164 THE MICROSCOPE AND ITS REVELATIONS. 



closure of their bodies in a ' tunic/ which is sometimes leathery or even 

 cartilaginous in its texture, and which very commonly includes calcare- 

 ous spicules, whose forms are often very beautiful. They present a strong 

 resemblance to the Polyzoa, not merely in their general plan of conforma- 

 tion, but also in their tendency to produce composite structures by gem- 

 mation; they are differentiated from them, however, by the absence of 

 the ciliated tentacles 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 anything 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 SalpidoB and other 

 floating species which are chiefly found in seas warmer than those that 

 surround our coast, and the curious Appendicularia to be presently no- 

 ticed ( 560), 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 gem- 

 mation at all, or, if this process does take place, the gemmae being de- 

 tached before they have advanced 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, resembling that shown in 

 Figs. 382 and 384; 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 expelled through the anal orifice. This action 

 may be distinctly watched through the external walls in the smaller and 

 more transparent species; and not even the ciliary action of the tentacles 

 of the Polyzoa affords a more beautiful spectacle. It is peculiarly re- 

 markable in one species that occurs on our own coasts, the Ascidid pa- 

 rallelogramma, 1 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 fissure makes two or three turns 

 of a spiral. When the cilia of all these spiral fissures are in active 

 movement at once, the effect is most singular. Another most remarka- 

 ble phenomenon presented throughout the group, and well seen in the 

 solitary Ascidian just referred -to, is the alternation in the direction of 

 the Circulation. The heart, which lies at the bottom of the branchial 

 sac, is composed of two chambers imperfectly divided from each other; 

 one of these is connected with the principal trunk leading to the body, 

 and the other with that leading to the branchial sac. At one time it will 

 be seen that the blood flows from the respiratory apparatus to the cavity 

 of the heart in which its trunk terminates, which then contracts so as to 

 drive it into the other cavity, which in its turn contracts and propels it 

 through the systemic trunk to the body at large; but after this course 

 has been maintained for a time, the heart ceases to pulsate for a moment 

 or two, and the course is reversed, the blood flowing into the heart from 

 the body generally, and being propelled to the branchial sac. After this 



1 See Alder in " Ann. of Nat. Hist.," 3d Ser., Vol. xi. (1863), p. 157; and Han- 

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



