BOOK NINTH. 



ACEP H ALA 



CLASSIFICATION. 



§ ITO. 



The Acephala are principally characterized in having a headless body, 

 and a very large mantle, which so envelops the body, that there is a 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. TUNIC AT A. 



Body wholly asymmetrical and so enclosed in the mantle, that there are 

 only two narrow openings. 



Family : Ascidiae. 



Genera : A. Compositae. 



Didemnum, Diazona, Aplidium, Botryllus, Botrylldides, Leptoclinum, 

 '• Eucoelium, Sy^ioeclum, Polydinum, SigilUna, Perophora, Pyroso?na. 



B. Simplices. 



Clavelina, Phallusia, Rhopalaea, Boltenia, Cynthia, Chelyosoma. 



Family : Salpinae. 

 Genus : Salpa. 



ORDER II. BRACHIOPODA. 



Animals which are symmetrical and bivalved, and whose widely-open 

 mantle encloses two fringed, arm-like, protractile tentacles. 



