5 66 



SEMIVERTEBRA TES. 



a jet of water; and that some of them, like the one figured on p. 561, are used as 

 articles of food. To the same subfamily as Pliallusia also belongs the extensive 

 genus Ascidia, in which the outer tunic is soft and flexible, instead of being 

 cartilaginous. A totally distinct subfamily is, however, indicated by the remark- 

 able deep-sea genus Hypobythius, of which the two known species were obtained 

 at depths varying from six hundred to two thousand nine hundred fathoms, 

 durino- the voyage of the Clmllenger. Here we find the cup-shaped or pear-like 



pear-shaped ascidiax, Jli/pobi/thius (^ nat size). 



body attached by a longer or shorter stem ; while the apertures are circular and 

 not closed by lobes. The outer tunic is cartilaginous, but soft and thin, although 

 thickened in places to form plates. The internal longitudinal bars usually found 

 in the branchial chamber are wanting in this genus ; the gill-slits are small and 

 irregularly placed ; and the viscera form a compact irregular mass on the dorsal 

 side of this chamber. In the species here figured {H. calycodes), which is from 

 the North Pacific, the stem is of great length, and the outer tunic thickened so 

 as to form a number of nodules or plates ; but in the South Atlantic form (H. 



