170 COMPARATIVE ANATOMY 



overgrowth of the yolk is complicated by the presence of the 

 amnion, but the course of events will be readily understood by 

 an inspection of the diagrams, fig. 41, A-D. The enteron is at 

 first of considerable size, owing to the bulk of the yolk con- 

 tained in it, but as the yolk is absorbed, it becomes smaller and 

 smaller, and is finally reduced to very small dimensions, the 

 larger part of the alimentary tract being formed from two epi- 

 blastic invaginations, the stomodseum and proctodaeum. 



The young cockroach is not hatched till it has assumed 

 nearly all the characters of the adult, and when it leaves the 

 egg it is an active insect, not, as is the case in so many other 

 insects, a fleshy grub or caterpillar. The young males are 

 wingless, like the females, and the tenth tergum is notched 

 posteriorly in both sexes. The young forms are generally 

 known as nymphs, and they attain their full sexual characters 

 after a number of moults or ecdyses, in each of which the 

 chitinous cuticle is entirely thrown off and replaced by a new 

 one. 



Insects which are hatched like the cockroach, with practically 

 the full adult characters, and undergo no metamorphosis, are 

 known as Homomorpha or Ametabola. Those which are hatched 

 out as grubs or caterpillars, and afterwards enter into a resting 

 stage from which they emerge as perfect insects, are known as 

 Heteromorpha or Metabola. Our common butterflies are good 

 examples of the latter division. The caterpillar is a larval 

 form produced from the egg. After a free life of some length, 

 during which it feeds on plants, the caterpillar forms a protect- 

 ing case or cocoon for itself and becomes a pupa or chrysalis. 

 During this stage it undergoes profound modifications, and 

 eventually emerges from the pupa-case as a perfect insect or 

 imago. 



It is clear, from the study of the development of the cock- 

 roach, that there is a closer correspondence between the 

 structure of insects and Crustacea than their adult anatomy 

 would lead one to suppose, and zoologists are abundantly 

 justified in classing them together in the phylum Arthropoda. 

 It is possible to go even further, and to compare the structure 

 of the Arthropoda with that of the Chsetopod worms. In both 

 these groups we find an elongate segmented body, bearing 

 lateral hollow appendages (parapodia in Polychaeta). The 

 alimentary tract is a more or less straight tube traversing the 



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