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Gonaree wesu eee as eee ye ante yin tinge 4 9 ier ge nl ede ng etre isa o ree ee 10) + tint et Shae Ue bp ah i} wens milion ne +i et as oe CAME sy icateel cleat tet prey nee wat che reeatarrs ares rate A ANAL Bria Aa ‘ owen oe : Britt hgh Ay yi 4 et tae Wee Aiton sior ot f ‘* My oe a Yeh Oe Or ate rub a are wee vey , Seacergceeiree rie hea seta ' rape ales Pernt it met genores Boatk owes An 4 oe Pet oe Sader ala! yaa by he Ora as 6-OtG0e bach oo. a 4. beat phy (Reprinted from the ‘Transuctions of the Edinburgh Field Naturalists’ and Microscopical Society,’ Session 1906-1907.) OBSERVATIONS ON SOME COPEPODA THAT LIVE AS MESSMATES OR COMMENSALS WITH ASCIDIANS. By THOMAS SCOTT, LL.D., F.L.S., Honorary MeEmser. (Read Feb. 27, 1907.) IN two previous papers read before this Society ——one in January 1903 and the other in March 1905—TI referred to a number of Copepoda which, in one way or another, are associated with fishes and with various marine invertebrates, as parasites, messmates, or commensals. In these papers I have shown that, in addition to fishes,—all of which now and then become, nolens volens, the hosts or entertainers of many kinds of Copepoda,—such organisms as Sponges, Echinoderms, Annelids, Crustacea, and Mollusca are also called upon to provide food and shelter for not a few of those guests which, in almost every case, may be correctly described as “un- invited.” As no special mention was made in my two last papers of the messmates of the Tunicata, my observations in this third paper will be confined chiefly to some of those Copepoda that are known to live in the company, and under the shelter and protection, of various kinds of Ascidians. To judge from the number of the organisms sometimes found located upon and within the test of one of the larger Ascidians or Tunicates, one might be led to imagine that the other invertebrata con- sidered the test of an Ascidian to be common property, and had proceeded to carry out the socialistic idea, as far as they could, to its logical conclusion hy taking forceful possession both of the outside and inside of the Ascidian’s dwelling,—a dwelling which it had built up by its own exertions and for its own convenience. Among the trespassers upon the preserves of the Tunicata is the Modiolaria marmorata. This dainty mollusc may frequently be found embedded in the tests of the larger Ascidians, such as, for example, Ascidia (Phallusia) mentula 358 Copepoda living as Messmates with Ascidians. (Sess. O. F. Miiller. It is sometimes buried so deeply in the substance of the test as to be almost entirely concealed, and where it can only communicate with the outside world by means of a small passage having an opening on the outer surface. Dr J. Gwyn Jeffreys, referring to this mollusc, remarks that sometimes a score of specimens may be extracted from a single large Ascidian. I have also found in cells formed in the tests of Ascidians several specimens of the curious little Amphipod, 7ritwta gibbosa ; while zoophytes of several kinds have been seen adhering to various other parts of its surface. But if, having finished the examination of the outside of the test, we proceed to investigate the inside of it, perhaps the first of the objects to be observed will be one or two specimens of Leucothoé spinicarpa Abildgaard—a moderately large, soft- bodied, and sluggish Amphipod of a pale flesh colour, and bearing green-coloured ova. Then there are nearly always present in the branchial chamber a number of Copepoda which may belong to several distinct species, and thus it sometimes happens that the capture of a large Ascidian may turn out to be a miniature treasure-house to the marine zoologist. The Copepod-messmates of Ascidians have had a good deal of attention devoted to them, especially by Continental zool- ogists, and the results of recent study seem to favour the opinion that in development and structure they exhibit a more or less near relationship with Cyclops. But this rela- tiouship, though obvious in some eases, is obscure in others. There are, for example, a few species that resemble Cyclops somewhat closely, both in their general appearance and habits. Like Cyclops, they carry two external ovisacs; they are active in their movements, gliding freely and quickly over the walls of the branchial chamber. These forms belong to the Licho- molgide, and their affinity with Cyclops is so evident that they have been classified along with that group and some other Copepoda in the division Cyclopoida. It may be observed in passing that while there are Lichomolgi which pass their lives as the messmates of these Ascidians, there are other species belonging to the same genus that live a free life amid the forests of Laminaria and zoophytes which in some places fringe our shores, and it has been suggested that the forms 1906-1907.