxilla. The dentary of this side bore 22 teeth, as against 3 on the right side. Maxilla.— Distinctly larger and more curved than the right but not so robust. At about a third from the head on the posterior edge is an eminence for the attachment of a stout tendon arising in connection with the M. adductor mandibule, and the action of which tends to draw the jaws towards the eyeless side. This eminence and tendon are not conspicuous on the ocular side, and indeed in the Sole, where the ocular is also the right side, the tendon is stated by Cunningham? to be absent on the eyeless side, although the muscle is said to be larger on that side. The eminence is figured and the muscle described by Tra- quair,t who calls the latter the Retractor Maxille.* At the head of the maxilla on the posterior side the bone articulates directly with the free anterior extremity of the palatine instead of by the interposition of a short liga- ment. The cap of cartilage gliding over and above the head of the vomer is larger and the terminal free facet is also more extensive. The whole action of the jaw appa- ratus is markedly asymmetrical owing to the unpaired mesethmoid prominence separating the two maxillary facets being obliquely set towards the right. Hence when the maxille are depressed they follow an oblique direction towards the left or eyeless side. The articulation of the maxilla with the pre-maxilla is also modified on this side. On account of the motion of the jaws towards the left the + The Common Sole. Plymouth, 1890, p. 48. * Op. cit., p. 279, Tab. 30. * Cp. also Allis, Jour. Morph., xii., pp. 552 and 576. 30 head of the maxilla is brought into closer connection with the posterior ascending process of the pre-maxilla. Then again the posterior articular process for the maxilla at the junction of the two arms of the pre-maxilla is smaller, and instead of overlapping the maxilla is overlapped by it. Pre-Maxilla.—Both arms are longer and stouter. The ascending arm is at right-angles to the oral arm instead of at an obtuse angle as on the right side, and passes over the inter-maxillary more to the middle line of that car- tilage. It bore in this specimen 17 teeth as against 4 of the other side, and set, as already stated, in a different plane. The asymmetry of the suspensory and jaw apparatus, whilst undoubtedly initiated by the torsion cf the cranium, has also been independently emphasized by the habits of the fish, as already described. The broad anatomy of this distortion is as follows: (1) The suspen- sory apparatus on the right side is mostly longer—thus thrusting the jaws over to the left; + (2) the motion of the maxille at the sides of the mesethmoid prominence and over the head of the vomer and the course of the inter- maxillary over the mesethmoid beak itself is an oblique one with a set towards the left, which the pre-maxillz and mouth must necessarily follow. The jaws themselves and the bones immediately related to them are naturally more robust on the left side, since their function is mostly per- formed on that side. Hence the practical absence of teeth on the right pre-maxilla and dentary. | 4.—THE OpEercuLAR Bones (Fig. 5). Ocular Side. Operculum (Op.).—A thin laminate bone containing no cartilage except at the articular cup. It is bifid pos- + Cp. Traquair, op. cit., Tab. 30 and pp. 276-7, dl teriorly, the apex of the lower arm overlapping the sub- operculum. Its anterior extremity is greatly strength- ened by a median ridge of bone deeply cupped in front and forming a strong ball and socket joint with a process on the posterior margin of the hyomandibular. Sub-operculum (S.0.).—Described in the Sole by J. T. Cunningham as the “ Inter-opercular.” A jeafy bone thinner than the operculum, sending upwards and back- wards a long process behind the bifid margin of the operculum. Ventrally it overlaps a small portion of the inter-operculum. The operculum and sub-operculum support the posterior free margin of the opercular fold, and the characteristic posterior process at the base of the pectoral fin (see fig. 23) is formed by the upper extremity of the sub-opereulum and the upper limb of the oper- culum. Inter-operculum (/.0p.).—The ‘ Sub-opercular ” of Cunningham. A thin bone but stouter than the sub- operculum. It stiffens the ventral free margin of the opercular fold. The whole of its dorsal edge Hes under the pre-operculum, and at about the middle of this edge there is a depression (and here, as in the operculum, the bone is thickest and most strong), providing a ligamentous articulation with the inter- and epi-hyals at the junction of the two latter—a somewhat similar condition to that found in Amia.t The connection of the operculum and inter-operculum (and especially the latter) with the hyoid arch, both apparently common in the bony fishes, confirms the view that these elements are modified branchiostegal rays. The sub-operculum is probably also another. Pre-operculum (P?.0p.).—This is usually considered to be a modified lateral line bone, 7.e., a bone developed primarily around a portion of the lateral line system, and + Allis, Jour. Morph., xii. Cp. also Shufeldt, Report U.S. F. C., 1883. 32 is therefore probably not homologous with the other oper- cular bones. In the Plaice it is L shaped and above over- laps a portion of the hyomandibular. Its straight anterior margins are closely connected with the hyomandibular, symplectic, and quadrate. Opposite the ventral edge of the hyomandibular it almost completely covers the inter- hyal. Ventrally it overlaps the posterior edge of the quadrate. It is the stoutest of the opercular bones—its anterior surface especially being strongly calcified. Eyeless Side. The relations of the bones are precisely the same, but the following differences in shape, &c., may be noted :— The operculum is slightly smaller and not quite so strongly calcified, and the sub-operculum, though smaller, is somewhat stiffer than the right. The pre- and inter- opercula are both distinctly smaller and less calcified, and hence the asymmetry is most marked in the anterior opercular bones. The bonest which enter more or less into ithe forma- tion of the entire skull of the Plaice may now be pro- visionally arranged in the following categories according to their manner of origin :— A.—Bones formed as ossifications within the primi- tive cartilaginous brain case of the embryo : —Alisphenoid (ALS.), Basioccipital (B.0.), Epiotic (L'p.0.), Exoccipital (Ev.0.), Mesethmoid (2.EL., M.E.1), Opisthotic (Op.0.), Prefrontal (P.Fr.), Prootic (Pr.O.), Pterotic (Pt.O.—less the fused dermal pterotic or squamosal), Sphenotic or Postfrontal (Sp.O.—less the fused dermal sphenotic), Supraoccipital (S.O.). + The only definite cartilages in the Plaice’s skull are the ethmoid cartilage (Hth.), inter-maxillary cartilage (J.M.C.) and Meckel’s cartilage. The remaining cartilage is not of an independent character, 33 B.—Membrane bones which have become secondarily incorporated into the cranium :— 1. Roof of Skull. Frontal (#/’r.—less the fused lateral line bone), Parietal (Par.). 11. Mouth. Ossifications in the mucous membrane— Parasphenoid (Pa.S.), Vomer (Vo.). C.—Lateral line ossicles and bones formed by the modification of such:—Lachrymal (Lc.), Nasal (R.Va.), Preoperculum (P.Op.), Squamosal (see pterotic—Pt.0.), Suborbital chain, Supratemporal chain. D.—Bones formed either within or in immediate con- nection with the mandibular arch of the embryo :— Angular (An.), Articular (Ar.—less the fused lateral line ossicle), Dentary (D.—less the fused lateral line bone), Maxilla (Mz.), Mesopterygoid (Ms.Pt.), Metapterygoid (M.Pt.), Palatine (Pa.), Premaxilla (P.Mz.), Pterygoid (Pt.), Quadrate (Qu.). E.—Bones formed in connection with the hyoid arch of the embryo :— i. By modification of the upper segment of the arch itsel{—Hyomandibular (Hm.), Symplectic (Sy.). ii. By modification of the posterior branchiostegal rays of the arch—Interopereulum (J.Op.), Operculum (Op.), Suboperculum (S.Op.). ¥.—Compound bones, 2.e., cranial bones to which ossiclés or bones originally developed in relation to the system of sensory canals have become secondarily fused to their superficial surface :—Articular (Ar.), Dentary (D.), Frontal (Fr.), Pterotic (Pt.0.), Sphenotic (Sp.0.). We shall now proceed to describe the visceral skele- ton proper, taking the hyoid arch first and the branchial arches afterwards. B4 5.—Hyorp Arcu* (Fig. 6). Ocular Side. Uro-hyal (U.Hy.).t—A short bony zod but car- tilaginous in front and behind, and articulating with the upper hypo-hyals and slightly with the first basi-branchial. Hypo-hyals (/7.Hy.)—Two pieces, as in the Cod, partly of bone and partly of cartilage and loosely articu- lating in the middle line with the same elements of the other side. The upper one is perforated in the middle, thus giving a false impression as to the whereabouts of the suture between the two. Cerato-hyal (C.Hy.).—A stout bar traversed in front (anterior face) by a longitudinal groove, the texture of the bone on each side of which running in different directions, thus seeming to indicate that the cerato-hyal, as well as the hypo- and epi-hyals, is either splitting or has been formed by fusion. Epi-hyals (/p.Hy.).—Also double, the lower piece being entirely cartilaginous and the upper partly so. The suture between the cerato- and upper epi-hyal is obscured by an overgrowth of bone as in Micropterus,t but may be seen on holding the hyoid bar up to the hght. *Tt is here necessary to explain the precise significance of the prefixes basi- and hypo- as applied to parts of the visceral skeleton. The term basi- can only be applied to a median unpaired ventral element, and the term hypo- to the pair (i.e., one on each side), immediately succeeding it. Now whilst these three elements may, and often do, exist side by side in any one species, the basi- element may be absent, and a median unpaired ventral piece formed by the two hypo- elements fusing together, the result being a secondary basi-segment. In the latter case the terms basi- and hypo- are synonyms (and are used indifferently) ; in the former, they are not. + The terms basi-hyal, glosso-hyal, ento-glossal and basi-branchiostegal have also been applied to this bone in Fishes by different authors, and the same terms, or some of them, have been applied to other elements in higher Vertebrates. The synonymy is too complex to be discussed here. { Shufeldt, op. cit., p. 819, and fig. 32. 35 Inter-hyal (/.fy., figs. 5 and 6).—Possibly a sesa- moid bone developed in the inter-hyal ligament and not homologous with the other segments of the hyoid. This bone is incorrectly called by some authors the stylo-hyal— a term really a synonym of the pharyngo-hyal, represented in most fishes by the hyomandibular. The inter-hyal of the Plaice consists of a central bony rod with cartilaginous extremities articulating with the hyoid and symplectic and inter-operculum, as already indicated. Below the attachment of the inter-hyal to the upper epi-hyal is seen the prominence which is also connected with the inter- operculum. Branchiostegal Rays (/7r./.)—There are the usual 7 of these rays, the first on each side meeting at their free extremities, and being closely bound together by strong fibrous connective tissue, appear to fuse. They are not all attached at the same plane as shown in the figure. The first 3 articulate with the cerato-hyal, the iast 4 at about the junction of the 2 epi-hyals. The last, however, is always attached to the upper or bony epi-hyal. The rays have cartilaginous extremities, but otherwise consist of a transparent milky coloured bone. The first two cross and lie under the others in the living state. Kyeless Side. The hyoid bar is slightly shorter and not quite so robust. The first pair of apparently fused rays are drawn over to the eyeless side as shown in the figure. ‘The most tonspicuous difference is in the branchiostegal rays, which are, except the first, uniformly shorter and not so curved. The length and curve are faithfully represented in the figure. 36 6.—Brancuiart ArcHEs (Fig, 7). Ocular Side. Basi-branchial I. (B.8r.1).—A triangular bone, with its apex wedged in between the upper hypo-hyals (see fig. 6). At its base it articulates with basi-branchial IT. and with the hypo-branchials of the first arch, but the latter articulation is not obvious dorsally, the head of the hypo-branchial fitting into a deep lateral socket formed by basi-branchials I. and II. According to Cunningham this bone is in the Sole completely wedged in between the upper hypo-hyals and does not articulate with the first branchial arch at all. Branchial Arch I.—The epi-branchial (#.Br.') bears a prominent tubercle on its anterior surface. The pharyngo-branchial (P.Br.') is a slender bone which articulates with the skull at the ventro-posterior margin of the jugular foramen in the prootic immediately below the hyomandibular cup. ‘his somewhat curious connec- tion with the skull also exists in the Sole according to Cunningham, and in Sebastolobus according to Starks. Basi-branchial II. (2.8r.*).—An hour glass shaped bone articulating in front with the basi- and hypo- branchials of the first arch and behind with basi-branchial III. and slightly with the hypo-branchials of its own arch. Branchial Arch I]—''he hypo-branchial is wide but compressed dorso-ventrally. Where it articulates with the second basi-branchial it sends down a prominent spine. Another well-marked spine is borne on its anterior edge. The cerato-branchial is longitudinally grooved ventrally. As in the first arch, the epi-branchial bears a tuberosity, but it is much more prominent on this arch. ‘the first and last gill rakers are very small. The superior pharyn- geal bone of the Plaice in medium-sized fish consists of 37 three pieces, which represent the pharyngo-branchials of the three arches to which they belong. These pieces are, however (and especially the anterior two), so closely bound together that they may, as we have known them to do in other forms, fuse up in old fish. The second pharyngo- branchial (P.Br.?) is a stout laterally compressed bone articulating with the third pharyngo-branchial posteriorly. It bore five teeth in one row. Basi-branchial III. (2.Br.°). A very thin laterally compressed bone, apparently wedged out of existence by the large hypo-branchials II. Its posterior extremity lies under and is covered by the two hypo-branchials III. Branchial Arch III.—The hypo-branchial is smaller than in arch II., but the ventral spine is both larger and longer, and articulating strongly with the same spine of the other side forms a bony arch traversed by the ventral aorta. The anterior spine in hypo-branchial II. is absent. The cerato-branchial is grooved ventrally as in arch IL., but more deeply. The epi-branchial bears two large tuberosities at its distal extremity. The posterior of these articulates with the pharyngo-branchial III. (P.Br.*), the anterior by two strong ligaments with the epi-branchial IV. The pharyngo-branchial (?.6r.%) bears a strong process behind for articulation with the pharyngo- branchial II., and bears eight teeth in two rows. Basi-branchial IV. (B.Br.')—A very small nodule of cartilage wedged in between the bases of arches III. and IV. It is only connected with the fourth arch on the ocular side, the basal elements of this arch on either side meeting in the mid-ventral line. The morphological value of this cartilage cannot be determined on adult material. It is obvious that the branchial arches have undergone reduction from behind forwards. ‘Thus there are only three segments in the fourth arch. Now it is 38 also obvious that the basal segment of this arch (C.Br.*) is serially homologous with the cerato-branchials, the missing element therefore being the hypo-branchial. Hence the fourth basi-branchial may represent either the two vestigial hypo-segments fused together (a primary basi-segment being absent) or it may have been formed by the basi- and two hypo- elements fusing up. Branchial Arch IY—The cerato-branchial (C.Br. ) is slightly grooved ventrally. The epi-branchial (#.Br.*) is a stout L-shaped bone, and is strongly connected with the same segment of the preceding arch. The pharyngo- branchial (P.Br.*) is small, and bore six teeth. Branchial Arch ¥.—This is more reduced than any of the other arches, and consists of a single bone on each side in which there are practically no traces of asymmetry. This is the inferior pharyngeal bone of Cuvier, and appears to represent the cerato-branchial segment only of the arch.* The inferior pharyngeals (J.Ph.) are stout triangular-shaped bones separate dorsally but bridged in front ventrally by a tract of cartilage. Two irregular rows of teeth are borne on the pharyngeal surface, and in the specimen now described there were 12 on the ccular side and 14 on the eyeless. At the side and at the base of the outer row are situated the replacing teeth, which be- come functional as their predecessors wear away. Gill Rakers.— These diminish both in size and number from before backwards. Their function is to pro- vide a rough filtering apparatus for the water passing out of the pharynx. ‘Their small size and number in the Plaice is due to the nature of the fishes’ food. In those fishes where the food might easily escape through the gill slits (e.g. Clupea), the gill rakers are much longer and more numerous. They are purely dermal and are not fused *Cp. Cunningham, op. cit., and W. K. Parker, Phil. Trans., 1873 (Salmo). 39 on to the arches, the only connection between the arches and the rakers being that in older specimens and in some places the position of the raker is indicated on the arch by a faint elevation. Their number and position, however, was in the few specimens examined remarkably constant and symmetrical, so that the following formula may apply to either side of most individuals :— pou Tal IV. V see ee | Se sees Ee ee Hypo-branchial | 2 | 8 : 0 0 0 Cerato-branchial .... 