474 



DEPTHS OF THE OCEAN 



ing- to greater depths in winter. The spawning females usually 

 repair to shallow places in the summer, the higher temperatures 

 being better suited to the development of the eggs and larvae. 



Several of the strange mask crabs {Hyas, see Fig. 2)2)7 > 

 Stenorhyncktis, Inachus) also inhabit the littoral zone, chiefly 

 where the bottom is overgrown with algae, bryozoans, and 

 hydroids, being rarely met with upon sandy bottom. They 

 are supplied with small hooks on the carapace and extremities, 

 by which they attach to themselves the algae or animal-colonies 

 around them. These crabs are extremely sluggish and inactive, 

 and they derive an advantage from this remarkable habit, since 

 they are difficult 

 todistinguish from 

 their surround- 

 ings, and conse- 

 quently they can 

 conceal them- 

 selves from their 

 prey as well as 

 from their ene- V 



The bottom 

 here chiefly con- 

 sists of what has 

 been called shell- 

 sand, made up Hyas arar,eus, L. 



entirely of shell- 

 fragments of molluscs, echinoderms, balanids and other creatures ; 

 it is usual to make a distinction between the coarse and the fine 

 shell-sand. This detritus is practically only met with in the 

 littoral zone of the skerries, and is undoubtedly due to the action 

 of the waves and breakers. Burrowing forms, for the most part 

 mussels, spatangids, clypeastrids, and worms, predominate. 

 The lancelet {Ampkioxtis) also makes this its principal home. 

 The loose formation is burrowed into quite easily, and a 

 lancelet can work its way down in the course of a few seconds.^ 

 We must also include the sand-eels i^Am.modytes) amongst the 

 vertebrate forms that burrow in this sandy bottom, though they 

 are somewhat local in their occurrence. 



Fig. 337. 



' This form burrows in a curving direction beneath the surface of the sand, finally pro- 

 truding its head very slightly a short distance from where it went in, and remaining stationary 

 in this position. 