| Copepoda living as Messmates with Ascidians. 359 which now seek the shelter and protection of the Ascidians were formerly as free-living as the others. But though these Lichomolgi and a few other species have retained their active habits along with a certain amount of freedom within the branchial chamber of the larger Ascidians, the majority of the Copepoda found in such situations are usually inactive and sluggish in their movements, and have their limbs more or less imperfectly developed. Dr Brady, the eminent British authority on the Entomostraca, referring to these Copepoda, observes that they are indeed, in all probability, Cyclopidz which have become modified in build by their inactive habits and the restricted boundaries of their dwellings; and it is evident that under these conditions the long antenne and limbs of the free-swimming species would be an encumbrance, or at any rate be of very small service, and one would almost be inclined to believe that these appendages must still be in course of degradation, owing to constant disuse. The Copepoda chiefly referred to in the foregoing remarks are those usually found within the comparatively roomy branchial chamber, but there are other species which hitherto have only been obtained in the alimentary tract, and are in consequence more restricted in their movements, and they also exhibit a rather more retrograde development. There also appears to be another and a rather interesting difference between a few of these forms whose environment is so limited and others that are favoured with a more liberal allowance of space. The females of the former species carry two external ovisacs, which are slender and sometimes considerably longer than the animal itself, as exemplified in Aplostoma affinis mentioned in my recently published ‘Catalogue of Forth Crustacea.’ On the other hand, the females of several of the species living in the more roomy branchial chamber are not furnished with external ovisacs, but have the fourth thoracic segment, with which the fifth is usually coalesced, enlarged on the dorsal aspect to form a pouch in which the ova are enclosed. This pouch when packed with ova becomes greatly distended, and forms a conspicuous part of the animal, owing to the colour of the developing eggs showing through the thin integument. It is because of this modification of the fourth 360 Copepoda living as Messmates with Ascidians. [Sess. thoracic segment that the name WNotodelphys was given by Professor Allman to what is made the representative genus of Sars’s division Notodelphyoida, Having seen how closely these Copepoda are associated with the life and fortunes of the Tunicata, the question may be asked, What is likely to happen to the Copepoda should the Ascidian die? Dr Canu, speaking of some of the more active species, says that, on the death of their host, they are able to quit their shelter and proceed in quest of another Ascidian, and while doing so, move freely about on the bottom of the sea. But as they are seldom captured in this free-living condition, it can only be on rare occasions that they require to adopt this form of life Yet though the more active and lively species may be able to look after themselves in the manner described by Dr Canu, it is prob- able that not a few will share the fate of their host. The alteration from a free-living to a semi-parasitic life has doubtless taken place long ago, seeing that the change of habitat has resulted in such a retrograde development as to cause the structure both of the body and its appendages to become so modified as to differ profoundly from that of the free-living species to which their ancestors appear to have been related. It is also interesting to find that in the early larval stages of even the most degraded forms they still retain some traces of their former free-living habits; and, moreover, it is during this early stage that such degraded forms usually seek for, and select, a suitable host, which, when selected, has henceforward to shelter and provide food for these unbidden guests. The number of Copepod species that have been recorded as messmates of the Tunicata is considerable——so much so, that I can claim only a limited acquaintance with them. The species of Tunicata which have been described as the hosts of these Copepoda are also fairly numerous, and include such large forms as Ascidia mentula—already referred to,—Ciona intestinalis, specimens of which have been obtained “ measur- ing as much as a foot in length,” ® and also the small Cynthias, Styelas, Botryllus, &e. 1 “Tes Copépodes du Boulonnais,’ by Dr Eugene Canu, p. 187 (1892). 