5 | 6 | 6 0 i 0 0 0 Epi-branchial .... 3 Gill Rays.—The gill filaments are supported by series of very delicate fragile gill rays fused together by their bases like a comb, which it is hardly practicable to dissect, but which are quite obvious in sections of the gills. They radiate out from the branchial arches as usual, and occur in pairs—one to each demibranch of the arch. Kyeless Side. Branchial Arch I.— ~ bel ) ad IN Z | Baseosts ~| Ja 5] {si iN bs a 3 by ai 8 4 fy 3 Z a ~| Im} SH [) s SJ dnl fe! H Axonosts |~ al | 8 my } sy | I) Is YW) 8 5B 3 Neural Spines IN) ba |) to be] io = No. of Vertebras a CI IE Accessory Ribs le Ribs i in) IN ieee! Spins eS | FP EI SI 8 a 4 % Axonosts L ST Ia [1] r to) LIN 9) SS : nN 5] al fy AN 3) IN iN % = ls im) Jt} |} le nN} jo! [a |S & 3) [2 8 a eal! in Mi | lf by Xt fs} lal (8) fs) is 3 S| 55 —— S Is.) | Fin Rays sey} |} |e} for JN} Je} |) JS S SR IS iN aN] [n a} 8 Cm NI fol (5) (st ll ls 8 5 ie FaJdoC. yd od baseost and axonost is a very slight non-ligamentous one, and hence the fin ray and baseost are only held down in the soft cartilage cup at the head of the axonost by the elevator and depressor and right and left abductor muscles (Z’.R. e) of the fin ray. The result is that the fin ray and baseost are capable of being moved on the axonost in any direction. The axonosts are held in position by two liga- ments: (a) by a longitudinal vertical ligament which keeps the axonosts at their correct distances from each other and separates the right and left series of the fin-ray muscles; (6) by transverse ligaments (Az. a) which keep the axonost from moving from side to side. The head of the axonost contains a triangular plug of typical hyaline cartilage (Aw. b) as before mentioned. The whole appa- ratus is somewhat asymmetrical as shown in the figure. The attached table, based on the examination of a 52cm. plaice, has been drawn up to show the number, position, and precise relations of the ribs, neural and haemal spines, and skeleton of the dorsal and anal fins. The specimen had 42 vertebre (cp. figs. 17, 18 and 19). The division into regions is somewhat arbitrary, since for example the boundary between the cranial and occipital regions is along the axis of fin ray 9 and baseost 8, @.e., between axonosts 8 and 9. In the anal fin the axonosts 1 to 7 are situated morphologically between vertebre 15 and 14 (cp. fig. 18), and hence at this region (as indicated by the line) the correspondence between the two regions of the table has no morphological value. Behind this ambiguous region (.e., behind and _ including vertebre 14) it will be noticed that although there is a very wide disagreement in the disposition of the fin skele- ton above and below the vertebral column, yet the numbers of the fin rays are practically the same, z.e., 45 in 58 the dorsal fin and 46 in the anal. Thus, although the physiological result is the same, the means by which it has been arrived at do not exhibit a serial agreement. 11.—PercroraL GirRDLE AND F'n (Fig. 8). Clavicle (C/.).—A large curved bone. The upper part or handle is the stoutest and bears a thin lamina behind for the articulation of the post-clavicle. The cuter face dorsally is flattened for the reception of the supra-clavicle. Below, the clavicle is connected in the mid-ventral line with the clavicle of the other side by a long symphysis. Post-clavicle (P.C/.)—A long thin curved bone articulating above by its upper enlarged extremity with the clavicle, and for the rest lying freely in the superficial muscles under the pectoral fin. Its position is partly indi- cated externally by a scar on the skin. In one specimen examined the post-clavicle was double, the two pieces uniting, however, at the clavicular articulation. Accord- ing to Cunningham’s figure this bone is not present in the Sole. Supra-clavicle (S.C/.).—A smal! triangular bone, thin below but stouter above. Ventrally it overlaps the clavicle, and above it is overlapped by the post-temporal. Its upper extremity bears a prominent cartilaginous knob, which fits into a deep pit on the inner face of the post- temporal. Post-temporal (?.7'p.).—This bone, sometimes called the ‘‘ supra-scapula,” differs from the usual Teleostean type in so far as there are only moderate indications of the forking, and there is only one direct articulation with the skull. Above and in front there is a prominent articula- tion (representing the upper or epiotic limb of the post- temporal) with the pterotic and epiotic. Somewhat below this, and also in front, is a moderate elevation (represent- 59 ing the lower or parotic limb), which is connected by a lhgament with the skull at the region of the junction of the pterotic and opisthotic. In the Sole, according to Cunningham, the forking is more marked, end ihe lower limb is connected with the opisthotic only. The post- temporal in the Plaice is tunnelled by the lateral sensory canal. Scapula (Sc.)—A thin plate, which for some time remains largely cartilaginous, but which is completely ossified in very large fish, having an oblique shelving articulation with the clavicle. Almost one-half of its outer surface lies internal to, and articulates with, the clavicle. It is perforated by the usual scapular fenestra for the R. ventralis of the first spinal nerve. Coracoid (Co.).—Consists of two parts which are, how- ever, continuous: a dorsal part (corresponding to the meso-pre-coracoid of W. K. Parker*), which calcifies late, is thicker than the ventral part, and gives articulation to fin rays; a ventral thin laminate part (the coracoid of Parker) which calcifies early and projects downwards for some distance as a ventral spine. Owing doubtless to the long articulation with the clavicle the connection between these two bones is here a simple and not a shelving one as is the case with the scapula. The ventral part of the coracoid is absent in the Sole according to Cunningham’s figure. Brachial Ossicles.—These are doubtless absent, but may be represented by three structures: (1) a wedge- shaped piece of cartilage attached mostly to the coracoid but partly also to the scapula (ep. fig. 8)—this is present in the Sole according to Cunningham’s figure; (2 and 3) two sub-cartilaginous pads, one of which works over the free surface of (1), and the other the free surface of the * ‘« Shoulder Girdle.’? Ray Society, 1868. 60 scapula (cp. fig.). It is these pads that really give articu- lation to the fin rays, and are in fact interposed between the extremities of the fin rays and the scapulo-coracoid. Judging from the analogy of the other fins there may only be one pad normally present in the pectoral fin (ep. pelvic and caudal fins). Fin Rays (/’.2.).—There were eleven fin rays in the pectoral fin of the specimen now described. ach ray consists of two pieces enclosing a central core of soft tissue, as commonly occurs among Teleosts. Hach ray is completely segmented for the greater part of its length, so that on maceration it falls into a number of very small pieces. The two portions of the ray diverge at the scapulo-coracoid articulation and embrace its sub- cartilaginous pad as already described. ach portion also sends down an articular process, and the two of each fin ray clasp the ray immediately below it (ep. fig.), thus giving a rigidity to the fin it would not have were the fin rays independent of each other. Three of the rays were bifurcated at their free extremity in this specimen, and where this obtains both halves of the ray split, the bifur- cation not being due to the two halves diverging. “Inter-clavicle” (/.C/.).—This bone, of questionable homology, is described last, as it is doubtful what claim it has to the name now given it. It is a median V-shaped bone—one limb of the V being horizontal and ihe other projecting forwards and downwards. It is situated in the muscular cone of tissue passing forwards to the hyoid arch from the clavicle in the middle line between the basal portions of the branchial arches. The horizontal limb is connected anteriorly by four long stout ligaments with the inner face of the lower hypo-hyal of each side— two ligaments passing to each hypo-hyal. Behind it is connected with the clavicle. The arms of the V are 61 densely calcified, and so is that portion of the lamina at the junction of the arms. The remainder or posterior part of the bone consists of a very thin plate or lamina. The “ inter-clavicle ” is called by Owen, Huxley and other anatomists sometimes the uro-hyalt and sometimes the basi-branchiostegal, and it is asserted by Cunningham that these names cannot correctly be applied to it. The term (.e., “‘ jugular’) used by Cunningham is, however, itself inadmissible, since it is Hable to be confounded with the jugal or with the jugular plates of “ Ganoids ”’—with the former of which it can have no possible connection. In the Sole, according to this author, it is apphed directly to the clavicle and first basi-branchial, and in this differs markedly from the Plaice, where it is placed some dis- tance from both these bones. Its position in the Plaice relative to that of the clavicle is correctly indicated in fig. 8, and it is somewhat further removed from the hyoid arch. Hence, whatever its position in other Teleosts, in the Plaice it is directly connected neither with the clavicle nor with the hyoid. In Sebastolobus according to Starks, and in Micropterus according to Shufeldt, what seems to be the undoubted homologue of the “ inter-clavicle ” articu- lates with the hyoid arch and is called by these authors the “uro-hyal.” Cunningham’s objection to this term, how- ever, seems to be valid, and hence the provisional name of “inter-clavicle ’—a bone with which it may not unrea- sonably be identified. On the other hand its connection in other forms with the hyoid arch indicates that it may have been derived from branchiostegal rays, in which case if may conceivably be homologous with the jugular +The supposed resemblance to the uro-hyal of the bird doubtless suggested this homology. Kyle has recently revived this name for the bone, but does not seem to be aware that the terms uro- and glosso-hyal are too often used as synonyms to justify any separation now. In any “case, we consider the term uro-hyal quite inadmissible. 62 or gular plate of Amia. ‘This, however, is so very problematical that to prevent confusion, Cunningham's term must be rejected. 12.—Pe vic GirpDLE anp Fin (Fig. 9). Innominate or Pubic Bone (/n.).—Situated just behind the ventral. extremity of the clavicle. It is strongly connected by its dorsal extremity with the inner face of the clavicle opposite the upper boundary of the clavicular symphysis. It is also connected more or less for the whole of its length, and especially ventrally, with the innominate of the other side. In medium-sized plaice it consists of three pieces, but the tendency is for the lower extremity to calcify. The dorsal piece (1) is the largest and consists of a fairly strong posterior rod, produced below into a ventral spine, to which is attached in front a thin bony plate. Ventrally there is an obvious piece of cartilage (2) which gives attachment to a terminal car- tilaginous epiphysis (3), which in its turn supplies the surface over which works the sub-cartilaginous pad giving articulation to the fin rays. Fin Rays (/’.#.).—Articulate directly with the innominate except for the intervention of a sub-car- tilaginous pad as in the case of the pectoral fin. The fin rays here resemble those of the pectoral fin, and consist each of two pieces, but the latter diverge more proximally, and the posterior articular processes are terminal instead of sub-terminal as in the pectoral fin. The fourth and fifth of these processes, further, projected backwards and downwards instead of straight backwards as with the others (cp. fig.). None of the fin rays bifurcated, and there were the normal six of them in the pelvic fin of the specimen now described. 63 C.—THE BODY-CAVITY AND ITS VISCERA. We propose describing under this section the alimen- tary canal, the digestive and ductless glands and the renal organs, leaving the reproductive organs to be described in Section G. 1.—Tus Cantomic Spaczs. The derivatives of the embryonic ccelom are (1) the body cavity, (2) the pericardium, (5) the cavities of the ovaries (in the female), and (4) the cavity of the ureters. The body cavity is bounded dorsally by the kidney, which les underneath vertebra 2 to 14, anteriorly by the pos- terior fibrous wall of the pericardium, the lower portions of the clavicles, the innominate bones and the muscles of both limb girdles, and posteriorly by the Ist haemal spine (HS. 1, fig. 21) and the 1st axonost (1. Az.). It contracts ventrally, so that only a small region surrounding the anus is bounded by the ventral body wall. The lateral body walls are strongly muscular, and the parietal peri- toneum is deeply pigmented. ‘There is no posterior exten- sion of the body cavity on either side, such as occurs in the Sole, and is stated by Kyle to exist also in the Plaice. The nature of the cavities of the ovaries and renal organs is best considered with the description of those organs. 2.—THE ALIMENTARY CANAL AND ITS GLANDS. The alimentary canal may be conveniently divided into the following regions: cesophagus, stomach, duo- denum, intestine and rectum. (isophagus and stomach are distinguished from each other and from the rest of the alimentary canal by the differentiation of the mucous membrane. ‘The duodenum is the proximal 64 portion of the post-pyloric intestine which bears the pyloric ceca and receives the bile and pancreatic ducts. There is no essential difference between intestine and rectum, but it is convenient to distinguish the terminal portion of the alimentary canal from that immediately preceding it. The appearance presented by the viscera cn opening the body cavity of a large plaice from the ocular side varies considerably with the condition of the reproductive organs and with the sex- Fig. 20, pl. V., represents the ‘ relations of the viscera in a “‘ripe’’ and mature female. The great increase in volume of the ovaries has crowded the greater portion of the intestine towards the dorsal part of the body cavity; the duodenum is pressed for- ward; the rectum being more fixed than the rest cf the post-pyloric intestine is not much displaced: the stomach occupies nearly its normal position. Fig. 21 shows the condition of the viscera in a mature female which has just spawned. The greater portion of the intestine has been removed, however, in order to display the deeper viscera. It would form two S-shaped loops overlying and hiding most of the structures indicated in the figure. The @sophagus is very short, and almost immediately on entering the body cavity expands into the stomach. Its walls are very thick and are composed almost entirely of a transversely disposed layer of striated muscle fibres. The external longitudinal muscle layer is thin, and appears to consist of unstriated fibres. The mucosa con- sists of a layer of columnar cells crowded with “ goblet ”’ cells. As observed in the dead fish, the lumen of the esophagus is greatly reduced, though it is evident from the nature of the food that it is capable of considerable expansion. The layer known as the muscularis mucosxe does not 65 appear to be present in the intestine of the plaice. Nor is the peculiar “stratum compactum” of the submucosa which Oppel has described in some other fishes certainly present. The Stomach (fig. 21) is sharply distinguished from cesophagus and duodenum by the strongly developed transverse musculature at its proximal and distal ends. The transverse muscle layer is less strongly and the longi- tudinal layer more strongly developed than in the esophagus. At its pyloric end the transverse muscle layer becomes much thicker and forms the prominent sphincter pylori, a valve which projects into the cavity of the duodenum. There is also a very marked differentia- tion of the mucosa. In the esophagus this consists of a simple columnar epithelium with goblet cells. In the stomach the goblet cells disappear and the epithelium is evaginated to form a closely-set series of gastric glands over the whole internal surface. Each gland is a tubule, the internal portion of which is straight and the deeper portion convoluted. The straight or condueting portion has a wall consisting of columnar cells with a cement substance between them, and the lumen is relatively wide. The deeper or secreting portion has walls made up of large cubical clear cells, whilst the lumen is narrow. The sub- mucosa consists of loose areolar tissue containing blood vessels. The stomach lies along the dorsal wall of the body cavity, and the pylorus is situated at the posterior end of the kidney. i: The Duodenum lies along the posterior wall of the body cavity towards the eyeless side of the body. Its proximal end is slightly folded over the distal end of the stomach. Its wall (and that of the succeeding regions of the alimentary canal) is thin and consists of an outer longi- tudinal and an inner transverse layer of unstriated muscle H 66 fibres, of a sub-mucosa lying internal to these, and of a mucosa which is a simple columnar or cubical epithelium containing goblet cells. Four pyloric ceca (Ce. fig. 21) are present. Day (“‘ British Fishes,” vol. II., p. 26) states that only two are present, and Kyle apparently also agrees with this. When the intestine is distended with food these ceca may be obscured, but their presence may always be determined, and in the young fish (of 2 to 4 inches long) they are usually particularly noticeable. One is present on the dorsal and proximal extremity of the duodenum, two on the ventral and proximal extremity, and one on the mid-ventral line about an inch distant (in large fishes) from the pylorus. This last caecum may be the largest of the four. They have a wide lumen freely