ABYSSAL BENTHAL AND PELAGIAL 



263 



bony fishes, Gigantura, Macruridae (Fig. 74) , and Gastrostomidae, are 

 characteristic of the abyssal waters. Band-like compressed forms such 

 as the deep-sea shark, Chlamydoselachus, and the fishes Regalecus and 

 Trachypterus, are likewise characteristic of the depths, in correlation 

 with their weak powers of locomotion. The absence of water movement 



Fig. 73. — Chimaera montrosa. After Boas. 



also makes possible the survival of many forms with weak and fragile 

 skeletons, whose existence is conditioned by the difficulty of lime 

 formation. 



The steady drop of temperature with depth leads to uniform low 

 temperatures in the great depths of the abyssal ocean. The difficulty in 



Fig. 74. — Macrurus rupestris. After Hentschel. 



the formation of lime produced by the low temperatures in the polar 

 seas (see p. 167) is thus repeated in the deep sea. Accordingly, animals 

 belonging to groups with a strong calcareous skeleton or shell in the 

 warmer waters are found with weakly or completely non-calcareous 

 skeletons in the deep sea. Thus calcareous sponges are wanting below 

 300 m. Numerous echinoderms, such as the echinothurid sea urchins, 

 have a leathery non-calcareous skeleton; the starfish Brisinga has a 