2 ¢ Jour. Plymouth Marine Biol. Assoc.,’ vol. vii. (N.S.), No. 2, p. 296 (Dec. 1904). 1906-1907.] Copepoda living as Messmates with Ascidians. 361 The Copepoda I now proceed briefly to describe may be arranged as follows :— A. NOTODELPHYOIDA Sars. 1, Females provided with a single dorsal incubatory pouch in place of external ovisacs :— Notodelphys Allman. Dorovxys Kerschner. Agnathaner Canu. Bonmierilla Canu. Doropygus Thorell. Gunenotophorus O. G. Costa. Notopterophorus O. G. Costa. Botachus Thorell. 2. Ovisacs, two, external, situated on the dorsal aspect, contiguous, and comparatively small, or situated laterally and more or less elongated :— Ascidicola Thorell. Eniteropsis Aurivillius. . Botryllophilus Hesse. Aplostoma Canu. Enterocola P. J. van Beneden. B. CYCLOPOIDA. LicHomo.ueus Thorell. In Lichomolgus the fourth pair of swimming -feet has the outer branches three-jointed, but the inner only two-jointed, and this forms one of the principal points of difference between it and other genera of the Lichomolgide. In Pseudanthessius, for example, the inner branches of the fourth pair consist of only one joint, while in Herrmannella they are three-jointed like the outer branches. I shall now proceed to make a few observations on the better known species belonging to the various genera men- tioned, and in the order in which they are given. Genus NOTODELPHYS. Four species of Notodelphys have been recorded for the British Islands — viz., Notodelphys Allmani, N. cerulea, N. agilis, and N. prasina. These have all been described by M. Thorell. The first-named appears to be the most common. JI have obtained it in the Firth of Forth, in Loch Tarbert, and at Tarbert Bank at the mouth of Loch Fyne: indeed, whenever large and aged Ascidians are brought up in the dredge or trawl-net we nearly always find these Copepoda in the branchial chamber, and frequently other 362 Copepoda living as Messmates with Ascidians. [Sess. species as well. Notodelphys Allmani, besides being the most common, appears to be also the largest, of the various species. According to M. Thorell, they sometimes measure five millimetres in length, but the largest Scottish specimens I have noticed scarcely reach four millimetres. This form has long tail-segments, and the dorsal egg-pouch in the case of adult females is usually coloured and distended with ova, so that the specimens are conspicuous enough. Notodelphys prasina is also of frequent occurrence, but it is not so common as the one just referred to. Though resem- bling that species in some respects, it is smaller, and is, besides, readily distinguished from it by the very short tail-segments. Rev. A. M. Norman, who collected this species at Oban, found it to be more abundant there than any other of the Entomo- straca taken from the branchial sacs of Ascidia mentula. He has also recorded it as occurring in the same Ascidian in Shetland I have taken a number of specimens in large Ascidians dredged in East Loch Tarbert, Tarbert Bank, and Kilbrennan Sound, Firth of Clyde. Notodelphys agilis is apparently less common than the others. Like WV. Allmani, the tail- segments are elongated, and its appearance being otherwise somewhat similar to that species, the one might be easily mistaken for the other were it not that WV. agilts is a distinctly smaller form. Dr Brady reports its occurrence off the coasts of Durham and Yorkshire, and at Shetland on the authority of Rev. A. M. Norman. I have obtained it very sparingly in the Firths of Forth and Clyde. Notodelphys cerulea—Dr G. S. Brady states that this species differs scarcely at all from Notodelphys Allmani, and that he cannot find any good reason for separating it from that species.” He records it as having been obtained in Corella (Ascidia) parallelogramma and