ZOOLOGY. 701 



IX. MOLLUSKS. 



Acephals; Gastropods; Cephalopods. 

 x. Vertebrates. 



Fish-like Vertebrates; Leptocatdians; Myzonts; Fishes; Am- 

 phibians; Eeptiles; Birds; Mammals. 



I. GENERAL ZOOLOGY. 



The deep-sea fauna. — Prof. T. Fuchs has sought to ascertain "what 

 is to be understood by the term 'deep-sea fauua,' and by what phys- 

 ical conditions is its occurrence governed." He recalls that (1) the limit 

 to which sea- weeds desceud is about 30 fathoms, that (2) the reef-build- 

 iug corals decrease perceptibly below 8 fathoms, "and a depth of 20 

 fathoms is generally regarded as their extreme limit," and that (3) " a 

 third important element of the littoral fauna consists of the beds of 

 large bivalves" which appear "no longer to occur below 20 fathoms." 

 It is claimed " that fully two-thirds of the whole of the littoral marine 

 auimals are more or less intimately connected with one or other of these 

 three assemblages," and that "it follows directly that the great ma- 

 jority of the littoral animals cannot desceud much below 30 fathoms in 

 the sea." It is then claimed that " the great mass of littoral animals do 

 not descend in the sea much beyond 30 fathoms, and, on the other hand, 

 that at a depth of 90 fathoms the fauna already everywhere shows the 

 marked type of the deep- sea fauna." As to the relations between the lit- 

 toral and deep-sea fauna?, Professor Fuchs thinks "we are in a position to 

 lay down a more exact boundary," and comes to the conclusion that the 

 "long series of facts from different seas indicate very accordantly a 

 depth of about 50 fathoms as that critical zone in which is situated the 

 great turning point that separates the littoral from the deep-sea fauna ; 

 and we are therefore justified in regarding the line of 50 fathoms as an 

 ideal boundary between the littoral and the deep-sea fauna." It is added 

 that " it is very interesting to observe that this depth is pretty nearly the 

 same in all seas," but that between the tropics the separation of the lit- 

 toral and deep-sea fauna? is " to a certain degree real." Moreover, in 

 those regions "below a depth of 30 fathoms there follows an extremely 

 sterile region, with few animals." Within the tropics, in fact, the littoral 

 fauna would be separated from the deep-sea fauna by " a comparatively 

 sterile region extending about from 30 to 90 fathoms." Such au inter- 

 mediate region is said to be unknown in temperate and cold seas. 



As to the cause of the limitation of the faunae, it is claimed that " light 

 is the most powerful factor amongst all the agents which influence 

 life upon the earth." It is urged that " the difference which is produced 

 in the fauna of the sea by its conditions of light is no other than that 

 which we distinguish as littoral fauna and deep-sea fauna; in other 

 words, that the littoral fauna is nothing but the fauna of light, aud the 