communicating with that of the duodenum; their wall is very similar in structure except that the muscle layers may not be so distinct. The Pyloric Ceca are only seen in Teleostomatous fishes. The number present is very variable, none being found in the Sole and 191 having been counted in Scomber. There has been much discussion as to their morphology and function. At one time they were regarded as the homologues of the pancreas—an organ which was then supposed to be absent in Teleostomi. They have been regarded as absorptive organs and as accessory digestive glands. Mordecai from observations on Clupea sapidissima supposed that they served to store up reserve food material. In the fishes ascending rivers to spawn, when presumably no food was being taken, the ceca were found distended with a brownish mucus-like substance which was absent at other times in the year. Edinger supposed them to exercise an absorptive function. Wiedersheim also held this opinion, and correlated their presence with the absence of a spiral valve (a device for increasing the 67 absorptive surface of the intestine). Many investigations have been made on their power of secreting enzymes, and the results obtained are confusing. Macallum* inyesti- gated the structures in Aczpenser, taking particular care to avoid the entrance of enzymes from the alimentary canal and pancreas, and found no certain evidence of a digestive action of their secretion on starch or proteid. Bonduoyt obtained the opposite results, finding the secre- tion of the ceca in many Teleosts to behave lke trypsin and to act strongly on starch and proteid. Macallum sup- poses that they represent the remains of a former series of digestive diverticula of the alimentary canal. These became restricted in most vertebrata to certain regions forming the digestive glands of the canal, but a variable number, however, persisted in Teleostomi as the pyloric ceca. The remaining portion of the intestine lies cn the ocular side of the body cavity. The duodenum passes into a tract of intestine which lies along the ventral and anterior walls of the body cavity to the left of the rectum. This is thrown into two S-shaped loops which terminate in the rectum. Near the anus the muscle layer becomes thicker, and the terminal portion of the rectum is also connected to the adjacent body wall by strands of connec- tive tissue. The Mesenteries are difficult to study on account of the convolutions of the intestine. They are best examined in a specimen well hardened with spirit. Two mesen- teric sheets appear to be present, though these may pos- sibly represent a single structure. One takes crigin from the dorsal and posterior walls of the body cavity in the * Jour. Anat. and Phys., vol. xx., pp. 604-636, 1886. + Arch. Zool, Exper., vii., pp. 419-460, 1899. 68 middle lines of the latter, and suspends the stomach and the greater portion of the intestine, but not the duodenum. It is curious that the anterior two-thirds of the stomach are attached to this mesenteric sheet along the mid-dorsal line, but towards the latter third the attachment is on the right side, as if the stomach had been longitudinally rotated from left to right. This mesentery is of course a double sheet of membrane which encloses the urocyst and ureter. A second, apparently distinct mesentery takes origin over the internal surface of the liver, and is attached to the duodenum and to the greater portion of the succeeding intestine. The latter is therefore attached to other parts by means of two mesenteric sheets. The second mesentery described above covers over the spleen and bile duct. The Liver (figs. 20 and 21) is asymmetrical. It con- sists of two lobes connected by an anterior isthmus of hepatic tissue. The larger of these lobes forms a flat cake lying on the eyeless side of the body cavity, and the smaller lies in the anterior and dorsal corner of the ocular side, its anterior surface being in contact with the pos- terior wall of the pericardium (Per. fig. 20). The organ is suspended to the body cavity wall by the two hepatic veins (V. hep. fig. 21) which penetrate the posterior wall of the pericardium, and by a fibrous sheet passing be- tween these and attaching the pericardial septum to the anterior surface of the liver. This anterior surface, as well as the lateral, is smooth, but the internal surface on the other hand is thrown into lobules (fig. 21) by deep furrows in which the factors of the hepatic portal system run, and along which they can be traced for considerable distances.. | The Gall Bladder (figs. 20 and 21) lies wedged in between the right hepatic lobe, the right surface of the 69 stomach and the right and dorsal surface of the spleen. It is a large thin-walled sac about one inch in diameter in large plaice. It is not imbedded in the liver in any way, and is attached to the latter organ by means of the bile duct only. Its posterior wall is thickened by a little nodular swelling. Its efferent duct leaves the anterior and ventral surface, turns back and runs on the internal surface of the right hepatic lobe partially imbedded in the tissue of the latter. Three groups of hepatic ducts enter it: one of these is situated near the proximal end of the duct, the other two are placed about midway on its course and enter it from opposite sides. Each is a group of three or four ducts. The eystic duct is the portion of the whole duct between the gall bladder and the opening of the first hepatic duct, the remaining portion is the common bile duct. ‘The walls of the bile duct are slightly iridescent, the distal extremity is thick and swollen, but encloses a very narrow lumen. It enters the duodenum between the paired pyloric ceca. Its opening into the duodenum is extremely small, and is very difficult to observe from the interior of the latter. The bile is a transparent slightly greenish fluid. The liver in the plaice, as in all other vertebrates, has a double blood supply, receiving blood from the veins of the intestine by the hepatic portal channels and from the dorsal aorta via the cceliaco-mesen- teric artery by the very small hepatic artery (A. hep. fig. 22). Of these sources the hepatic portal system of veins is by far the most important, and the system of intra- hepatic vessels containing venous blood is exceedingly striking in sections of the organ. The liver is essentially a tubular gland, but the hepatic tissue is disposed in strings of cells in which as a rule the lumen or bile capil- lary is only apparent from the radiate arrangement of the hepatic cells in transverse section of the strings. Within 70 the liver pancreatic tissue surrounding the bloodvessels is also present. The Pancreas is apparently absent in Plewronectes, but is really present.in a diffuse form. It will be noted that a perivascular tissue is stated to occur round all the visceral blood vessels and particularly round the factors of the hepatic portal system. ‘This tissue appears on naked eye examination as a band on either side of the vessel equal to or greater than the diameter of the latter. It lies of course within the thickness of the mesentery in which the blood vessels travel. It is particularly abun- dant round the vessels in the vicinity of the pyloric ceca, where it forms nodular masses which are also associated with fatty tissue. This diffuse perivascular tissue is the pancreas, which nowhere has the massive form charac- teristic of Hlasmobranch fishes. It recalls the extended form of organ characteristic of Mammalia, except that here the gland acini have a constant association with the blood vessels, forming an investment round the latter. No proper pancreatic blood vessels are accordingly present. Text-fig. 1, B. is a section through a small portion of the mesentery including two branches of the cceliaco-mesen- teric artery and a small factor of the hepatic portal system. It will be seen that the mesentery is greatly thickened round the blood vessels, and this thickening is really due to a mass of gland acini. No attempt is made in the figure to represent the structure of these acini, but their radiate grouping round the portal vein is indicated. Hlse- where in this section they had no definite arrangement. Nodular masses of pancreatic tissue are also present round the paired pyloric ceca, and from these some small ducts enter the ceca. Probably a number of such efferent ducts are present, but we have not determined this exactly. The gland acini have the structure characteristic of the (a Spinal cord i] FiGe2. Longitudinal f- 3) \WX& , Muscles 7 ae: be Dorsal aorta Salto (ae ke Segmental op eee [i> auick a A She EL ot a \ Se Palos ed aS 2 | \s_(\. -4Qesophagus \ ), ) } pS cate ar RMepaticiissuc=:_ss-5—< 9 Fic ILA K, .- Portal vein '~\ Mesenteric ~\? Arteries /-Pancreas ———— Muscle layer = Pigment layer J.J del. Section of liver, showing pancreas round a vein, x 225. Fic. 1. B. Section of a part of the mesentery, x 22. Fic. 2. Part of a transverse section of a 12 days’ old larva. Fic. 3. Transverse section of ovarian wall, x 26. ree AN. 72 pancreas. A lumen is not generally apparent, but the presence of such is generally associated with a particular phase of the activity of the gland which doubtless did not coincide with the fixation of our material of the organ. They are small and in a single transverse section are composed of few polygonal cells. Not only does the pancreatic tissue form a peri- vascular investment in the vessels in the body cavity, but it extends along the portal veins into the interior of the liver. Text-fig. 1, A. represents a section of a part of the latter organ, and shews a small portal vein cut in trans- verse section with two veinules opening out from it and passing between the hepatic cells. A single layer of pancreatic gland acini forms an investment for the vessel, and the whole les within a space in the hepatic tissue which is probably natural. The acini are elongated per- pendicularly to the surface of the vessel, and the whole is surrounded by a fibrous sheath which sends in partitions between the acini, becoming continuous with the fibrous wall of the vein. Here also a lumen is generally wanting, or is only with difficulty apparent in the acini. In Acipenser, Amia and Lepidosteus Macallum* has described very similar relations for the pancreas, and the description of the organ given by Gullandt for Salmo answers in all essential respects to that stated above. Macallum} has described the pancreas in Améurus as being imbedded in the liver round the interlobular veins. The diffuse condition of the pancreas seems to be characteristic of most Teleostomatous fishes hitherto investigated, and is most probably quite general. * Loc. cit. | Life history of the Salmon; Rep. to the Fishery Board of Scotland, 1898. } Proc. Canadian Institute, N.S. vol. ii., No. 3, pp. 387-417, 1884. 73 3.—IlIHE DucriEess GLANDS. It will be most convenient to consider here a group of glands (the thyroid, thymus, spleen and suprarenal bodies), though these structures have a widely different morphological significance and have most probably very different functions. They agree in being glandular bodies devoid of efferent ducts, and acting in modifying the com- position of the blood either by adding to it some sub- stance (internal secretion), or by withdrawing some por- tion of its constituents. The lymphatic portion of the kidney is also supposed to function in some such way, but this structure will be most conveniently considered together with the renal organs. The Thyroid in P/ewronectes is not a compact gland, and is relatively very small in mass. It consists of a number of separate alveoli situated along the course of the ventral aorta. It is difficult to find by dissection in the full-grown specimen, and must be identified in a fish sufficiently small to section as a whole, or by microscopic examination of the tissues surrounding the vessel in question. The separate alveoli of which it is composed are not bound together in any way, but lie loosely in the connective and fatty tissue in which the ventral aorta is imbedded. They are most abundant in the immediate neighbourhood of the origin of the Ist and 2nd efferent branchial vessels, and lie mostly ventral to the aorta, but may be found lateral, and even dorsal, to it. In a trans- verse section through the region indicated in a small fish 1} to 2 inches long there may be about a dozen alveoli present in a single section. Round the ventral aorta between the places of origin of the branches referred to and between the 3rd and 4th vessels few alveoli are pre- sent, though one or two may be found here and there. 74 Each alveolus is a small rounded or oval closed sac; the largest is about O°‘O7mm. in diameter, but they vary in diameter within wide limits. The wall is made up of a single layer of columnar cells outside which a delicate sheath may often be distinguished. Hach alveolus is filled with the colloidal substance characteristic of the thyroid, which usually stains slightly with eosin. It may be con- tracted away from the wall in section, but this appearance is most probably artificial and in life the alveolus is un- doubtedly filled. _ The thyroid originates in Salmo* (and probably in all Teleosts) as a median evagination of the ventral pharyn- geal epithelium which has no connection with the gill clefts. This evagination forms a little vesicle, the cavity of which at first communicates with that of the pharynx by a tubular stalk. Later on the stalk becomes solid and the vesicle separates entirely from the pharyngeal wall. It then shifts backwards towards the heart, and its wall begins to form hollow buds, which later on separate and become closed. These persist as the definitive thyroid alveoli. Paired rudiments do not, as in higher verte- brates, contribute to the formation of the adult gland. The Thymus (Zim. fig. 21) lies internal and slightly posterior to the posterior and dorsal corner of the oper- culum. On dissecting away the skin in this region a sheet of muscle fibres is seen originating at the cranial ridge connecting the 4th and 5th tuberosities (7'6. 4; Jb. 5) and inserted into the dorsal border of the opereulum. When this is dissected off the thymus is seen lying under- neath and immediately in front of the supra-clavicle (S.Cl. fig. 8), between this and a slip of muscle which originates in the pterotic at the base of the 4th “Maurer, Schilddriise u, Thymus der Teleostier. Morph. Jahrb., Bd. xi., pp. 150-175, 1885. 75 tuberosity and is inserted into the upper end of the clavicle. It is a flattened glandular mass occasionally covered with black pigment spots. In a fish of about 20 inches in total length it is about 14cm. in length, half that in breadth, and about 2mm. in thickness. It lies with one edge uppermost. Its artery appears to be a branch of the subclavian, its vein opens into the superior jugular. It lies immediately external to the roots of the vagus, and this association of the gland and nerve appears to be a constant one in fish of all sizes from the stage at which the structure is definitely formed. Its histological struc- ture is that generally characteristic of the thymus gland of vertebrata. It consists of small rounded cells closely packed together in a narrow-meshed reticulum of connec- tive tissue. The cells have large nuclei and attenuated cell bodies. The whole gland is surrounded by a loose capsule of connective tissue which is continuous with the internal reticulum. ‘There is no well marked difterentia- tion into cortical and medullary regions except that in adult specimens a narrow peripheral zone stains more intensely than the rest of the gland. Fatty tissue is little developed, and concentric corpuscles are apparently absent. The thymus in Salmo* (and probably in all Teleostean Fishes) develops, hke the organ in Elasmobranchs, from proliferations of the epithelium clothing the dorsal ex- tremities of all the gill clefts. These originally separate thymus buds fuse together while still in connection with the gill clefts, and for a time their cells can be traced continuously into the epithelium of the cleft. The whole organ then separates from its parent tissue and undergoes a backward shifting into its adult position. The Spleen (S//. fig. 21) les on the internal surface of the left lobe of the liver, usually wedged in between * Maurer, Loc. cit. 76 ° the pylorus, the lower surface of the stomach and the distended gall bladder. The bile duct (Zd.) lies imme- diately underneath it. It is covered over or attached by the mesenteric sheet connecting the duodenum and suc- ceeding portion of the intestine to the liver. A prominent branch of the cceliaco-mesenteric artery (A.sp. fig. 22) supplies it with blood. It is oval in shape, and in a large fish is about 1 to 2cm. in longest diameter. It is black in colour, and is very soft and easily torn. Its structure is the characteristic splenic one. ‘There is a strong fibrous investment which is continuous with strong trabeculee passing inwards: and dividing the whole gland up into lobules. The reticular formation within the lobules is filled with the characteristic splenic pulp, and in the centre of each lobule is a mass of densely aggregated lymphoid cells round which the texture of the pulp is looser. A prominent vessel passes to each of these nodules. ‘This structure is best seen in the organ of quite small fish, as in larger specimens it is much more obscure, and granular masses of black pigment are abundantly present. These pigment masses are composed of rounded granules of variable diameter. The Supra-renal Bodies are situated on the morpho- logical dorsal (spinal) surface of the kidney on the perpen- dicular surface which is apposed to the Ist haemal spine at about 4 of the length of this surface from the extreme tip of the kidney. To display them the kidney must be removed from the body, and since this is difficult on account of the soft pulpy nature of the organ in the fresh condition, the dissection is most conveniently carried out on preserved specimens. ‘The structures in question are then seen as two oval or rounded bodies lying close together, one on each side of the middle line to right and left of the common genital artery (A.gen.) at the point 77 where the latter vessel leaves the kidney. ‘They are yellow or pink in colour, and contrast strongly with the pigmented kidney. In a specimen of about 22 inches in total body length, the largest measured 5°5mm. in longest diameter. They le in little cavities in the kidney tissue, but project slightly above the surface of the latter, in the capsule of which they are enclosed. Their blood supply is from a twig of the common genital artery, and their minute structure is somewhat similar to that of a lymphatic gland. The capsule is continuous with a system of fibrous trabeculee traversing the whole organ. ‘These trabeculze form the coarser bars of a reticulum the meshes of which are crowded with small cells which may be described as lymphoid in appearance. According to Vincent they are secreting glands affecting the composi- tion of the blood by furnishing an internal secretion. Until comparatively recent times it was supposed that the suprarenal bodies were absent in Teleostean fishes.t It has, however, been shewn by Vincent* that they are probably universally present. As is well known, the suprarenals of mammalia consist of two morphologically distinct portions—the cortex and medulla. The latter has been stated to have been derived from certain of the sym- pathetic ganglia, and concerning the former an interesting suggestion has been made by Weldon and Grosglik. It is certain, however, that the most important relations of the suprarenal bodies are with the vascular, not the nervous system. 