450 ZOOLOGY 



solitary forms. The sexual forms are always protogynous, the 

 ovum reaching maturity before the Spermatozoon ; thus self -im- 

 pregnation is prevented. 



All the members of this order are pelagic, and, as is usual 

 with those animals which live at the surface of the sea, they 

 are very transparent, or of a bluish hue. 



Order 3. AsoiDiACEA. 



This order includes some forms, such as Ciona, which are 

 fixed, and rarely reproduce by budding ; but it also includes 

 certain free-swimming pelagic colonies of Ascidians, and certain 

 fixed colonies the individuals of which have no separate tests, 

 but are embedded in a common mass, which increases by gem- 

 mation. In the free -swimming forms the individuals are 

 arranged in a cylinder with their mouths on the outer side 

 and their atrial openings abutting on the hollow of the cylinder. 

 There is but one genus of these pelagic forms, Pyrosoma, which 

 includes, however, many species. 



