146 THE FAUNA OF THE DEEP SEA 



with a very long stalk. Furthermore, the only known 

 stalked forms of the very large family Ascidiidas are 

 the abysmal genera Gorynascidia and Hypobytliius. 



The most remarkable character of the genus 

 Hypobythius is the simple condition of its branchial 

 sac, reminding one of the structure of this organ in the 

 shallow- water genus Clavelina. 'There are no folds 

 and there are no internal bars,' to quote the descrip- 

 tion given by Professor Herdman ; ' only a single 

 system of vessels can be recognised, branching and 

 anastomosing so as to form a close network, the small 

 rounded meshes of which are the stigmata. The 

 tentacles and dorsal lamina cannot be made out.' 



Among the compound Ascidians only four families 

 extend into the abysmal zone, namely, the Botryllidae, 

 Polyclinidas, Didemnidae, and Ccelocormidas, and of 

 these only one species, Pha/ryngodictyon niirabile, of 

 the family Polyclinidas. extends into water of greater 

 depth than 1,000 fathoms. In Pharyngodidyon we 

 find the same curious simplification of the branchial 

 sac that we have just referred to in the genera of 

 simple Ascidians, Cvleolvs, Fim'ji<l>i.i?, and Bathyuncus. 

 Coelocormus Huxleyi from a depth of 600 fathoms is 

 a very peculiar form and the type of a separate family, 

 the Ccelocormidse. 



