584 TUNICATA : — RESPIRATORY APPARATUS. 



scribed by Mr. Busk, no fewer than-126 are furnished either with 

 Avicularia, or with Yibracula, or with both these organs. * 



452. Tunicata. — The Tunicated Mollusca are so named from 

 the enclosure of their bodies in a ' tunic,' which is sometimes 

 leathery or even cartilaginous in its texture, and which very com- 

 monly includes Calcareous spicules, whose forms are often very 

 beautiful. They present a strong resemblance to the Polyzoa, not 

 merely in their general plan of conformation, but also in their 

 tendency to produce Composite structures by Gemmation ; they are 

 differentiated from them, however, by the absence of the Ciliated 

 Tentacula which form so conspicuous a feature in the external 

 aspect of the Polyzoa, by the presence of a distinct Circulating 

 apparatus, and by their peculiar Respiratory apparatus, which may 

 be regarded as a dilatation of their Pharynx. In their habits, too, 

 they are for the most part very inactive, exhibiting scarcely any- 

 thing comparable to those rapid movements of expansion and 

 retraction which it is so interesting to watch among the Polyzoa ; 

 whilst, with the exception of the SalpidcB and other floating 

 species which are chiefly found in seas warmer than those that 

 surround our coast, and the curious Appendicularia to be pre- 

 sently noticed (§ 457), they are rooted to one spot during all but 

 the earliest period of their lives. — The larger forms of the Ascidian 

 group, which constitutes the bulk of the Class, are always solitary ; 

 either not propagating by gemmation at all, or, if this process 

 does take place, the gemma? being detached before they have ad- 

 vanced far in their development. Although of special importance 

 to the Comparative Anatomist and the Zoologist, this group does 

 not afford much to interest the ordinary Microscopist, except in 

 the peculiar actions of its Respiratory and Circulatory apparatus. 

 In common with the Composite forms of the group, the Solitary 

 Ascidians have a large Branchial sac, with fissured walls, resem- 

 bling that shown in Figs. 302 and 303 ; into this sac water is 

 admitted by the oral orifice, and a large proportion of it is caused 

 to pass through the fissures, by the agency of the Cilia with which 

 they are fringed, into a surrounding chamber, whence it is ex- 

 pelled through the anal orifice. This action may be distinctly 

 watched through the external walls in the smaller and more trans- 

 parent species ; and not even the ciliary action of the Tentacula of 

 the Polyzoa affords a more beautiful spectacle. It is peculiarly 

 remarkable in one species that occurs on our own coasts — the 

 Ascidia pared/ el cx/rammaf — in which the wall of the branchial 

 sac is divided into a number of areolae, each of them shaped into 

 a shallow funnel ; and round one of these funnels each branchial 



* See Mr. G. Busk's ' Remarks on the Structure and Function of the 

 Avicularian and Vibracular Organs of Polyzoa, 'in "Transact, of Microsc. 

 Sue," Ser. 2, Vol. ii. (1854), p. 26. 



t See Alder in "Ann. of Nat. Hist.," 3rd Ser., Vol. xi. (1863}, p. 157; 

 and Hancock in " Journ. of Linn. Soc," Vol. ix., p. 333. 



