264 MARINE ANIMALS 



reduced skeleton; and the pelagic deep-sea holothurian, Pelagothuria, 

 has no calcium bodies in its skin. Numerous deep-sea barnacles of the 

 genus Rcalpellum have an incompletely calcified shell. Such forms all 

 belong to great depths, mostly below 1250 m. Even in the same species, 

 the skeleton may be reduced in deep water ; Scalpellum stearnsi in 200- 

 450 m., with a normal shell, is identical with S. polymorphum from 

 below 400 m., with a reduced shell. 23 Mollusk shells from great depths 

 are mostly fragile. Many deep-sea fishes exhibit uncalcified or weakly 

 calcified skeletons, as in Chimaera. Some deep-sea forms, on the con- 

 trary, are not deficient in calcium carbonate. 



Faunal affinities of abyssal animals. — The occurrence of surface 

 forms of polar seas in deep waters in the warmer zones, where they 

 are absent at the surface, is explained by the temperature relations. 

 Among many animals the connection between the polar surface forms 

 and the tropical abyssal ones can be followed continuously. This is 

 true of many snails and bivalves of the northern Atlantic, living in 

 the Arctic littoral to a depth of 50 m., while they may be followed at 

 steadily increasing depths on both sides of the ocean, to the Canaries 

 and St. Helena (to 2000 m.) and to the West Indies and Pernambuco 

 (to 800 m.). 24 A few further examples may be cited. The northern 

 starfish Brisinga is found in the abyssal Indian Ocean, which also has 

 the characteristic northern snails Pleurotomidae, Trochidae, and 

 Naticidae. The lamellibranch genera Yoldia, Nucula, Lima, and Abra 

 are littoral in the Arctic, abyssal in warm seas; the same is true of 

 decapod crabs; 25 typical northern genera of shrimps,* of crabs,t and 

 of anomuranst inhabit the deep sea in low latitudes. The deep-sea 

 genera of shrimps, Hymenodora and Pontophilus, have at least one or 

 a few species near the surface in the Arctic Ocean. The sharks have 

 similar distributions, especially among the Squalacea; Centroscyllium 

 is also found nearer the surface in polar waters than in the tropics. 26 

 The great majority of deep-sea fishes, however, do not enter polar 

 surface waters. 27 On the whole, the difference between littoral and 

 abyssal animal life is greater in the tropics than in arctic seas. 



Uniformity in abyssal waters. — Uniformity of environmental 

 conditions in the deep sea is greater than in any other division of the 

 ocean. At great depths the water is uniformly cold, with little motion, 

 without sunlight, the bottom covered with ooze whose only variation 

 is chemical, and uniform over wide areas even in this respect. The 



* Crangon, Pandalus, Pasiphaea. 

 t Homola, Latreillia, Maia. 

 t Lithodes. 



