252 MARINE ANIMALS 



the weight to widely separated points, are adaptations for this purpose. 

 Thus flattened forms such as Sperosoma grimaldi are found among 

 abyssal sea urchins. Holothurians have a specially broadened sole. 

 Almost all deep-sea sponges are stalked or provided at the base with 

 brushes or collars of spines. 7 The root-like growths at the base of the 

 stems of stalked forms serve the same purpose, as in the hydroid 

 polyp Branchiocerianthus, horn corals such as Chrysogorgia, and cer- 

 tain crinoids (Fig. 63) and brachiopods (Fig. 64). As compared with 

 their relatives elsewhere, crustaceans living on the ooze usually have 



Fig. 65. — Deep-sea isopod, Eurycope novae-zelandiae, from 2000-m. depth, on 



blue ooze. After Beddard. 



very long limbs, the terminal joints of which are expanded by hairs to 

 increase their supporting surface (Figs. 65-66) . 



The abyssal pelagic community. — The upper limit of the lightless 

 pelagial varies from 200 to 600 m. in depth according to the latitude. 

 No sharp faunal division at this point is possible, especially as many 

 of the inhabitants of the abyssal waters rise to the surface at night, 

 while others undergo their development at the surface. There are forms 

 which remain permanently in the abyssal waters; thus whereas 90% 

 of the fish larvae and juvenile fishes collected by the Michael Sars 

 in the Atlantic were caught in depths of 0-150 m., Alepocephalus 

 appears to develop entirely below the lighted level. It is unusual for 

 a surface animal to develop in the depths. The siphonophore Velella, 

 whose larvae are taken at depths of 1000 m., is such an exception. 



Nocturnal net hauls on the surface frequently obtain fishes with 

 every mark of the inhabitant of abyssal waters. In the herring fishery, 

 the bottom trawl is used by day and surface nets at night. Cod caught 









