132 THE LIFE OF CRUSTACEA 



Thaumastocheles (Fig. 44), mentioned above, only 

 four specimens are known — one dredged by the 

 Challenger in the West Indies, and three others 

 more recently brought from Japan. 



The low temperatures prevailing in deep water, 

 even in tropical seas, render it possible for many 

 Crustacea to live there which are closely allied to, 

 or identical with, species occurring in shallow water 

 in the colder seas of the North and South. Many 

 examples of this are mentioned by Dr. Alcock in his 

 discussion of the deep-sea fauna of Indian seas ; for 

 example, the Lobster Nephrops andamanicus, found at 

 depths of 150 to 400 fathoms in the Indian seas, is 

 very closely allied to the Norway Lobster {Nephrops 

 norvegicus) of our own coasts. To some extent this 

 fact affords an explanation of the phenomenon that 

 has been called " bipolarity " in the distribution of 

 marine animals. It has been observed that certain 

 families, genera, and even species, are found in the 

 Arctic and Antarctic seas, although they seem to be 

 entirely absent from the intervening tropical zones. 

 In some cases, however, it has been found that these 

 forms occur in the deep sea in the warmer regions 

 where the cold water offers them a connection 

 between North and South without any great differ- 

 ence of temperature. 



In the early days of deep-sea exploration, when 

 naturalists were becoming aware of the rich fauna 

 inhabiting the abysses of the ocean, which till then 