4.—Tnr Renat Oreans. The Kidney (figs. 20 and 21) lies along the whole dorsal, and part of the posterior wall of the body cavity, + Cp. Weldon—Head Kidney in Bdellostoma—Studies Morph. Lab. Cambridge, vol. ii., pt. 1, 1884. * Contributions to Anatomy and Histology of the Suprarenal Capsules. Trans. Zool. Soc., London, vol. xiyv., pp, 41-84, 1896-8, 78 from the 2nd to the 14th vertebra. Its greater portion is covered laterally by the transverse processes and ribs of those vertebra. (Its ventral surface forms the roof of the body cavity. Posteriorly it increases very much in thick- ness dorso-ventrally and oceupies the angle formed by the vertebral column and the nearly perpendicular 1st haemal spine and axonost, and laterally by the 7th to 10th ribs. It is for the greater portion of its length a single un- divided mass, but anteriorly is produced into two tapering and diverging horns—the head portions of the kidney, which he laterally and dorsally from the esophagus. The right unpaired cardinal vein runs along the middle line as far as the thickened portion, and is visible on its ventral surface. Dorsally the aorta lies in a groove in the middle line, and this with the cardinal vein separates the uriniferous tubular tissue into two paired masses. Only in the thickened posterior portion of the kidney is this tubular tissue continuous across its whole breadth. At the dorsal posterior corner of the kidney the caudal vein enters as a single vessel which almost immediately divides into paired portions. Apparently it does not become continuous with the cardinal vein, but breaks up round the uriniferous tubules, though there are doubtless anastomoses between the two vessels. The extreme ventral portion of the kidney is produced downwards into paired tips, and into these the paired genital veins (V.gen. fig. 21) enter. Along the dorsal surface of the crgan other paired venous trunks (parietal veins) also enter. The most anterior of these vessels is shewn in fig. 22 entering the extreme anterior tip of the head portion of the kidney. The caudal, genital and parietal veins, with the renal arteries are the afferent vessels of the kidney. The paired posterior cardinal veins are its efferent vessels. The Ureter (U/ret. fig. 21) leaves the ventral and pos- qe 79 terior surface of the kidney between the paired terminal processes into which the genital veins open. It is a single tube which immediately on entering the kidney divides to form the paired segmental (or Wolffian) ducts which traverse the entire length of the organ. The ureter rapidly expands into the urocyst (urinary bladder), a large thin walled sac lying between the ovaries (or testes) in front of and rather to one side of axonost 1. Its most expanded portion is near the rectum. Its cavity then rapidly diminishes, and the efferent ureter is a tube with an extremely contracted lumen. It passes to the right side and runs forwards in the dense connective tissue of the body wall, surrounding and posterior to the anus. It then curves laterally at a sharp angle and opens externally on to the surface through the urinary papilla. The latter (Ur. pp.) is an unpaired prominent projection of the body wall to the right side and immediately posterior to the anus. Detailed observations of the development of the Teleostean urocyst are few, but it seems most probable that it is of hypoblastic origin, and that its cavity, unlike that of the segmental duct, which is ccelomiec, is really a cloacal portion of the hind gut. McIntosh and Prince* give a description of the early condition of the vesicle in Molva, though its origin or latter fate is not described. In the youngest plaice (one week after hatching) of which we have made serial sections the united segmental ducts appear to open into the hind gut, which does not yet open to the exterior. In plaice a fortnight old the hind gut opens externally, and the urocyst ceases to have any connec- tion with its cavity, but opens independently in the middle ventral line of the body immediately behind the anus. The secretory tissues of the kidney are the uriniferous * Development and Life Histories of Teleostean Fishes. Trans. Roy. Soc., Edinburgh, vol, xxxy., pt. 3, No. 19, 80 tubules, which are somewhat sparsely distributed in the anterior part of the organ, but are very abundant in the thickened posterior portion. They are greatly convoluted tubules of varying diameter opening into the ureters. It is somewhat remarkable that Malpighian bodies are very difficult to find, and indeed seem to be absent, in some parts at least, of the kidney. This condition is connected with the vascular arrangements of the organ. By far the greater portion of the blood entering it is venous, and the arterial supply is very scanty. Two, or at most three, very small vessels originating directly in the dorsal aorta enter at the dorsal surface, and the common genital artery (A. gen., fig. 22) gives off several very fine arterial twigs which ramify in the posterior portion. The whole arterial blood supply is very small compared with the amount of venous blood entering by the renal portal veins. The lymphoid tissue which is so frequently met with in the kidneys of fishes is most abundant in the middle and anterior regions of the plaice kidney. It consists of very small cells, supported by reticular connective tissue, and filling up the interspaces between the blood vessels and the uriniferous tubules. Groups of pigment granules are scattered throughout this lymphoid tissue and give the organ its black appearance. ‘l'hey are small rounded granules of variable diameter, and of a greenish-black colour. ‘They lie freely among the lymphoid cells. The Pronephros and Head Kidney (Text-fig. 2).— The kidney in Pleuronectes is a mesonephros, and _ its paired ducts are segmental or archinephric ducts. The most common mode of origin of these structures in Teleosts is by a longitudinal evagination of somatopleure forming a groove which afterwards closes by constriction of its lips, giving rise to a tube. McIntosh and Prince state, however (/oc. czt.), that im Gadoids and Pleuro- 81 nectids the segmental ducts originate as solid rods, which afterwards acquire a lumen by the radiate arrangement of their cells. This condition, however, is most probably a secondary one, and the mode of development as a longi- tudinal groove seems most primitive. The cavity of the paired ducts of the adult kidney and that of the tubules is accordingly celomic in its nature. During larval life these ducts are the efferent channels of the pronephros— the larval excretory organ. The Pronephros is probably formed before the larva hatches from the egg. Text-fig. 2 represents the condi- tion of the organ in a plaice 12 days old. It is part of a transverse section through the anterior part of the trunk immediately behind the gill-bearing region. Here the segmental duct makes two or three convolutions and opens by a non-ciliated nephrostome into a small chamber. The right and left pronephric chambers lie side by side, separated by a thin septum. The dorsal aorta lies between them in the dorsal thickened part of the septum. A vascular tuft, the glomus, projects from the lateral wall of the aorta into each pronephric chamber opposite the nephrostome. The whole organ lies between the noto- chord and the csophagus. It has no connection, at least in the stage studied, with the body cavity, but there can be little doubt that the pronephric chamber is simply an enclosed portion of the general celom. ‘The whole organ is essentially similar to the pronephros of Lepidosteus as described by Balfour and Parker, except that in the latter form the pronephric chamber still com- municates with the body cavity by a richly ciliated funnel. The glomus is really a tuft of capillaries in communica- tion with the aorta. ‘The resemblance of the whole struc- ture to a Malpighian body of the kidney with its contained glomerulus will be noted. I 82 The pronephros degenerates at a variable age. We have seen it in a plaice of about 4 inch in length, that is at an age when the metamorphosis of the larva is com- plete and the adult asymmetry thoroughly established. The Head Kidney.—In a continuous series of sections from a plaice of about one inch long passing through the whole length of the kidney, the latter organ can be seen to be divided into three ill-defined parts. The posterior or thickened portion of the kidney is crowded with uriniferous tubules cut in various planes, Malpighian corpuscles can be seen, though these are very few, and the lymphatic tissue is (relatively) not abundant. Anterior to this, and occupying the thinnest middle portion of the kidney, is a region where the segmental duct, slightly expanded, alone persists, but no uriniferous tubules are present in the sections. his is the intermediate portion of the kidney. Anterior to this, and beginning at the plane of transition of stomach into cesophagus, is a region where the segmental duct becomes thrown into convolu- tions. Here, too, the kidney divides into the two anterior horns which lie on either side of the cesophagus. This is the head kidney, and it contains lymphatic tissue. This tissue is present through all the length of the kidney, but is more abundant in the anterior portion, and here it is aggregated into nodules with well-marked blood chan- nels between : that is, it has characters intermediate be- tween a true lymphatic and haemolymph gland. In the oldest specimens investigated this swollen anterior portion of the kidney has no traces of uriniferous tubules or segmental duct. It consists only of a modified form of lymphatic tissue with large blood vessels. In it are nests.of black pigment in the form of irregular granules, and its wall also is deeply pigmented. These anterior swollen portions are the degenerate 83 remains of the larval pronephros. This identification is confirmed by a study of various early stages. In the youngest forms examined (symmetrical larve one to two weeks hatched), the mesonephric tubules are absent, or are only just forming, while the segmental duct extends anteriorly as a straight line which becomes convoluted in its anterior extremity forming the pronephros. In a later stage (asymmetrical fish 3/5th inch long) the three regions of the kidney described above are well marked, the latter portion presenting all the characters of a mesonephros, while the nephrostomes and glomi of the pronephros are still recognisable, though much reduced. In an asym- metrical form about one inch in length, the intermediate lymphatic portion is relatively shorter, the mesonephric portion with its Malpighian corpuscles has extended fur- ther forward, the pronephric convolutions of the seg- mental duct are still present, but glomi, pronephric chambers and nephrostomes have disappeared and lym- phatic tissue and blood spaces are largely developed. Finally in the oldest adult specimens examined the head kidney contains only lymphatic tissue. The pronephros probably degenerates in all Teleostean fishes with the exception of one or two specialised forms. Organs formerly supposed to represent a persisting func- tional pronephros such as are present in Lophius have been shewn to be anterior extensions of the mesonephros, and others such as those present in Anguilla and Hsow are lymphatic organs with degenerate remains of the pronephric convolutions. Only in Fverasfer, and perhaps Zoarces, is the evidence conclusive that a functional pronephros exists in adult life. In Dactylopterus Calder- wood* has described an organ which he regards as a func- tional pronephros, but in this case it is still probable that * Jour. Mar. Biol. Assoc., vol. ii. (N. S.), pp. 43-46, 1891-2. ~ 84 we have to deal with an anterior extension of the mesone- phros, and conclusive proof of Calderwood’s hypothesis would only be afforded by tracing the embryonic pronephros into the structure so termed in the adult. Concerning the representative of the lymphatic head kidney of Teleostean fishes in higher vertebrata, Weldont put forward the suggestion that they exist in the supra- renal bodies. But he thought that the latter bodies were very generally absent among Teleosts, whereas it is now known that they are present in all forms sufficiently in- vestigated. It is only the medullary portions which are present, however, and Grosglikt has suggested that the homologues of the cortical parts of the suprarenal bodies of higher vertebrata are present in Teleosts as the lymphatic portions of the head kidney. According to Kimery, this lymphatic tissue is to be derived from the peritoneal epithelium. It is a formative blastema which remains 7 statu quo on the reduction of the pronephros. D.—THE BLOOD VASCULAR SYSTEM. The heart in Pleuronectes, as in all fishes, is a respira- tory one, and consists of a single auricle and ventricle. The de-oxygenated blood which it contains is propelled into the gills, and after passing through the respiratory capillary network in the branchial lamelle reaches a great loop-shaped vessel—the circulus cephalicus—lying beneath the base of the skull. From the circulus cephalicus, which contains oxygenated blood, the carotid arteries pass forwards to supply the brain and head, the cceliaco-mesenteric artery enters the body cavity and supplies the viscera, while the rest of the body is supplied by the dorsal aorta. De-oxygenated blood, t Loe. cit. { Anat, Anz., Jahrg., viii., pp. 605-611, 1885. 85 after traversing the systemic capillaries, returns through three main channels. The blood from the head returns directly to the sinus venosus by the jugular veins, but two portal circulations are interposed in the course of the blood returning from the viscera and the body. The caudal vein, the genital veins and other smaller vessels convey blood returning ‘from the great muscles of the trunk and from the reproductive organs to the kidneys, where these afferent veins break up into a network of capillaries, which are in close association with the renal tubules. From the kidney the blood reaches the heart again va the two ductis Cuvieri, er precaval veins; the blood from the stomach, intestine and spleen, containing the absorbed products of digestion, is conveyed to the liver by several afferent vessels known as the hepatic portal veins, and after traversing the hepatic capillaries enters the sinus venosus by the hepatic veins. The Pericardium and Heart.— The pericardial cavity (Per. fig. 20) is displayed by dissecting away the pectoral girdles with their muscle masses, which cover it laterally and in front; behind, it is bounded by a strong fibrous septum which separates it from the body cavity. Its walls contain black pigment. The heart, which nearly fills its cavity, is suspended by the hepatic veins traversing its posterior wall, by the ducttis Cuvieri above, and by the bulbus arteriosus in front. It lies in a curved position, so that the sinus and auricle are nearly vertical, the ventricle oblique and the bulbus nearly horizontal. The Cuvierian Ducts or precaval veins (V. pe. fig. 22) are wide thin-walled vessels passing slightly obliquely over the lateral surfaces of the esophagus. Their union beneath the latter forms the sinus venosus (Sin. V. figs. 21 and 22). Sinus and precaval veins together form a horse-shoe shaped chamber surrounding the cesophagus 86 on its lateral and ventral surfaces. In the middle of its ventral wall is the sinu-auricular orifice through which its cavity communicates with that of the auricle. This open- ing is guarded by a rather weak valve consisting of two membranous flaps, anterior and posterior in position, which hang down slightly into the cavity of the auricle. The large vessels opening into the sinus are the paired precaval veins into which open the paired posterior car- dinal veins (V. card.), the paired hepatic veins and the unpaired inferior jugular vein. The Auricle (Awr.) lies dorsal and anterior to the ventricle which it partly enfolds. Its external surface is lobulated, the postero-dorsal portion being produced into two notable lobes. Its walls are thin, but are strength- ened internally, especially on their dorsal and ventral por- tions, by interlacing muscle bands—the musculi pectinati. A deep auriculo-ventricular groove separates it from the ventricle. Its cavity communicates with that of the latter by the auriculo-ventricular orifice, which is guarded by three semi-lunar valves—pocket-shaped membranous flaps, the cavities of which face the cavity of the ventricle. ‘wo of these valves are large, and are nearly anterior and posterior, whilst the third is much smaller, and is situated laterally. The Yentricle (Ven.) les ventral and posterior to the auricle. Its walls are very thick, and are produced internally into ridges—the column carnee, which largely reduce its cavity. It is separated by a deep con- striction from the bulbus arteriosus (.A4.), which is a flask-shaped dilatation of the proximal end of the ventral aorta. Its cavity communicates with that of the bulbus by an opening which is guarded by two strong semi-lunar valves. The wall of the bulbus is composed of fibrous connective tissue free from muscle fibres. It is very 87 thick, and its internal surface is produced into longi- tudinal folds. The Afferent Branchial Vessels.