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. ASCIDIACEA. 
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. 
