836 MANUAL OF THE MOLLUSCA. 



first flexure of the intestinal canal in the tunicaries is always to the haemal 

 side, but it is usually turned again in the opposite direction. 



The food of the ascidians, judging by the contents of their stomachs, 

 consists chiefly of minute particles of the articulated sea-weeds and diato- 

 macece ; and it is a remarkable circumstance that the outer tuuic of 

 these animals contains ceUidose, a ternary organic substance formerly sup- 

 jjosed to be peculiar to vegetables.* They also contain radiated concretions, 

 sometimes silicious, but more frequently calcarious, like the bodies found iu 

 alcyonium and gorgonia. 



All the Tunicata appear to possess the power of reproduction by buds — or 

 gemmation; but in one group the individuals, however produced, become 

 entirely distict, in another they remain connected by a vascular canal, and 

 in a third they become blended into a common mass. These three 

 groups are the "solitary," " social," and " compound ascidians " of Milne- 

 Edwards ; these are all fixed in their adult state, whilst the two remaining 

 families swim freely in the open sea, Pi/rosoma being compound, and Salpa 

 alternately aggregated and solitary. The separate individuals of these com- 

 posite masses are termed Zoids. 



The sexes are united in all the Tunicata but DoUolum and Jppendi- 

 cularia. The young produced from eggs undergo a metamorphosis, which 

 has been observed in many genera. The larvae are shaped like the tadpole 

 of the frog ; the body is oval and furnished with black eye specks, short 

 tentacular processes, and a long tail by the vibrations of which they swim 

 (PL 24, fig. 18). Ultimately they fix themselves, the tail is absorbed, and 

 the young ascidian, or first zoid of a compound tunicary, is developed. 



The tunicata are found in all seas, from low-water to a considerable 

 depth. Pour genera are pelagic, and several belong to the Arctic province 

 viz., BoUe/iia, Cliehjosowa, Si/ncecium and Cyst'mgia. 



Mr. Huxley divides the Tunicaries into three groups — 



1. Ascidla Branchiales. Branchial sac occupying the whole, or nearly 

 the whole, length of the body ; intestine lying on one side of it. {Ascidiadae 

 — Perophora — Botryllus — Pyrosoma.) 



2. Ascidia Intestinales. Alimentary canal completely behind the bran- 

 chial sac, which is comparatively small. (Other genera.) 



3. Ascidla Larvales. Permament larval form. {Appendiculana.)\ 



guided by the analogy of tlie ganglionic side of the tunicata to the dorsal region of 

 the lainelUbranchiafa. Still more confusion exists in the employment of the terms 

 anterior and posterior ; the inhalent orifice is anterior if compared with the mouth of 

 a polype, but Milne-Edwards makes it posterior. 



* Discovered by Dr. Schmidt, in 1S45. The observation has been confirmed by 

 M. M. Ldwig and Kolliker, and by M. Payen, who gives the followiug as the chemical 

 composition of the ascidian tunic; — Cellulose 60.34, azotised matter 27.00, inorganic 

 12.66. The cellulose portion is not acted upon by soda or hydrochloric acid. 



t See Knight's " English Cyclopaedia," article Mollusca. 