—The ventral aorta (Ao. V.) continues forward the bulbus arteriosus. It runs forward in the middle line of the body beneath the esophagus and the ventral extremities of the gill arches. Its wall is composed of fibrous connective tissue appa- rently without muscle fibres. Like all the larger blood vessels in the plaice, it contains black pigment. Three afferent branchial vessels are given off at nearly equal intervals on each side. The first of these almost immedi- ately divides into two vessels of equal calibre (Af. Br. 4; Af. Br. 3) which supply the 4th and 3rd_holobranchs. Separate vessels (Af. Br. 2; Af. Br. 1) are given off to the 2nd and lst holobranchs. ‘The ventral aorta ter- minates by dividing to form the Ist afferent branchial vessels. Each afferent branchial vessel enters the gill at about one-third of the length of the latter from the ventral extremity, and immediately divides into two branches which traverse the whole length of the gill, running on the concave surface of the gill arch. The Structure of the Gills. —It will be convenient to describe here the minute anatomy of the gills before con- sidering their vascular arrangements. In the Plaice, as in most Teleostean fishes, there are four functional gills. Each gill is a holobranch, and consists of two separate series of gill filaments borne on the same branchial arch, each of which represents the demibranch or single series of filaments found on the one side of a gill pouch of an Elasmobranch fish. In the Teleostomi the septum which in the Elasmobranch separates the two adjacent demi- branchs has disappeared, with the result that the two series of filaments borne by the same arch have become closely opposed. 88 Each branchial arch (Text-fig. 4, A.) consists of several pieces, and is for the most part a densely calcified tube, the ends of which are cartilaginous. The interior of the tube is strengthened by bony trabecule. The gill fila- ments are borne on the posterior and convex borders of the gill arches. On first inspection it may appear that there is only one series, but closer study shews that there are really two. ‘This is particularly noticeable in the anterior gills, where the two series of filaments are of unequal length, so that all the anterior (or external) are longer than the posterior or internal ones. The bases of all the filaments borne on one branchial arch are fused together, but the greater portions of them are free from each other. In section (Text-fig. 4, B.) each filament is an isosceles triangle. They are so disposed that the apices of the triangles are directed towards each other and those of the one series alternate with those of the other. Obviously this arrangement secures the greatest economy of space consistent with the size of the filaments. Text-figs. 4 are a diagrammatic representation of the structure of the gill filaments. Fig. A. is a diagrammatic transverse section of a gill arch, and shews two filaments belonging to adjacent demibranchs. Hach filament is supported by a cartilaginous rod—the gill ray which runs down in its axis. These gill rays are super- ficially calcified ; their proximal ends are swollen and are all fused together, but the connecting portions are not calcified. The skeleton of a demibranch is therefore a comb-like structure. The rays of the adjacent demi- branchs are placed alternately, so that the knob-like calei- fied proximal end of one ray is placed opposite the car- tilaginous connecting portion of the iwo opposed ones. Dense ligamentous bands connect the fused heads of the gill rays with the branchial arch and with each other. 89 Text-Fic, 4. Structure of the Gills: x R Post-trematicus dorsalis JX 7 Branchial arch. _fEfferent branchial! vessel. wR Pre-trematicus primus X. __--R Post-trematicus ventralis IX. AE ae Afferent branchial vessel blige ets ae Base of gill ray. _- Filamenter muscle 2 ~>=~ Afferent-~ filamentar vessel. Capillary plexus. woe resee ny wonwenesneenennanennem-----Gill ray. -=--=----.--~-~------Efferent filamentrar vessel: Ext Int. : Gill ray. : Capillary plexus we Vascular lamella. A. Diagrammatic transverse section through BranchialZarch I. B. Trans- verse section of a double filament, showing the surfaces of two lamelle, C, Longitudinal section of a single filament in a plane at right angles to A, 90 These structures, the convex surface of the gill arch, the proximal ends of the gill rays and the ligaments, form a tunnel through which pass the efferent branchial vessels and certain of the nerves of the gill. A very definite little muscle takes origin in the cartilaginous connecting portion of every two gill rays, passes obliquely downwards into the tissues of the opposite gill filament, and is in- serted into the upper portion of its gill ray. This is part of an extremely pretty mechanical arrangement. It has been stated that each half filament is in section an isosceles triangle, and that the two series dove-tail into each other on account of their alternate arrangement. Obviously the contraction of the little muscles described above must have the effect of approximating the two series and ebliterating the spaces between all the separate filaments; conversely the relaxation of the muscles and the elastic recoil of the gill rays must separate all the filaments attached to a single arch, leaving a space between each two, and this is effected without any alteration in the total length of the gill. It seems extremely probable, though we have no experimental evidence on the point, that these movements do actually take place in life, and that they aid in the movement through the gill of the respiratory water. Text-fig. 4, C. is a longitudinal section through a gill filament in a plane at right-angles with that of 4, A. It shews that each filament consists of a flattened axis which bears on either side a close-set series of lamelle. The axis is strengthened by dense connective tissue, and con- tains the gill ray and the filamentar blood vessels. Text- fig. 4, B. is a transverse section of a double filament between the centres of two lamelle. It shews the position of the axis and the gill ray. If the branchial blood vessels are injected from the ventral aorta, a series of vessels become apparent on the 91 internal (with respect to the middle line of the gill arch) sides of the filaments. These are the afferent filamentar vessels (4, A. and B.), and they are connected with the afferent branchial vessels which run in the fused bases of the filaments outside the tunnel referred to. If, on the other hand, the system is injected from the dorsal aorta, a second series of vessels which run down on the outer surfaces of the filaments becomes visible; these are the efferent filamentar vessels, and they are vonnected with the efferent branchial vessels which run in the tunnel on the convex surface of the arch. At regular intervals along its course the afferent filamentar vessel gives off an arterial twig on either side of the axis of the filament which passes into the respiratory lamelle (4, B). Text-fig. 4, B. represents a surface view of two lamelle, the transverse section of the filament passing through the axis between two such lamelle. In a fortunate injection of the branchial system it will be seen that the lamellar branches of the afferent filamentar vessel on entering the lamelle immediately break up into very close capillary networks. This capillary network, seen from the side, is represented by the transverse black lines connecting the two filamentar vessels in 4, A. Hach lamella has a wall which at the base is composed of cubical cells, but which over the flat surfaces is a thin squamous epithelium. Within the space enclosed by this wall is the capillary network, and no other tissues. According to Plehn* the blood flows in spaces hollowed out of adjacent closely-fitting cubical cells. The capillaries have not the ordinary epithelial wall characteristic of such vessels. After having traversed this network the blood is received * Zum feineren Bau der Fischkieme. (Vorl. Mitth.) Zool., Anz, No. 648, 24 Bd., pp. 439-443, 1901. 92 by a very short vessel which opens into the efferent filamentar vessel. Respiration is effected by rythmical swallowing movements of the mouth and co-ordinated lftings of the opercula. ‘he water swallowed passes from the pharynx through the gill slits and over the surface of the gill fila- ments. Probably movements of the latter in the mode already suggested assist in this circulation of the sea water. ‘The gaseous interchange between the blood in the respiratory lamelle and the water takes place through the extremely thin walls of the latter and the walls of the capillaries. Only two thin epithelia separate the two liquids, and through these carbon dioxide passes from the blood to the sea water, and oxygen from the sea water to the blood. The respiratory area of the gills can be approximately calculated. The lengths of the gill filaments vary, and the greatest number of lamelle counted on any one side of a filament was 225; probably 150 will represent the average number to a side of a filament; there are two series of lamellae, of course, on each filament. Therefore we have :— | _Holobranchs. | No. of Filaments.| No. of Lamelle. is | 88 x 4 99600 ile | 78x 4 93600 II. | ae 86400 Feee werraree so 69600 | ae | “Total ...| 349200 } es ees ee Ja u 1164 93 Now the area of each lamella can be approximately caleu- lated, since it is nearly triangular. It is roughly 0°365 square millimetre. But since both the flat surfaces of the lamella are in contact with the water, the respiratory sur- face is double this, and is 0°730 sq. mm. x 349,200 = 254,916 sq.mm. ‘That is over } square metre. The total respira- tory surface of the gills is therefore that of a square, the length of the side of which is } metre. These calculations apply to a plaice of about 22 inches long. The area of the skin of such a fish is approximately 2,340 sq. em., or nearly } sq. metre. The respiratory surface of the gills is therefore about equal to the total area of the skin. The Efferent Branchial Vessels—The blood, after having passed from the heart and afferent vessels through the lamellar capillaries, is collected by four trunks on each side—the efferent branchial vessels. These open into the epibranchial arteries of each side. Posteriorly the two epibranchial arteries (A. ep.) unite to form the dorsal aorta; anteriorly they are connected together by a short anastomosing vessel (C2. ¢.). The loop thus formed is the circulus cephalicus. It is the reservoir into which the blood, after having undergone oxygenation in the gills, is poured, and from which it is distributed over the body. The efferent branchial system is best injected from the dorsal aorta after tying the coeliaco-mesenteric artery. It can be displayed after cutting away the greater portion of the operculum of one side, removing the opercular, sub-opercular and inter- opercular bones. ‘The remaining dorsal portion of the operculum is then forced outwards and held in position by a hook. The gill filaments should be cut away close to the arches. The vessels themselves are then seen, after dissecting apart and removing most of the muscles, pass- ing dorsally from the gill arches. The circulus cephalicus 94 and carotid arteries can be dissected by removing the greater portions of both opercula as indicated above, and then cutting away the gills, having previously divided the efferent vessels as far away from their attachments to the epibranchial arteries as possible. The head is then placed ventral side uppermost, and held in that position by hooks. The ventral portion of the parasphenoid must be removed in order to study the course of the internal carotid trunks. The first and 2nd _ efferent branchial vessels (Ef. Br. 1, Hf. Br. 2) open separately into the epi- branchial trunk. The 3rd and 4th (Hf. Br. 3, Hf. Br. 4) unite together to form a short trunk. On the left side this opens into the left epibranchial, on the right it opens into the cceliaco-mesenteric artery (A. cm.). It may appear, however, that the cceliaco-mesenteric, instead of taking origin from the epibranchial as represented in the figure, springs from the common trunk of 3rd and 4th efferent branchials. Immediately behind the union of the epibranchial trunks the subclavian arteries are given off from the dorsal aorta. Hach of these vessels (A. scl.) passes out trans- versely, then bends down ventrally and runs along the internal surface of the corresponding pectoral girdle, the muscles of which it supplies. Behind these vessels trans- verse arteries are given off from the dorsal aorta on either side, one to each segment. These vessels are not repre- sented in the figure. Several arteries leave the epibranchials to supply the muscles of the gill arches with blood. The most impor- tant of these is a paired vessel which leaves the epi- branchial immediately anterior to the place of entrance of the 2nd efferent vessel. It passes at first dorsally, then backwards and downwards over the 3rd and beneath the 4th efferent trunks. Approaching the middle iine it runs 95 ventrally among the muscles on the anterior border of the pericardium, where it apparently breaks up; a much smaller vessel takes origin from the dorsal portion of the Ist efferent vessel and runs backwards and downwards among the muscles of the Ist and 2nd gill arches, where it breaks up. These vessels are represented but not lettered in fig. 22. Two fairly large trunks take origin on each side from the ventral portions of the Ist and 2nd efferent vessels. Apparently they do not anastomose in Pleuronectes. The first, which is the hyoidean artery (A. Ay.), leaves the efferent trunk while still within the arch, and after giving off a small twig, which breaks up on the internal surface of the operculum, turns round dorsally and runs on the internal surface of the operculum externally to the cerato- hyal and symplectic bones. At the level of the upper extremities of the gill arches it breaks up into a number of branches which end in the filaments of the pseudo- branch (Ps. Br.). The Afferent Pseudobranchial Vessel.—The precise disposition of the afferent pseudobranchial vessels varies among T'eleostean fishes. In the greater number the afferent vessel is the hyoidean artery, which, moreover, anastomoses with the circulus cephalicus, so that the blood in the minute vessels of the pseudobranch may be derived from that in all the efferent branchial vessels. This is the arrangement in Gadus. In others, of which Salmo is an example, the hyoidean artery is the sole afferent vessel, and does not anastomose with the circulus cephalicus. In addition to these types of blood supply, Maurer* has deseribed another in //sov, where the afferent vessel of the pseudobranch is a twig of the circulus cephalicus and the * Beitr. zur Kenntniss der Pseudobranchien der Knochenfische, Morph. Jahrb., 9 Bd., pp. 229-252, 1883-4. 96 hyoidean artery contributes nothing. In Pleuronectes the organ is supplied by the hyoidean artery, and there is no evidence of an anastomosis of the latter with the circulus cephalicus beyond a doubtful communication between branches of the hyoidean and external carotid arteries. The Efferent Pseudobranchial Vessel is the ophthalmic artery. Blood, after traversing the capillaries in the pseudobranchial filaments, passes into this vessel (A.op.), which runs forwards along the roof of the pharynx covered ‘over by the mucous membrane. The two ophthalmic arteries approach each other in the middle line of the body, then separate and perforate the prootics together with the superior jugular veins, passing through the jugular foramina (f. jug. fig. 2). Hach vessel accom- panies the jugular vein and optic nerve of its side, and reaching the eye perforates the sclerotic and breaks up in the choroid gland. This peculiar arrangement is common to all Teleostean fishes, and has not received any satis- factory explanation. Joh. Miiller suggested that the pseudobranch was a gland furnishing an internal secre- tion, and that the object of the included capillary system of the pseudobranch was to equalise the intra-optical pressure by smoothing down the pulsations of the heart. But the blood in the ophthalmic artery has already passed through the branchial capillaries before reaching the pseudobranch, and there is no evidence of the elaboration of any internal secretion. The Pseudobranch.