270 LECTURE XX. 



Radiated to the Molluscous series would be effected. But the Bryozoa, 

 after their larval locomotive stage of existence, became roofed, like 

 plants, and were aggregated together, forming compound animals ; 

 and you may be disposed to ask whether any Molluscous animal can 

 present these Zoophytic conditions. Such do in fact prevail among 

 the Ascidian Tunicaries. All these low-organised Mollusca, after 

 a brief period of natation, become adherent to foreign bodies, and 

 many form groups of individuals united together by a common or- 

 ganised external integument ; the present order of Tunicata consists, 

 in fact, of compound and simple Ascidians. I shall first demonstrate 

 the organisation of this group by these large examples of the simple 

 or solitary Ascidians *, which do not essentially differ in anatomical 

 structure from the compound species^ whose small size renders this 

 a subject for microscopic investigation. 



The exterior tunic of the solitary Ascidian is a thick gelatinous or 

 coriaceous elastic substance adhering by its base, or by a long flexible 

 peduncle to some foreign body, and perforated at the opposite end or 

 at the side by two apertures. The exterior of this tunic is sometimes 

 rough and warty, the inner surface always smooth and lubricous. 



The second tunic is muscular ; it adheres to the outer tunic at the 

 circumference of the two orifices, and is connected to it by blood- 

 vessels at a few other points ; elsewhere it is quite free, and the op- 

 posed surfaces of the intervening space between the muscular and 

 elastic tunics has the aspect of a serous cavity. Its fine fasciculi of 

 fibres are remarkably distinct, and are arranged in two layers, the 

 external circular, the internal longitudinal. The fibres or fasciculi 

 of the outer layer are smaller than those of the inner one, and less 

 regularly disposed. They describe regular circles around the pro- 

 cesses leading to the orifices of the shell. Other fibres of the outer 

 layer pass transversely from one tube to the other. The longitudinal 

 fasciculi radiate from the two orifices, and decussate each other, 

 winding round the bottom of the sac. Deeper, again, than this layer 

 there is a sphincter surrounding the base of each tube, or orifice, from 

 which a third more delicate layer of longitudinal fibres are given off. 



Of the two more or less protuberant and stellate apertures in the 

 outer tunic, one leads directly into the muscular sac, the other into a 

 wide vascular sac contained in the muscular one. The entry to the 

 vascular sac is defended by a circle of short tentacles. The inner 

 surface of the sac is marked by parallel and equidistant transverse 

 lines, the interspaces of which are divided into a series of narrow 

 vertical compartments : the surface is beset with vibratile cilia. An 



* Preps. Nos. 614, 615,616. 785. 898. b. 998. 1303. c. - 



