136 THE LIFE OF CRUSTACEA 



It is in the deep sea also that we find the curious 

 Hermit Crabs of the family PylochelidcB (Fig. 37, p. 94), 

 which are perfectly symmetrical and show no trace 

 of having ever adopted the habit of living in Gastro- 

 pod shells ; so primitive, indeed, are these forms that 

 it is not easy to find characters by which to define 

 them from the lobster-like Thalassinidea or from the 

 true Lobsters themselves, and, although no fossil 

 representatives are yet known, there seems no reason 

 to doubt that the Pylochelidae are nearly related to 

 the primitive stock from which the other Hermit 

 Crabs have been evolved. Among the deep-sea 

 Prawns there are many forms, both of Penseidea and 

 of Caridea, which are more primitive than most of 

 their relatives from shallow water ; and although in 

 these cases also the geological records are faulty, we 

 may assume, if we cannot prove in detail, a general 

 similarity to the fossil Prawns of Mesozoic rocks. 



When all has been said, however, perhaps the most 

 surprising thing about the deep-sea fauna is, not that 

 the animals are unlike those living in shallow water, 

 but that they differ from them so little. When we 

 consider the physical conditions of the oceanic 

 abysses — the absolute darkness, the freezing cold, 

 the pressure measured in tons on the square inch — 

 it would seem inevitable that the physiological pro- 

 cesses of deep-sea animals must differ greatly from 

 those of animals living in shallow water ; yet in 

 very many cases these differences of function are 



