CRUSTACEANS 367 



here serve for swimming-organs, and in front of these will be 

 seen the sprouting jaws and feelers. At a somewhat later stage 

 (b) the feelers and jaws have become much larger, the locomotor 

 foot-jaws have attained a relatively enormous size, and behind 

 them stump-like outgrowths have made their appearance which 

 are destined to become pincers and legs. Long spines, that 

 serve to balance the body, are now present on the strong shield 

 which covers the front part of the body. There can be no doubt 

 that Crabs, the most specialized of all Crustaceans, with their 

 broad cephalothorax and absurdly small tail, have been derived 

 from lobster-like forms, in which there was not the same dis- 

 proportion between these two regions of the body. And the 

 Zosea, by completion of its set of appendages, loss of its balanc- 

 ing spines, and comparatively slow further growth of the pre- 

 cociously-developed foot-jaws, passes into the Megalopa stage (c), 

 which would certainly seem to be reminiscent of such ancestors. 

 Swimming is now effected lobster fashion, by strokes of the tail, 

 but this mode of progression is soon abandoned in favour of 

 walking. The tail now develops but slowly, while the front part 

 of the body grows vigorously, especially in breadth, and thus 

 the proportions of the adult are at last attained (d). The little 

 tail is represented in the figure as projecting backwards, but is 

 in reality folded up under the front of the body. In the adult 

 female this method of disposal largely conduces to the safety of 

 the eggs. 



Particular interest attaches to the development of the Fresh- 

 water Crayfish {Astactts flitviatilis). The animal population of 

 rivers has, for the most part, been originally derived from the 

 sea, and this change of habitat has led to a number of special 

 adaptations to new conditions of life. The development, for 

 one thing, has in many cases undergone considerable modifi- 

 cation. For marine animals the possession of a larval form 

 presents certain advantages, e.g. it helps escape from unfavour- 

 able surroundings and promotes the dispersal of the species. 

 But it would be positively dangerous for creatures which live in 

 rivers to hatch out as larvae, since these being small and weak 

 would be liable to be swept away by the current, and would 

 certainly perish if they reached the sea, for life in which their 

 species have gradually become entirely unsuited. It is therefore 

 not surprising to find that the freshwater representatives of marine 



