TUNICATA. 



1193 



genus Salpa, Forskahl ; synonyms Thalia, 

 Brown ; Holothuria, Linneand Pallas ; Dagysa, 

 Banks and Solander ; Biphora, Brugiere ; 

 Tethis, Tilesius ; Pegca and lasts, Savigny. 

 The characteristic features of this genus are 

 detailed above. 



Quoy and Gaimard * established for some 

 animals nearly allied to the Salpes and inha- 

 biting the coasts of Amboina, the genus Do- 

 lio/ium, the characters of which are, its having 

 the form of a little cask open at the ends; 

 from two to ten lines in length ; the anterior 

 extremity a little prominent ; marked with 

 circles in relief on the external surface ; and 

 having internal branchia, divided into two 

 branches; and a heart and a dorsal vessel, si- 

 tuated near the union of these branches. This 

 name had also been previously given by Otto -f- 

 to a genus established by him on a Mediterra- 

 nean Salpa, mutilated by a crustacean of the 

 genus Phronymc, that had made it its habita- 

 tion. This form, like Salpa triangufaris and S. 

 polymorpha (by Quoy and Gaimard, and Bory 

 de Vincent), has been erroneously regarded as 

 belonging to the family of Diphydes. MM. 

 Lowig and Kb'lliker, however, who found the 

 tissue of Doliolium to be identical with the pe- 

 culiar substance of the test of other Tunicate, 

 have pointed out its true affinities, and placed 

 it with the Salpidce. 



Family PELONAIAD.E, Forbes. This fa- 

 mily is represented by two rare animals, both 

 inhabitants of the Scottish seas, constituting 

 two species of a single genus, Pelonaia, esta- 

 blished by Professor E. Forbes and Professor 

 Goodsir.j We have derived our description 

 of the general and anatomical characters of 

 these interesting Molluscs from the detailed 

 account given of them by the original dis- 

 coverers. 



Genus Pelonaia, Forbes and Goodsir. 

 Animal simple, unattached. Test more or 

 less cartilaginous, smooth or wrinkled, elon- 

 gated, and cylindrical ; anterior extremity 

 bearing two orifices, four-cleft, without ten- 

 tacles, and placed on the same plane, on two 

 equal, approximate, papillose eminences ; 

 posterior extremity ending in a blunt point ; 

 mantle adherent to the test. The Pelonaia: 

 live buried in mud, quite unattached to any 

 other body, and are extremely apathetic ani- 

 mals, presenting scarcely any appearance of 

 motion. 



ANATOMY AND PHYSIOLOGY OF THE TUNI- 

 CATA. 



It will be most convenient to describe 

 the test generally for the whole tunicate class, 

 and the rest of their anatomy under the 

 heading of each of the families. 



Test or Shell. The test or external enve- 

 lope of the AscidiadfE is subject to considerable 

 variation of shape ; from the bottle-like form 

 of the Ascidia (fg. 766.), to the flat, patella- 

 like form ofChclyosoma (Jig. 767.) ; it is elastic, 



* Voyage de 1' Astrolabe, Zoologie, torn. iii. 

 f Nova Acta Academ. Nat. Curios, t. xi. 

 j Jameson's Edinb. New Philosoph. Journal, 

 vol. xxxi. 1841. 



varying very much in its thickness and con- 

 sistence in different species. In colour it 

 varies considerably, being occasionally nearly 

 black, sometimes red, orange, yellow, or 

 milky white. Its surface is sometimes smooth, 

 often tuberculated, covered with hairs or 

 spines, or otherwise roughened. Sometimes 

 minute patches of horny tissue or hardened 

 epidermis rise up irregularly on the surface of 

 the test ; in other cases these are placed in a 

 tessellated arrangement. In the peculiar and 

 unique form, Chelyosoma, first noticed by 

 Messrs. Broderip and Sowerby, and since more 

 fully described by Professor Eschricht, the 

 upper surface of the test is occupied by eight 

 large, horny, polygonal plates arranged some- 

 what like the shell-plates of a tortoise, and by 

 several smaller triangular plates, which form 

 two circles, one around the branchial, and the 

 other round the anal aperture of the animal 

 (Jig. 767.). The large plates are so disposed 

 that the branchial orifice is surrounded by 

 three plates, and the anal by four, besides that 

 which is intermediate and abuts upon both : 

 this latter plate is hexagonal, the sides in con- 

 tact with the orificial valvules are lunated. 

 The three plates near the branchial orifice are 

 much larger than the four which are near to 

 the anal orifice. Each of the plates is marked 

 with three or four elevated striae, which are 

 near to the edges of the plate, and parallel with 

 them, leaving an area in the centre, and giving 

 rise to a general resemblance to the external 

 plates of the shell of a land tortoise. The 

 orifices are very small, and are surrounded by 

 six triangular valvules, each transversely stri- 

 ated, and, when shut, rising from the sur- 

 rounding surface in the form of a cone. The 

 lower or adherent part of the test of Chely- 

 osoma is coriaceous, with occasional slight 

 traces of separation into plates. 



The test of Ascid'ns is frequently covered 

 with innumerable smaller animals and their 

 spawn. Modiolas and Annelids burrow in it ; 

 Cirrhipeds, naked Molluscs, and Actiniae lodge 

 upon it ; and Corallines cover it sometimes 

 with a little forest ; a condition fully justify- 

 ing the denomination of " microcosmus," be- 

 stowed by Redi * on a Mediterranean species. 

 Occasionally, as in Ascidia conchilega and Mol- 

 gula oculata, the animal works up extraneous 

 matter, as gravel, fragmentary shells, &c., with 

 the external surface of the test. It is by this 

 shell or test that the animal fixes itself. In 

 the sessile species, the tissue of the base or 

 the side of the test interlaces with the stems 

 of sea weeds and corallines, or closely adheres 

 to the surface of another ascidian sac, or of a 

 stone, a shell, a crab, or other object. In the 

 peduncled forms this tunic is at one point pro- 

 longed into a tubular process or stem, the dis- 

 tal extremity of which is attached to marine 

 bodies in the same manner as the base of the 

 sessile tests. In Cystingia and Bipapittaria, 

 however, this process or stalk appears to be 

 less perfectly developed, and not to be al- 



* Opuscula varia Physiologica, 3 vols. 12mo. 

 Lugd. Batav. 1725. 