—It will be convenient to give some account here of the structure of this organ. It is situated on the inner surface of each operculum in a little concavity which lies behind the strong transverse muscles forming the roof of the pharynx, and which is formed by the abrupt termination of these in a posterior transverse ridge. It is situated mostly on the hyomandibular, but 97 its ventral extremity lies on the preoperculum. It is so situated that its attached base is exactly opposite to the dorsal portion of the first branchial arch, and ihe direction of its filaments is almost exactly that of those of the dorsal portion of the first holobranch, that is, posterior and slightly dorsal. The first branchial cleft is therefore bounded posteriorly by the anterior demibranch of the Ist branchial arch and anteriorly by the pseudobranch. The afferent pseudobranchial vessel or hyoidean artery runs along the external or deep-seated part of the base of the pseudobranch, and gives off a vessel to each filament. The efferent pseudobranchial vessel or ophthalmic artery runs along the internal or visible part of the base, and receives a vessel from each filament. The filaments of which the pseudobranch is composed are strikingly similar in appearance to those of any one of the true demibranchs, and their structure is the same in all essential points. Hach is made up of a flattened axis, on each side of which are borne a number of lamelle. The afferent filamentar vessel runs down the internal (with respect to the attachment of the organ to its arch) edge of this axis; the efferent vessel runs up the external edge. Small twigs are given off from the afferent vessel into each lamella, and in each of the latter they break up into a capillary plexus, as in the true gills, which empties its blood into a corresponding twig opening into the efferent filamentar vessel. Only about one-half of each filament projects freely into the opercular cavity. The basal halves are all attached to each other and to the epithelium clothing the inner surface of the operculum. The lamelle are mostly free, but many are attached together by their edges. They differ from the lamelle of the true gills in that their wall instead of being a squamous epithelium is made K 98 up of nearly cubical cells and contains numbers of goblet cells, in this respect resembling the general wall of the pharyngeal cavity or the epithelium covering the bran- chial arches. Probably the pseudobranch is not a functional respiratory organ, though its structure is very similar tc that of any demibranch of the posterior series of true gills. The walls of the vascular lamellz resemble mucous epithelia rather than membranes through which gaseous interchange may take place. And the blood reaching the organ has already passed through respiratory plexuses in the first holobranch. Undoubtedly it is part of the holo- branch which was formerly situated on the hyomandibular arch, and its situation suggests that it is the posterior demibranch of that gill. There are no traces of the pre- sence of a vestige of the anterior demibranch of this gill, and the structure of the organ is exactly that of a demi- branch, the respiratory surfaces of which are greatly modi- fied. Since no traces of the afferent vessel originating in the ventral aorta, which would have suppled a functional hyomandibular gill, are present, the vascular supply gives no certain indication of the homology of the organ.* The Pelvic Artery.—EHach of the 2nd efferent bran- chial vessels gives origin to an artery which almost imme- diately unites with its fellow of the opposite side, and the azygos trunk so formed runs backwards in the floor of the pharynx in the middle line of the body. Various small vessels are given off to the ventral portions of the branchial arches. This pelvic artery (A. pe.) then gives origin to a small vessel supplying the pericardium, the pericardial artery (A. per.), and continues backwards between the * Cp. the cranial nerves for a discussion of the nerve supply of the pseudobranch. 99 pelvic arches almost to the musculature of the body wall surrounding the anus. The Carotid Arteries—The portion of each epi- branchial artery anterior to the entrance of the Ist efferent branchial vessel may be spoken of as the common carotid artery. It is a very short trunk which divides into two vessels. The outer of these, the external carotid (A. Car.*), curves round behind the pharyngo-branchial segment of the Ist branchial arch, and runs forward on the ventral surface of the skull. Several branches are given off which break up on the internal surface of the operculum and on the base of the skull. The internal branches of the common carotids, the internal carotid arteries (A. car.), after perforating the skull at the junction of the prootics and parasphenoid by the carotid foramina (f. car. fig. 2), communicate by a very short anastomos- ing vessel (Cir. c.) which completes the circulus cephalicus. From this transverse anastomosing vessel three arteries take origin, which run anteriorly in the trough of the parasphenoid. The two external vessels, which are the internal carotid arteries, run forwards towards the nasal region of the skull. The internal median vessel divides, and the two vessels so formed run forwards in the trough of the parasphenoid, or eye muscle canal, accompanying the eye muscles. Each passes out with the corresponding optic nerve, and runs forwards towards the eye. The Visceral Arteries.—The cceliaco-mesenteric artery (A. em.) is an unpaired vessel lying entirely to the right side of the body. After leaving the right epibranchial artery it passes over the external surface of the right precaval vein, and gives off a small branch—the cesopha- geal artery (4. @.), which breaks up on the wall of the cesophagus. It then almost immediately bifurcates, and 100 from the point of division the ccelac artery (A.c@.) is given off. From this artery a small vessel arises (A.hep.) which turns backwards and enters the liver on the pos- terior surface of that organ. The two cceliaco-mesenteric trunks then pass internally to the right lobe of the liver. One vessel runs in ‘the mesentery, giving origin to branches which supply the greater portion of the intestine from the anus forwards. The other courses in the mesen- teric sheet connecting the liver with the duodenal loop, and supplies that portion of the intestine and the stomach. The dorsal aorta gives origin to a pair of arterial trunks in each segment, which supply the muscles of the trunk. Towards the posterior extremity of the kidney, a large median vessel—the common genital artery (A. gen.) —is given off, and passes downwards through the posterior portion of the kidney, sending small branches to the kidney and suprarenal bodies. This divides into two branches, one of which goes to each ovary or testis and the adjacent portions of the body wall. The dorsal aorta then passes backwards to the tail in the tunnel formed by the haemal arches. With regard to the venous system, we propose to describe the larger venous trunks only. All the blood from the head is returned to the heart va the paired superior and the unpaired inferior jugular veins. The Superior Jugular Veins (V. Jug.) are large thin walled vessels which will have been exposed in dissecting for the branchial vessels. They receive the blood from the eyes and adjacent parts, and accompany the eye muscles in the eye muscle canal, emerging from the latter through the jugular foramina (f. jug. fig. 2). ‘They then run backwards on the ventral surface of the skull over the dorsal extremities of the branchial vessels slightly dorsal LO1 and external to the epibranchial arteries, receiving in their course vessels conveying blood from the head and brain, and enter the precaval veins at the dorsal and anterior extremities of the latter. ; The Inferior Jugular Vein (V’. /ug.') is an azygos trunk running backwards under the ventral wall of the pharynx immediately above the dorsal aorta. It then passes upwards on the anterior wall of the pericardium, and may enter either the right or left side of the sinus venosus, though its ending on the right side seems to be the more common one. The Hepatic Veins (VV. hep.) are short wide trunks coming from the liver, which penetrate the posterior wall of the pericardium and enter the posterior part of the sinus venosus. The Renal Portal System.— The afferent vessels of this system are the parietal veins, the caudal vein and the genital veins. The caudal vein (V. cd.) runs forwards from the tail in the haemal canal immediately beneath the dorsal aorta. It enters the kidney at the most dorsal and posterior angle of the latter organ, and divides into two vessels which run forwards in the kidney and breaix up, but do not apparently anastomose with the cardinal vein. A short venous trunk comes from each ovary (or testis) and the adjacent portions of the body wall, and enters the kidney on each side near the extreme ventral tip of the latter organ. A series of veins from the muscles of the trunk enter the dorsal portion of the kidney on each side; these are the parietal veins. One such vessel is represented in fig. 22 as entering the anterior tip of the head kidney. The efferent vessels of the system are the cardinal veins, which run forward in the kidney. The right car- dinal vein (V. card.) runs along the middle part of 102 the kidney, and is visible on its ventral surface. We have said that the kidney at its anterior extremity is divided into two horns, which reach forward towards the heart. The right cardinal vein emerges from the kidney through the right horn and enters the posterior side of the right precaval vein. The left cardinal (as in the Cod) is a short vessel which begins at about the anterior third of the kidney, traverses the left horn, and enters the posterior side of the left precaval vein. ‘The two cardinals do not apparently anastomose with each other. The Hepatic Portal System. —The afferent vessels of this system are the portal veins carrying the blood from the stomach, intestine and spleen. ‘The smaller factors of this system have much the same course and distribution as the branches of the celiac and cceliaco-mesenteric arteries. They do not, however, unite to form a single hepatic portal vein, but enter the liver as a variable number of separate portal veins. Commonly there are (1) a trunk receiving the blood from the spleen and the greater portion of the intestine, and anastomosing with (2) a vein receiving the blood returned from the loops of the intestine posterior to the pylorus; (3) a smaller vessel draining the region of the pylorus, and (4) a vein coming from the stomach. ‘These vessels enter the internal sur- face of the liver principally on the larger left lobe, and run for some distance parallel to and immediately beneath the surface, so that their ramifications can be easily traced. Their precise number and distribution in the liver varies ; five such trunks are represented in fig. 21, cut off close to the liver surface (Vp.) The apparent calibre of the intes- tinal veins, and to a less extent the arteries also, is increased by. the presence of the perivascular glandular tissue referred to above. The efferent vessels of the hepatic portal system are the two large paired hepatic —_—s 108 veins (V. hep.) which enter the lower portion of the sinus venosus on its posterior side. E.—THE NERVOUS SYSTEM. We shall commence our description of the nervous system with the brain and spinal cord, then proceeding to the cranial and spinal nerves, and finally to the sympathetic nervous system. 1.—TuHer Bratn anp Spinat Corp.* (Figs. 28, 30, 51). The brain of the Plaice may be conventionally divided into four regions, including the following structures : — A. Hind- Brain —This comprises the medulla oblongata, which itself includes many structures that can only be regarded as the continuations of corresponding ones in the spinal cord, and the cerebellum. The latter consists of a body and the anterior valvula cerebell. B. Mid-Brain. —Formed by a base (crura cerebri) and side wall, and the tectum opticum or tectum mesencephali (optic lobes). C. IT ween-Brain.—Represented by three parts: (1) the epithalamus (epiphysis generally and the ganglia habenule); (2) the thalamus (optic thalami—thalamence- phalon); (3) the hypothalamus (corpus geniculatum, * The following works will be found to contain references either to the brain of the Plaice or to allied Pleuronectids :—Cattie, Arch. Biol., iii., p. 150; le Roux, ‘‘ Recherch. Syst. Nerveux Téleostéens,’’ Caen, 1887 Mayne, ‘‘ Optic Nerves,” Todd’s Cyclopedia, part xxvi.; Malme, Bihang K. Svens. vet.-akad Handlingar, xvii.; Mayer, Verhand. K. Leop.-Carol., xxx.; and Steiner, ‘‘ Entstehung d. asymmetrischen Baues der Pleuronec- tiden,” 1886; a recent important work, by J. B. Johnston, on the brain of Acipenser (Zool. Jahrb., Abth. Morph., xv.), may be used as a starting point in studying the brain of Fishes in detail. 104 corpus mamiillare, infundibulum, lobi inferiores, saccus vasculosus and pituitary body). D. Fore -Brain.—This may be considered as including the epistriatum, striatum proper and the membranous pallum, together with the bulbus olfactorius. The roots of the cranial nerves will be described in the section on the nerves. In a dorsal view of a well-preserved brain we note the following characters :—First the relatively small size of the brain. This is seen also in the Cod and in Teleosts generally. The small brain lies in the large cerebral cavity, surrounded by a packing of areolar connective tissue loaded with fat, and seems to be very dispropor- tionate to the size of the fish. Then the asymmetry of it is at once striking. The spinal cord, on entering the brain case, turns slightly to the left, but opposite the cerebellum it swerves markedly to the right, so that a median line would pass through the left striatum instead ot between the two striata. In the medulla the great reduction of the terminal bud system that has taken place involves the absence of the lobi vagi. Also the lateral line system is not suffi- ciently robust to have produced that exaggeration of the tuberculum acusticum known as the lobus linee lateralis. The medulla is therefore smooth, and presents no obvious traces of its ganglia. On removing the vascular covering of the fourth ventricle known as the choroid roof, the ventricle itself is seen to be apparently divided into two parts by the partial union over its roof of the medio-lateral portions of the tuberculum acusticum, forming an elliptic- shaped opening behind (calamus scriptorius) and a triangular one in front, with its apex directed backwards. The cerebellum, of which the body only is visible in the undissected brain, is small and globular. This is what 105 one would expect, seeing that it is connected with the general activity of the organism, and the Plaice is sluggish in habits. : The mid-brain is represented on the dorsal surface on each side by the tectum opticum (optic lobe). These are very large bodies almost spherical in shape, and charac- terised in dead and preserved specimens, and doubtless in life also, by a deep furrow, which extends backwards in a curve from the anterior margin of each lobe for about half its antero-posterior diameter. As is usual in Teleosts, the “tween-brain hardly appears at all on the dorsal surface of the brain, being excluded from it by the meeting of the two striata and optic lobes. However, a small portion of its membranous roof is visible, and from this there is seen emerging by the triangular space formed immediately in front of the median apposition of the two optic lobes, the extremely fine pineal tube. In sections it is seen to arise as an evagination of the roof of the third ventricle almost behind the ganglia habenule and in front of the posterior commissure. It then passes forwards over the pallium of the left striatum and swells into the large pineal gland lying on the pallium near the anterior extremity of the left striatum. By pressing apart the optic lobes there may be seen immediately in front of the exit of the pineal tube the ganglia habenule and the plaited choroid roof of the third ventricle. In a well-preserved brain the membranous pallium of the fore-brain is very obvious. It is a large oval sheet, with its long axis at right-angles to that of the brain, and almost equal to that of the optic lobes. It is a very thin membrane, and appears thicker than it really is on account of the coagulated cerebro-spinal fluid in the ventricle. The corpora striata are also visible through the pallium. 