ABYSSAL BENTHAL AND PELAGIAL 267 



of the dead shells of the relatively few surface forms, which predomi- 

 nate on the sea bottom on account of their great numbers in the lighted 

 zone. Among coelenterates, Hydrozoa are scarce in the abyssal. The 

 stony corals of deep water are completely different from the reef corals 

 and are almost always simple. Sessile Alcyonaria, like Mopsea and 

 Primnoa, are not rare. Certain sea pens (Umbellula) reach great depths. 

 The abyssal sponge fauna is composed of siliceous sponges, and among 

 these the glass sponges are almost confined to the lightless depths. 

 Turbellarians appear to be absent. The Challenger Expedition secured 

 only six species of nemerteans from the sea floor, while the family 

 Pelagonemertinidae is represented by numerous genera and species in 

 the abyssal pelagial, to which it is confined. 34 



The number of annelids is relatively small, and among them are 

 only the tubicolous forms which feed on detritus ; predaceous forms are 

 rare. The Gephyrea of the deep sea are also tube-dwelling forms such 

 as Phascolosoma. Among crustaceans the barnacles are represented 

 primarily by the genera Verruca and Scalpellum, which are mainly 

 abyssal though not wholly absent from the upper zones. Isopods and 

 amphipods are relatively well developed, with numerous species. 

 Stomatopods seem to be wholly absent. Numerous deep-sea decapod 

 crustaceans are known, although the brachyuran crabs do not go below 

 800 m.; the family Eryonidae is now confined to the deep sea; Anomura 

 are not rare; Galatheidae are for the most part deep-sea forms; 

 Paguridae (hermit crabs) are represented by a number of peculiar 

 genera. Pantopoda are poorly represented, only 6 of the 27 genera 

 being known from the abyssal. Among mollusks, about half the species 

 of scaphopods are deep-sea forms. Lamellibranchs are few and snails 

 still fewer; they have no characteristic abyssal genera. 



The number of abyssal cephalopods is relatively large, the benthic 

 forms fewer than the bathypelagic. Of Bryozoa, the chilostomes have 

 numerous abyssal representatives, some going to great depths, Far- 

 ciminaria delicatissima to more than 5000 m. Cyclostomes and cteno- 

 stomes are poorly represented. The most abundant deep-sea animals are 

 echinoderms. Of the 49 genera of holothurians, 19 are littoral, 20 are 

 abyssal, and 10 are found in both zones; the family Elasipodidae is 

 confined to the deep sea with the exception of two species; it has 66 

 abyssal species, double the number of the abyssal species of other 

 families. 35 Among starfishes, 272 species, out of the total of 832 known 

 forms, descend below 400 m. The proportion of abyssal sea urchins 

 and brittle stars is somewhat smaller. The stalked crinoids are almost 

 wholly confined to the deep sea. Tunicates are few in the deep sea, with 

 the exception of ascidians, which have 12 genera confined to the 



