THE ARTHROPOD A OF THE DEEP SEA 143 



Among tlie simple Ascidians we find no family 

 that is peculiar to deep water ; but the Cynthiidse and 

 Ascidiida? both contain genera that are abysmal, and 

 the Molgulida3 have one species, Molgula 2^y'^ifoTmiSj 

 that extends into the abysmal zone to a depth of 600 

 fathoms. 



In the genus Culeolus and in Fimrjulus cinereus 

 and Bathijoncus, all deep-water Ascidians, there is a 

 very curious modification of the branchial sac, the 

 stigmata being apparently not formed, in consequence 

 of the suppression of the fine interstigmatic vessels. 

 This peculiar feature is only found in the deep-sea 

 simple Ascidians and, as we shall see presently, in one 

 species of the deep sea compound Ascidians, but it is 

 not apparently an essential character of those living 

 in the abysmal zone, notwithstanding the fact that 

 it is found in such widely separated genera ; for 

 Cory7iascidia, Abyss ascidia, and Ilypohyilims, living in 

 depths lying between 2,000 and 3,000 fathoms below 

 the surface, have branchial sacs of the ordinary type. 

 Professor Herdman is of opinion that this simple form 

 of branchial sac is not a primitive form, but most 

 probably a modification of a more complicated type. 



In Culeolus Murraiji there is a remarkably abun- 

 dant supply of blood-vessels to the tunic, and these 



