BOOK NINTH. 
ACEPHALA. 
CLASSIFICATION. 
§ 170. 
Tue Acrpuata are principally characterized in having a headless body, 
and a very large mantle, which so envelops the body, that there is 4 spa- 
cious and more or less closed cavity in which the oral and anal orifices are 
often entirely concealed. 
Their body is either wholly asymmetrical, or divided into a right and a 
left side. In this last case, the organs, excepting the digestive canal, are 
in pairs; and the two sides are perfectly symmetrical, or one is developed 
at the expense of the other. All Acephala are aquatic; many are perma- 
nently attached during life; others creep about, and a few only can swim 
freely. Copulatory organs are wanting throughout. 
ORDER I. TUNICATA. 
Body wholly asymmetrical and so enclosed in the mantle, that there are 
only two narrow openings. 
Famity: Ascrpraz. : 
Genera: A. Compositae. 
Didemnum, Diazona, Aplidium, Botryllus, Botrylloides, Leptoclinum, 
Eucoelium, Synoecium, Polyclinum, Sigillina, Perophora, Pyrosoma. 
B. Simplices. 
Clavelina, Phallusia, Rhopalaea, Boltenia, Cynthia, Chelyosoma. 
Famity: SaPinar. 
Genus: Salpa. 
ORDER II BRACHIOPODA. 
Animals which are symmetrical and bivalved, and whose widely-open 
mantle encloses two fringed, arm-like, protractile tentacles, 
