1137 



TUNICATA. 



TUNICATA. 



11J9 



the whole of the , Tunicata. But Aristotle afterwards proceeds to a 

 more definite description of the Tethya : " But of all these animals 

 (the Ostraeoderms), those which are called Tethya have the most 

 remarkable nature; for with them alone is the body entirely con- 

 cealed in the test. This test or envelope (TO oo-Tptu<oi>) is between the 

 texture of leather aud shell, and may be consequently cut like a piece 

 of tough hide. The animal adheres to the rocks by its test, and has 

 two passages or orifices (iropovs), distant from each other, and so small 

 as not to be easily visible : by means of these it imbibes and discharges 

 the water. On opening one of these animals, the inside presents, in 

 the first place, a membrane composed, as it were, of nervures (ijueVo 

 ffvpuSri), and communicating with a fleshy intestine (TO o-apKwSfs) ; so 

 that the intestine of the Tethyon appears contained in the reticulated 

 membrane. Although indeed the flesh is alike in all testaceous 

 animals, this intestine resembles in form that of none of them. It is 

 suspended at two places, namely, to the above-mentioned membrane, 

 and to the muscle which proceeds from the side (literally, 'to the 

 skin from the side') ; and wherever it adheres to either of these it is 

 narrowest. At each point of suspension this intestine tends towards 

 those orifices which lead to the outside of the test, and by which it 

 receives and discharges the food and water ; so that if one of these 

 apertures be the animal's mouth, the other must be its anus. One of 

 these orificial processes (the branchial orifice) is thicker than the 

 other. Within the cavity also of one or other of them there is a 

 certain small cohering substance which divides it." 



Mr. M'Leay, who, in his ' Hor;o Entomologies,' thus translates a 

 passage presenting almost insuperable difficulties to any but a zoolo- 

 gical scholar, observes, that " the membrane, composed as it were of 

 nervures," is a good description of the beautifully reticulated mem- 

 brane which forms the branchia; of the Tethya. Aristotle, he adds, 

 appears however by some mistake to have considered the branchial 

 pouch as surrounding the intestine ; aud he remarks that he suspects 

 that the small cohering or continuous substance which Aristotle 

 alludes to immediately after the mention of that part which Mr. 

 M'Leay considers to be the branchial orifice, is the valvule of the 

 anal orifice. 



Dr. Fleming has given a very good view of this natural group in 

 the article ' Mollusca ' of the supplement to the ' Encyclopedia 

 Britounioa.' 



Synoptu of the Sub-Gene>'a. 

 Jformal Group. 



I. Branchial pouch with more 

 than eight folds. Tentacula 

 compound. Liver distinct. 



Aberrant Group. 



2. Branchial pouch with only 

 eight folds. Tentacula simple,' 

 Liver none. 



" The Tunicala," says Dr. Fleming, '' agree with their contiguous 

 group the Molltuca in the remarkable variation that exists in their 

 system of generation. Like every other solitary character that can 

 possibly be adopted for the groundwork of a zoological system, the 

 mode of generation ought to rise in importance only in inverse pro- 

 portion to its degree of variation. In a group of animals for instance, 

 where it varies according to the species, it is evidently of less import- 

 ance, as affording natural characters, than among those groups where 

 it remains less subject to variation. When the naturalist happens to 

 consider that he ought always to obtain his group before he attempts 

 to find its character, he is sure to perceive this truth ; and it is on this 

 very principle that Savigny, with his usual discrimination, has pro- 

 ceeded in the above natural arrangement of the genus Ascidia, which 

 I have done little more than borrow from him. To this naturalist, 

 whose works I cannot too often recommend to the careful attention 

 of zoologists as models for imitation and true examples of the method 

 in which natural history ought to be studied, I would willingly have 

 dedicated the following genus " [DENDBODOA], " but his name happens 

 to have been employed in other branches of the science." (' Linnrean 

 Transaction!!,' vol. xiv.) 



Mr. Brodcrip and Mr. G. B. Sowerby have described a curious form 

 belonging to this natural group. 



Group, Tunicata. Family, . . . ? Genus, Chdyosoma. 



Generic Character. Body sessile, fixed, involved in a coriaceous test 

 or envelope dividedly laminate above ; orifices conical, each closed with 

 six trigonal valvules. 



C. StacLtayanum is elongate-ovate, affixed at the base, flat above, 

 octopartite, the lamina; striated ; orifices prominent. It is found in 

 the Arctic seas, adhering to stones. 



This animal comes nearest to the Tethya above noticed, but there 

 re no trace* of tcntacula surrounding the branchial orifice. It differs 



*AT. H1OT. WV. VOL. IV. 



from the Thalida inasmuch as the mantle seems to adhere to the 

 orifices only, and, instead of a simple valvule, each orifice of Chelyosoma, 

 is furnished with a complicated one. From the Ancididoe it differs 

 inasmuch as both its orifices are surrounded by six valves, instead of 

 being quadrifid. 



Chelyosoma MacLeayanum. 

 a, side view ; b, seen from above ; c, interior of valvule. 



Forbes and Hanley, in their ' History of British Mollusca,' describe 

 three genera of Tunicata : 



1. Aicidia. Body sessile, covered with a coriaceous or gelatinous 

 tunic; branchial orifice 8-lobed and 6-lobed. There are 1C species of 

 this genus found on the British coasts. 



2. Molgula, E. Forbes. Body more or less globular, attached or 

 free, with a membraneous tunic, usually invested with extraneous 

 matter ; orifices on very contractile and naked tubes ; the branchial 

 6-lobed, the anal 4-lobed. Two species are British. 



3. Cynthia [CYNTHIA], of which there are 13 species. [CLAVELINIDJE ; 

 BoiRYLLJDa: ; MOLLUSCA ; MALACOLOGY ; BOWENIA.] 



The compound (and most probably all the) Ascidians, in their first 

 state of development, after leaving the egg, assume the form of Tad- 

 poles, and are locomotive by means of a vibratile tail, which they cast 

 off when they quit the larva state and assume the sessile condition. 



Tadpole of Amaruunium pmliferum, newly hatched (highly magnified). 

 A, tegumentary body of the trunk seen within the pouch j a, pouch inclosing 

 the vitellus, and forming the tunic of the body of the larva ; b", appendages 

 terminating in suckers and enabling the animal to fix itself; a", tail formed by 

 a prolongation of the tegumentary body, and inclosing a tubular appendage of 

 the vitellinc sac, (Milne-Kdwards.) 



4 D 



