126 A BIOLOGY OF CRUSTACEA 



of the ocean floor. The long setae on the front legs of Platymaia 

 may be used as a rake for separating edible material from the 

 ooze, and the fantastic chela of Thamnastocheles might also be 

 used for a similar purpose. 



Another peculiarity of the deep-water Crustacea is that they are 

 often larger than their close relatives from shallower water. The 

 most striking example of this is the isopod Bat hy nonius giganteus, 

 which reaches a length of ten inches; among other isopods a length 

 of three inches is exceptional. Among the Tanaidacea too the deep 

 water forms are the largest. Ncotanais giganteus and Herpotanais 

 kirkegaardi reach lengths of 20 and 25 millimetres respectively. A 

 length of 10 millimetres among the tanaids from shallower water 

 is exceptional enough to elicit admiration. 



Fig. 56. Platymaia alcocki (Brachyura). A bottom dwel- 

 ling spider crab from the depths of the Indian Ocean. 

 Actual width of carapace about 9 cm. (After Doflein). 



Relatively large eggs are also of frequent occurrence among the 

 deep-sea Crustacea. Large eggs are usually associated with the 

 elimination of early larval stages so that the young hatch at an 

 advanced stage of development. It is not known why this should be 

 advantageous in the deep sea, but it may well be due to a shortage 

 of food suitable for larvae. 



A special name is given to the deepest parts of the oceans; depths 

 greater than 6,000 metres are known as the hadal region (from 

 Hades, the mythical ruler of the Greek underworld). Such regions 

 occur as faults or trenches in the sea floor. The Crustacea of this 

 region are not well known, but it is possible to say that the isopods, 