106 On removing the pallium, it will be noted that there are no lateral ventricles, but a large single median prosocoele. In the floor of this are raised up the large corpora striata separated dorsally by a wide fissure, but connected below by the anterior commissure, and constituting the solid cerebral hemispheres of older authors. ‘These are con- siderably smaller than the optic lobes, and the dorsal surface of each is marked by a somewhat complex furrow (“sulcus ” of former authors—see fig. 30). In front of and below the striata are the olfactory bulbs, from which the olfactory nerves originate. ‘The left is smaller than the right. On the ventral surface of the brain the most notice- able structures are the appendages of the “tween-brain. ‘The lobi inferiores are a pair of large bean-shaped bodies opposed by their median surfaces. In the middle line immediately in front of these is the spherical pituitary body. The apposition of the pituitary body and lobi inferiores is not complete, and a triangular space is left by which there emerges on to the ventral surface of the brain the red thin-walled saccus vasculosus. This is at its origin a very narrow tube, but 1t expands and passes straight backwards in the middle line over the opposed lobi inferiores. It is dilated behind, and ends blindly shghtly posterior to the hinder border of the lobi inferiores. In the adult the pituitary body (hypophysis cerebri) and saccus vasculosus are essentially glandular organs receiv- ing a marked nervous supply from the infundibulum. According to most recent authors the saccus at least “probably forms part of a mechanism for secreting, or otherwise controlling the pressure of, the cerebro-spinal fluid. It may affect the heart beat and blood pressure by way of the vagus” (J. B. Johnston). The crossing of the optic nerves is very obvious in the 107 Plaice, as it is effected some distance in front of the pituitary body, and is not hidden by the olfactory nerves on the ventral surface. It is also quite clear that they merely cross and do not exchange fibres, whilst their plaited nature is at once revealed by a little simple dissec- tion. On removing the optic nerves the two small and asymmetrical olfactory bulbs are well seen lying largely under the anterior extremities of the two striata. In the medulla the ventral fissure of the spinal cord is continued as far forwards as the base of the lobi inferiores, where it slightly expands. Regarding the ventricles of the brain, the central canal of the spinal cord appears in the sections as a pin hole. It begins to widen rapidly into the fourth ventricle (myelocoele) at about the posterior region of the auditory organ. The ventricle is at first very deep from above downwards and very narrow from side to side. It soon opens above, and is only closed in by the choroid roof. The peculiarity of the roof of this ventricle has been already mentioned. In front of the expanded portion it becomes completely roofed over by the tuberculum acusticum, and at the same time is reduced to a very small size. Opposite the junction of the medulla and cere- bellum it again expands, but does not communicate with a cerebellar cavity (metacoele), the cerebellum being solid. In front of the body of the cerebellum it passes into the aqueductus Sylvii (mesocoele—iter a tertio ad quartum ventriculum), roofed over behind by the valvula cerebelli and communicating on each side and in front with the large space enclosed by the tectum opticum (optocoele). In front, the latter opens below into the third ventricle (thalamocoele), bounded laterally and below by the thalamus (optic thalami) and above in front by the choroid roof. The third ventricle is prolonged downwards and 108 somewhat backwards into the hollow infundibulum. If this be now traced posteriorly, it is found first of all to communicate with the cavity of the pituitary body. Almost at the same time, but more dorsally, it is pro- longed on each side into the large cavities of the lobi_ inferiores, whilst finally it communicates with the cavity of the fine stalk of the saccus vasculosus. The third ventricle therefore is continuous with the cavities of the pituitary body, lobi inferiores and saccus vasculosus. The infundibulum of the Plaice is difficult to delimit, as it is largely merged into the floor of the thalamus. Inciden- tally we may draw attention in the lattér to the very large paired nucleus rotundus, which is very striking in sec- tions. Anteriorly the third ventricle passes into the large median ventricle of the fore-brain (prosocoele), roofed over by the pallium. The prosocoele is not prolonged into the bulbi olfactorii as a rhinocoele, the bulbs being solid. Cunningham makes two assertions on the brain of the Sole that appear to us to require confirmation. One is that the “position of the brain is almost entirely unaffected by the change which has taken place in the normal position of the fish,” and the other is that “ the left olfactory lobe is somewhat larger than the right, a differ- ence which is related to the great development of the left olfactory capsule.” On the other hand, Malme states of the Sole (op. cit., p. 34) that “imsbesondere . . 2 Saas ist der rechte Lobus [striatum ] (derjenige der Augenseite) viel grésser als der linke,” and again that in Pleuronectids generally ‘‘der Bulbus der Augenseite ist stets der grésste.” Malme’s observations agree with ours on the Plaice. Again, Cunningham apparently overlooks the work of Rabl-Riickhard on the brain of Teleosts, and describes what are really the corpora striata as receiving prolongations from the third ventricle. 109 In the Spinal Cord we wish to direct attention to two pecuharities only. The first is the giant ganglion cells that are found in the dorsal fissure. Transverse sections of the cord will demonstrate these quite easily. They have been studied especially by J. B. Johnston,* Sargentt and Dahlgren.t The latter author, who has devoted his attention particularly to the Pleuronectide, states that these very peculiar cells are the first ganglion cells to be differentiated in the embryo flat-fish, and that they become an important and permanent apparatus in the adult. In an adult fish they are seen to form a row of very large nerve cells in the median dorsal fissure, and their neurites pass backwards to form an isolated fibre tract on the median side of each dorsal horn. Their exact distribution and function are unknown, but Dahlgren suggests that the neurites pass out with the dorsal roots of the spinal nerves and are connected with the sensory supply of the unpaired fins. Sargent finds in Ctenolabrus that the giant cells are connected with a fibre bundle passing forwards through the cord and medulla, and emerging by the ventral root of the trigeminus nerve. If this be true then the fifth nerve of this fish possesses a nerve component not hitherto recognised, and it would be interesting to study the giant cell apparatus from the point of view of the component theory. The second peculiarity of the cord is one which it shares with all Teleosts, and that is in the presence of the very interesting rod or fibre within the lumen of the central canal known as Reissner’s fibre. This fibre has been investigated recently by Sargent,§ who finds that it “extends through the whole length of the canalis centralis * Jour. Comp. Neurol., x., p. 375. t Anat. Anz., xv., p. 212. + Anat. Anz., xili., p. 281. § Anat. Anz,, xvii., p. 33, and Proc. American Acad. Arts and Science, xxxvi., No. 25, 110 of the cord and continues cephalad through the 4th and Srd ventricles to the anterior end of the optic lobes,” where it passes into the brain tissues. It is not a single fibre, but a collection of axis cylinders, and is therefore a fibre tract. Some of the fibres in the tract originate in cells situated at the posterior extremity of the central canal, and pass forwards to the tectum opticum. Others originate in cells in the tectum opticum and pass back- ‘ wards as far as the “ posterior canal cells.” The tract, therefore, contains fibres coursing in two opposite direc- tions. According to Sargent this unique apparatus forms a “short circuit between the visual organs and the muscu- lature, and has for its function the transmission of motor reflexes arising from optical stimuli.’ It is most highly developed in active fish, and is entirely absent in the blind vertebrates of the cave fauna. 2.—TuHE CrantaL Nerves (Fig. 23). In spite of the fact that the cranial nerves of Fishes have been more or less investigated for about two and a half centuries, it is only within the last few years that our knowledge of them has assumed a form likely to be at all lasting. Although these results were made possible as long ago as 1811 by the enunciation of Bell’s law, and although this law was very ingeniously developed and applied to Fishes in 1849 by Stannius, who has never received due credit for his work, it was only in the eighties that Gaskell stated his “ four root theory ’’ of the spinal nerves, which showed that there were represented in each spinal nerve four kinds of fibres instead of the two assumed by Bell’s law. The attempt to strictly apply the four root theory to the cranial nerves of lower vertebrates has not only been Tt] unsuccessful, but it has actually retarded knowledge by diverting the energies of investigators into an unprofitable channel. The work on the cranial nerves of the frog’s tadpole, published in 1895 by Strong, distinctly proved this, for he showed that, for example, there were three systems of sensory fibres in the cranial nerves of the larval frog, one of which must be considered characteristic of the head and not represented in the spinal nerves at all, and another only partly so. One of the first results of Strong’s work was to show that the old system of classifying the cranial nerves of Fishes into ten formal pairs was essentially unsatisfactory, and that attention should be concentrated rather on the various definite systems of nerve fibres characterised by their structure, central origin and peripheral distribution, than on those heterogeneous collections of nerve rami ‘known as the “cranial nerves.” We must, however, in the meantime adhere to the old classification, until suffi- cient work has been carried out on the new lines to justify a revision of the cranial nerves, and to ensure for its findings some permanent value. The new theory of the cranial nerves is known as the “component theory.” It takes advantage of the fact that the fibres forming them, and omitting the olfactory and optic nerves and the sympathetic, which present problems of an altogether special nature, fall by reason of their functions and certain structural relations into five fibre systems, three of which are sensory and two motor. Hach system is delimited by a uniformity of peripheral ter- mination and a special and characteristic origin in the brain, and each system may appear in a variable number of cranial nerves as a component of those nerves. It is therefore indispensable, as we have done in the Plaice, to work out the whole course of the nerves by means of serial 112 sections. Microscopie work either on the brain or the peripheral nerves only is inadequate, and dissection, as a means of research, has but a very doubtful value. The only fish which has been thoroughly investigated according to the component theory is J/enzdia—in-an important work published recently by C. J. Herrick, who truly remarks: ‘‘ Until each component can be isolated and treated as a morphological unit, and then unravelled in its peripheral courses through the various nerve roots and rami—until this is possible, no further great advances in cranial nerve morphology can be looked for even among the lower vertebrates, still less in man.” The five systems of fibres which variously compose the cranial nerves of the Plaice are as follows :— 1. General Cutaneous or Somatic Afferent System.— These fibres, which undoubtedly correspond to the cutaneous fibres of the spinal nerves, are derived from continuations of the dorsal horns of the spinal cord, which form two longitudinal bundles in the medulla known as the spinal vth tracts. These fibres in the Plaice leave the brain by the roots of two cranial nerves only—the vth and the xth. In the former case their ganglion is the Gasserian ganglion, in the latter the jugular ganglion. The cutaneous fibres in the facial nerve are distinctly derived from those of the fifth. The fibres of this system are distributed generally to the skin, and do not end in any specialised dermal sense organs. Hypertrophy of this system produces a corresponding hypertrophy of its centre in the central nervous system, as witness the remarkable lobes at the anterior extremity of the spinal cord of Prionotus (Morrill). 2. Somatic Efferent System.—Represented by the heavily myelinated eye muscle nerves (i11., iv. and vi.). This system is of course largely present in the so-called - 115 “hypoglossal” nerve, or first spinal, but we do not con- sider this to be a cranial nerve in fishes. 3. Communis (Viscero Afferent?) System. — Partly synonymous with the fasciculus communis system of Osborn and Strong. A striking feature about this sensory system is that it may innervate both ecto- and endo-dermal surfaces, and it may therefore be disputed whether it is a visceral nerve that has invaded the skin, a somatic nerve that has invaded the visceral surfaces, or a complex of more than one component. The latter seems perhaps the most probable. The fibres of the communis system are fine and lightly myelinated, and are _ distributed peripherally as follows :—(qa) to the special sense organs in the outer skin called “terminal buds,” 7.e., to all the definite sense organs of the skin not belonging to the lateral line system. This part of the component has been reduced in the plaice; (+) to taste buds in the mouth; (ce) to the general mucous surfaces without the interven- tion of sense organs at all. The ganglia and cranial nerves into which the system enters are: (a) the genicu- late ganglion (vii.), glossopharyngeal ganglion (ix.), and the intestinal and four branchial ganglia of the vagus (x.). Any communis fibres in the trigeminus arise from the communis facialis. The central origin of the component is the Lobus vagi, and the enormous vagal lobes of Carpiodes are simply due to the hypertrophy of the com- munis vagi component in this fish (Herrick). Further the so-called Lobus trigemini of some fishes (Amzurus) is due to the hypertrophy of the communis facialis, and hence it should be called Lobus facialis. 4. Viscero Efferent System.—This comprises the motor roots of the vth, viith, ixth and xth cranial nerves. Each of the first two has its own motor nucleus in the brain, but the two latter arise from collections of cells L 114 supposed to represent the nucleus ambiguus. The fibres of this system are heavily myelinated. 5. Acustico Lateral System.—Includes the auditory and lateral line nerves. Let it be emphasised at once, what is taking a long time to filter down into the text- books, that the lateral line nerves can only be associated with two cranial nerves—the vith and xth. The lateral line fibres in the fifth nerve are always derived from the facial, those in the ixth (when present) from the vagus. The fibres of this system are distributed to the ear, to the sense organs in the lateral line canals, and to those lateral line sense organs lying free on the skin, and known as pit-organs. Its ganglia are the dorsal and ventral lateral ganglia of the facial, and the lateralis ganglion of the vagus, and its fibres are very large, being in fact the coarsest in the body. Its central termination is the tuber- culum acusticum of the antero-dorsal region of the medulla—associated with the cerebellum. The hyper- trophy of the lateral line nerves produces an exaggeration of the tuberculum acusticum well marked on the surface of the brain and called by Johnston the Lobus, linez lateralis. This lobe has also been called the Lobus trigemini by the older writers, and when associated with lateral line fibres it may well receive the name given it by Johnston. Otherwise it should be called the Lobus facialis (see above). Nervus Olfactorius*—I. (Figs. 25 and 28.) As considerable asymmetry is exhibited by these nerves, both sides will be described. *For the cranial nerves of Teleostean Fishes compare especially the following works ;—Allis (Amia), Jour. Morph., ii. and xii. ; Cole (Gadus), Trans. Linn. Soc., ser. 2, vii.; Desmoulins and Magendie (nerves of Rhombus), Anat. Syst. Nerveux, Paris, 1825; Herrick (Menidia, Gadus, and Amiurus), Jour. Comp. Neurol., ix., x., and xi.; Juge (Silwrus), Rev. Suisse Zool., vi; and Stannius (general), Rostock, 1849. The works of Herrick are most important so far as the Plaice is concerned, and should certainly be consulted. We have purposely adopted, as far as possible, the same reference letters, in order that the comparisons may be facilitated. aes 115 The right Bulbus olfactoriust lies mostly under the corpus striatum (the latter is the cerebral hemisphere of older authors). Behind, it is free, unconnected with the striatum and ends bluntly, but in front it acquires a firm connection with the striatum. Anterior to this again it separates once more from the striatum. So far it has been increasing in size, but it now begins to taper down, its ventral portion becomes fibrous, and its dorsal divided into two. The upper or cerebral portion disappears in front, and the remainder narrows down into the cylindrical nervus olfactorius. Both olfactory nerves lie to the right of the upper or left optic nerve. As the nerve passes forwards it becomes divided by connective tissue strands into two or more fasciculi, each of these again being further subdivided into small bundles of fibres. The right olfactory passes through the foramen olfactorium in the right prefrontal, turns up at once and breaks up in the olfactory lamine of the right nasal chamber. The left Bulbus olfactorius is not free behind like the right, but passes imperceptibly into its striatum. Nor is it situated below the latter, but between the two striata (see fig. 28). The appearance therefore of this portion of the brain is very asymmetrical, and suggests a rotation towards the right side of the ventral axis of the brain only. The left bulbus is perceptibly smaller than the right, but the left striatum extends further forwards than its fellow. The bulbus separates from the striatum in front, becomes fibrous at its right ventral corner and gives off the left olfactory. nerve, which passes at once to the right side, so as to lie near the right bulbus. The left + This structure is also called by some authors the Tuberculum olfactoriwm (Stannius) and Lobus olfactorius. We have no space to discuss the precise significance of each of these three terms, if indeed they have any (but see Elliot Smith, Jour. Anat. and Phys., xxxv., 1901). 