STRUCTURE OF THE MOLLUSCA. 205 



The Salpcc have the most complex structure of all the 

 Tunicata. Their mantle is tubiform, with transverse mus- 

 cular bands, and enclosed in a transparent cartilaginous 

 envelope. Both are open at each end, and their posterior 

 orifice has a valve, so disposed as to allow water to enter, 

 but not to escape. The mouth is placed in the interior of 

 the tube formed by the mantle, towards its anterior part; 

 and the heart, liver, and other viscera are collected into a 

 small mass near that aperture ; the anus is situated pretty 

 far behind, and a single branchia, composed of a membrane 

 transversely plaited, extends obliquely from the upper to 

 the lower wall of the pallial cavity ; the water which tra- 

 verses this tube consequently bathes the respiratory appa- 

 ratus, and it is by forcibly expelling it that the animal shifts 

 its place. When adult, these animals are free, but at first 

 they are often joined together so as to form a long chain, 

 and swim about a long time in this maimer. 



The Simple Ascidice have no locomotory power, but live 

 affixed to rocks. Their mantle has the form of a bag, fig. 

 7, a, with two orifices, fig. 7, b c, generally near each other ; 

 and the interior of this cavity is lined by the network of 

 branchial vessels. The mouth and the mass of viscera are 

 fixed at the bottom of the branchial sac, and the intestine 

 ends near one of its apertures. Of this extensive series of 

 animals there are numerous genera. 



Other Tunicata, very nearly allied to the preceding, live 

 united into a common mass, and are for this reason desig- 

 nated by the name of Compound Ascidice. A tissue of gel- 

 atinous or cartilaginous consistence contains a great number 

 of these little creatures, and presents at its surface a multi- 

 tude of small six-rayed stars formed by their apertures. 



The name of Botrylli is given to small aggregated Tuni- 

 cata, of an oval form, which differ somewhat from the pre- 

 ceding, inasmuch as they have their branchial sac open at 

 both ends, and the anal orifice opening in a central cavity, 

 around which ten or twelve of these Mollusca are grouped 

 in a radiating manner. 



Lastly, the Pyrosomata are aggregated Mollusca pretty 



