THE CYCLE OF LIFE 447 



compared to a microscopic six-legged easel, and the 

 same type occurs in Brittle-stars. Those of starfishes 

 and sea-cucumbers baffle brief description. Those of 

 feather-stars or Crinoids are not so divergent. 



But even more remarkable than the shape of the larvse 

 is the fact that they do not develop directly into the adult, 

 in the way in which a tadpole develops into a frog. The 

 development is circuitous. Within the larva a new forma- 

 tion begins, on a fresh architectural plan, utihzing some 

 parts and rejecting others, and the result is the adult 

 form (Fig. 21). The curious arms or processes characteristic 

 of the larva are in part absorbed and in part thrown ofi. 

 The wandering amoeboid cells which play so diverse and 

 important a role in the animal kingdom are very active, 

 at once as sappers and miners in breaking down, and as 

 builders in the re-construction. 



Mermis and Horse-hair Worms. — ^A curious sight is 

 sometimes seen in gardens, especially after heavy rains 

 in summer — a thin thread of a worm raising itself into the 

 air from the top of a cabbage plant and writhing as if in 

 search of something. That is a female Mermis, and it is 

 supposed to be seeking out a place for egg-laying more 

 suitable than the very damp earth. This is an episode in a 

 curious hfe-history. The mature Mermithidse hve in the 

 earth or in fresh water, and so do the first larval stages. 

 From the earth or water, the young larvae migrate and 

 bore actively into beetles, caterpillars, milhpedes, slugs, 

 and so on. When they become mature, the worms leave 

 their hosts. Now it is noteworthy that no food is taken 

 either by the adults or by the young larvae. All the 

 feeding is done by the second larval forms during the 

 parasitic period. Many adult insects are non-nutritive 