116 bulbus has no direct connection with its nerve in front as on the right side, nor does it extend as far forwards as the right one. There is much more difference in the size of the two olfactory nerves than one would expect from the sizes of their bulbs; the left being only } the size of the right. Nor do its fibres take such an intense stain with the osmic acid. For some distance the two olfactory nerves course together, but finally the left separates from the right and passes upwards towards the eyeless side of the body. It then traverses the olfactory foramen in the left prefrontal, and at once passes straight upwards to break up in the olfactory lamine of the left nasal chamber. The left nasal organ is much smaller than the right (cp. fig. 25, n. olf., n. olf.1), and hence the small left nerve. It is also situated somewhat behind the right, and therefore the left olfactory is the shorter of the two. Nervus Opticus—ll. As in all lower vertebrates, the fibres of the optic nerve arise mostly from the roof of the mid-brain (tectum opticum), and as is usual in Teleosts they pass forwards over the ventricle to collect at the anterior extremity of the optic lobe, and then course sharply downwards and forwards to reach the surface of the brain. The optic chiasma is a simple crossing without any intermingling of fibres, so that the nerve to the right eye, for example, arises exclusively from the left side of the brain. As in Menzdia the nerve to the left eye is uppermost at the crossing. Kach optic nerve, as is usual in Teleosts, consists of a thin wide ribbon so thrown into longitudinal folds as to form a round nerve, and each exhibits 3} folds.* If the optic nerve could be flattened out the width of the ribbon would “The number of the folds increases with the size of the nerve, judging from our sections of young plaice at different stages, and also from the condition in the adult (see fig. 28). Lee be about 7 of the maximum thickness of the body. For a time the right optic nerve lies directly under the left, and both immediately under the right bulbus olfactorius. The right passes straight out to its eye, but the left curves over towards the left side. Both reach the eye at about the same level, perforate the sclerotic and retina, and spread over the concave surface of the latter in the usual way. Owing to the fact that the left eye is situated over the right, the optic chiasma is less emphasised in the Plaice than in a symmetrical fish. We now proceed to the description of the eye muscle nerves (fig. 23), taking them in their numerical order. Nervus Oculomotorius—lIll. The nucleus of the third nerve (iii.) lies dorsally on the floor of the mesocoele very near the middle line, and mostly just over the fasciculus longitudinalis dorsalis. There is no crossing as in the case of the patheticus. The fibres of the right oculomotor curve round the fasciculus and pass backwards and downwards through the brain substance, to emerge as a large nerve on the ventral surface of the brain just above the lobi inferiores. Imme- diately it leaves the brain the nerve takes a sharp turn forwards, and in due course fuses with the patheticus. It passes downwards on the outer side of the lobus inferioris and between this and the v.-vii. complex. After liberating the patheticus again it courses downwards inside the skull, passes through the meninges, and enters the cup-shaped cavity formed by the parasphenoid and known as the eye muscle canal. Here it divides into a smaller upper and a larger lower nerve. Now the oculomotor consists mostly of large and well myelinated fibres, but it also contains 118 some small lightly staining fibres. These are for the most part handed over to the lower nerve, and collect at its outer side. Finally they pass over into the ciliary ganglion and hence form the radix brevis (fig. 26, rv. 6.) of that ganglion. The upper division of the oculomotor (7.s.) passes into the Rectus superior muscle of the eye. Its small fibres are distributed to the smaller fibres of its muscle. The lower division soon after leaving the ciliary ganglion, to which it has been closely opposed, passes sharply downwards and forwards accompanied at first by the ramus ciliaris brevis from the ciliary ganglion (fig. 26, cil. b.). It splits into three almost equal branches, which soon take up the following positions in the vertical plane, and are as below: — (a) Dorsal branch (r. zt.). To rectus internus. Passes upwards and inwards and reaches the ventral surface of the lower or right optic nerve. It subsequently breaks up in its muscle between and below the two optic nerves. (0) Intermediate branch (7. zf.). To rectus inferior. Divides into three principal twigs which enter their muscle in the order shown in the figure. (c) Ventral branch (02.). To obliquus inferior. Descends and crosses the palatinus vii. internally and for some distance les just below and internal to it. It then rises, crosses the palatine again, and now lies to the inner side of the rectus inferior. From this point it courses almost straight forwards at the right side of the parasphenoid and ethmoid cartilage, and finally splits up to enter its muscle in the way shown in the figure. As regards now the left side, it may be noted at once that the distribution of the eye muscle nerves, except those coursing far forwards like the patheticus and the branch of the third to the inferior oblique, is not much 119 affected by the torsion of the head, since the parasphenoid and its eye muscle canal are simply rotated en bloc to the right along their longitudinal axes. The distribution of the nerves is therefore the same, except that those of the left side have been swung upwards so as to he nearer the dorsal edge of the body, and hence above those of the right side. In the case of the branch to the inferior oblique this rotation has caused the left one to be situated at first much above the right. In front, however, it begins to turn downwards towards the parasphenoid, and the right one at the same time rising, they eventually take up cor- responding positions at the sides of the parasphenoid and ethmoid cartilage. Finally the left turns upwards to reach its muscle in which it breaks up in much the same way as the right. Neither of the long eye muscle nerves (patheticus and the branch just described) of the left side | reaches, at its final distribution, a much higher transverse level than that of the right. If a comparison be made with the eye muscle nerves of Menidia, as described by Herrick, it will be seen that in the two forms the relations of the nerves are essentially the same. Nervus patheticus s. trochlearis—lIV. The fourth nerve of the right side (iv. 0.s.) consists of many large and a few small fibres all heavily myelinated. It has no connection with the communis vii. as described by Herrick in Menidia. The nucleus of the pathetic is situated dorsally close behind that of the oculomotor. The two pathetic nerves cross over the mesocoele as in all hitherto investigated vertebrates, so that, for example, the right nerve arises from the left side of the brain. The two nerves pass first backwards, then rise sharply over the mesocoele, cross, and leave the brain almost in the same 120 section so as to lie wedged in between the axis of the brain and the optic lobe. It at once turns sharply downwards and forwards, and becomes closely opposed to the dorsum of the oculomotorius. For some sections the two nerves can be distinguished, but in front they appear to fuse completely, and cannot be distinguished even under the high power. The pathetic is, however, given off again from the dorsum of the oculomotor, passes forwards and downwards, pierces the membranous wall of the brain case obliquely in front, and breaks up in the superior oblique muscle of the eye as shown in the figure. On the left side the relations of the nerve to the brain and for some distance in front are essentially the same as on the right side. As, however, it approaches the eyes (section 392 of chart), it begins to pass towards the lower or right optic nerve. Subsequently it takes up a position above and to the left of the upper or left optic nerve, having now crossed over the top of the parasphenoid and lying distinctly to the right of the morphological middle line. The two optic nerves having dipped down the left pathetic crosses over the left optic to its right side. The left optic now turns upwards towards its eye, so that the left pathetic hes considerably below it. The latter after- wards passes upwards to the left side of the frontal bridge, and is seen below the right pathetic. It finally breaks up in the left superior oblique in much the same way as the right. Nervus abducens—Vl. The sixth nerve (vi. 7.e.), which consists mostly of large well myelinated fibres together with some small ones, arises from the medulla by two small rootlets some distance from the middle line. Both these rootlets have apparently a common nucleus situated far from the middle 121 line and not far from the ventral surface of the brain. Soon after leaving the brain the abducens passes sharply downwards to reach the floor of the brain case. In front it passes downwards and forwards, perforates the meninges, enters the eye muscle canal, and at once reaches the rectus externus muscle which it supplies. The abducens is the most posterior of the eye muscle nerves (cp. chart), and on this account the two nerves exhibit practically no traces of asymmetry. Before we can proceed to describe the trigeminal and facial nerves separately, it is necessary to interpolate an account of the roots and ganglia of the trigemino-facial complex as a whole (fig. 25). As in Teleosts generally the fifth and seventh nerves at their exit from the brain, and also their ganglia, are so disposed that it is quite impossible to completely analyse them by dissection. Hxamination, however, of a series of Weigert sections enables us to do this without much difficulty. Macroscopically there are two roots to the facial nerve and one to the trigeminal, and three of the four ganglia of these two nerves are compacted together into one mass. Analysis by serial sections reveals the following facts :— The most anterior root of the complex (r.v.) is that of the trigeminus. It lies, however, largely internal to and below the second root, so that it is at first not obvious on dissection, and emerges from the brain just below the cerebellum. It is the only root of the trigeminus, and consists of a general cutaneous and a motor component. The nucleus of the latter les in the floor of the fourth ventricle, and the fibres pass right through the Gasserian ganglion first into the Truncus infraorbitalis (t2nf.) and then into the R. Mandibularis V (man. v.). On account 122 of the fact that the large ventral nerve emerging from the Gasserian ganglion contains lateral line and communis components from the facial, the term 7’. mawillo-mandibu- laris, which refers to the unspht RR. maxillaris and mandibularis V, cannot strictly be applied to it. The cutaneous component of the trigeminus arises from the spinal v. tract, and its ganglion is the Gasserian ganglion. It is distributed to the skin of the face and operculum. The second root of the complex (r.1.vi.) belongs wholly to the facial, and consists of 3 roots so closely packed together that it is difficult to separate them by dissection. These roots are the dorsal and ventral lateral line roots and the communis root. The whole arise together at the same level high up on the medulla and much higher than and external to the exit of the trigeminus. The ganglia of the lateral line roots are respectively the dorsal and ventral lateral line ganglia, and that of the communis root is the geniculate gangiion. The dorsal lateral line root splits into the Ramus ophthalmicus superficialis vu. and the R. bucealis vii., whilst the ventral lateral line root is continued into the Truncus hyomandibularis as the R. mandibularis externus vu. The communis root splits into the communis v., R. palatinus vil., the R. Posttrematicus vil. and the R. mandibularis internus vil. Although the three com- ponents in this root are very compacted they retain their individuality under the microscope. The communis root enters the brain first, and then the other two fuse and enter together behind and above it. The communis root in the brain passes at once into the fasciculus communis tract, and the fused two lateral line roots terminate in the tuberculum aeusticum. The third root of the complex (r.2.vé.) is also entirely facial and constitutes its motor root. It arises 128 behind the second root and much ventral to it. Its nucleus lies in the floor of the fourth ventricle very near the middle line, and the root below joins the ventral lateral line root proximal to its ganglion as in Menidza. After leaving the brain the motor root, which consists of deeply staining heavily myelinated fibres, becomes so confused with the anterior part of the auditory root that their separation is difficult even with the microscope. The auditory nerve, however, passes dorsally into the tuber- culum acusticum, whilst the motor vii. enters the brain below and immediately in front of it. The motor vii. passes into the Truncus hyomandibularis. Of the four ganglia of the complex (g. v.-vii.) only one remains distinct macroscopically. ‘This is the ganglion of the dorsal lateral line root, which in front is situated just dorsal to the Gasserian ganglion, and behind overlaps externally the root of the trigeminus. It is entirely intracranial and is partly shown in the chart as the cells at the base of the R. buccalis vii. The other three ganglia are crowded between the brain and the skull wall and apparently form one mass also entirely intra- cranial except for the narrowed anterior extremity of the Gasserian ganglion which extends outside the skull along the R. ophthalmicus superficialis v. as far forward as sec- tion 472 (cp. chart). When these three ganglia are examined in serial sections it is seen that the most anterior is the Gasserian ganglion. This overlaps exter- nally the geniculate ganghon situated behind it, which in its turn overlaps externally the ventral lateral line ganglion—the most posterior of the three. Although the three ganglia form a single very compact mass, it is not difficult to define their boundaries, even where, as in the case of the first two, the character of their cells is much the same. In the ventral lateral line ganglion, the cells, 124 as is usual in these ganglia, are very small and not crowded, being scattered among the nerve fibres. The dorsal lateral line root, as already described, has a discrete ganglion, and a reference to the chart will show that the communis, ventral lateral line and motor roots enter the ganglionic mass above by two nerve bundles. The anterior one is the communis root and the posterior represents the ventral lateral line and motor roots fused together, the motor fibres of course having no connection with the ganglion cells. The entry of the trigeminus root into the Gasserian ganglion is described above. Leaving the compound ganglionic mass are three large nerve trunks: (1) the Truncus supraorbitalis; (2) the Truncus infraorbitalis; and (3) the Truncus hyo- mandibularis—all compound trunks into which both trigeminal and facial nerves enter. It will be seen on reference to the chart that the last is formed by three nerve bundles from the ganglionic mass uniting together. The most anterior of these arises from the Gasserian ganglion and thus forms the trigeminal cutaneous vii. component of the hyomandibular trunk. In Menzdia the cutaneous vil. is extracranial, and is formed by two bundles from the Gasserian ganglion fusing together. In the Plaice there are one large and two very small bundles —all intracranial. The middle of the three nerve bundles above is the communis root, and the posterior the fused ventral lateral line and motor roots. The motor vii., as mentioned above, joins the ventral lateral line root proxi- mal to the ganglionic mass. At first they remain distinct, the motor vii. lying on the outer face of the ventral lateral line ganglion. Before leaving the ganglion, however, the two roots become almost too intermingled to be distinguished. Before proceeding to describe the divisions of the vth and viith nerves, it may be mentioned that the Gasserian ganglion is the only one to receive a prominent R. com- municans from the sympathetic. There also arises from the same ganglion a motor nerve which has traversed the ganglion and passes to the M. depressor opereuli (m. d. op.). This is the most posterior nerve passing through the trigemino-facial foramen. The trigemino-facial foramen (represented by a ring in the chart) transmits the trigeminal nerve + the dorsal lateral line root of the facial + a communis vil. com- ponent. The jugular foramen (the posterior ring in the chart) transmits the hyomandibular trunk, comprising the remainder and greater part of the facial + a cutaneous component from the trigeminus. The various nerve rami may now be described under the names of the nerves to which they belong. Nervus Trigeminus—V. 1. Nervus ophthalmicus profundus (fig. 26, 0. pr.).— The root of this nerve (Radix ophthalmici profundi) arises on the right side from the root of the trigeminus near the brain, and proximal to the Gasserian ganglion. It passes downwards and forwards over the inner face of the latter ganglion between it and the brain, and enters the pro- fundus ganglion, which, though closely opposed to the inner face of the Gasserian, is absolutely distinct from it. From the profundus ganglion an apparently single nerve arises which leaves the skull cavity with the rest of the vth and becomes intimately attached to the sympathetic. We could not be certain whether a few fibres were not given off to accompany the R. ophthalmicus superficialis y., thus constituting a Portio ophthalmici profundi. The nerve from the profundus ganglion passed with the sym- pathetic through the skull wall again by a special small 126 foramen into the eye muscle canal. The two bundles separate in front, but for a time the union is so close that they could not be satisfactorily analysed. However, most of the profundus fibres proximal to the ciliary ganglion separate out as the Ramus ciliaris longus (cd. l.), but a few of them accompany the sympathetic to the ciliary ganglion (ec. g.) as its Radix longa (re. /.). The R. ciliaris longus leaves the eye muscle canal in front and accompanies the right rectus superior muscle to the eye, which it enters from above.