THE CLASS OF INSECTS. 



Stages, from the time of their exclusion from the egg, imtil 

 they pass into mature life. A more careful study of this 

 period than we are now able to enter upon would show us how 

 much alike the young of all Arthropods are at first, and how 

 soon they begin to differ, and assume the shape characteristic 

 of their branch. 



Most Worms, after leaving the egg, are at first like some 

 infusoria, being little sac-like animalcules, often ciliated over 

 nearly the entire surface of the infinitesimal body. 

 Soon this sac-like body grows longer, and con- 

 tracts at intervals ; the intervening parts become 

 unequally enlarged, some segments, or rings, 

 formed by the contraction of the bod^^-walls, 

 greatly exceeding in size those next to them ; and it thus 

 assumes the appearance of being, more or less equally ringed, 

 as in the young Terebella (Fig. 5), where the 

 ciliae are restricted to a single circle surrounding 

 the body. Gradually (Fig. 6) the cilise disap- 

 pear and regular locomotive organs, consisting 

 of minute paddles, grow out from each side ; 

 feelers (antenna), jaws, and eyes (simple rudi- 

 mentar}^ eyes) appear on the few front rings 

 of the body, which are grouped b}^ themselves 

 into a sort of head, though it is difficult, in a 

 large proportion of the lower worms, for un- 

 skilled observers to distinguish the head from 

 the tail. 



Thus we see throughout the growth of the 



worm, no attempt at subdividing the body 



into regions, each endowed with its peculiar 



I , functions ; but only a more perfect s^^stem of 



' rings, each relatively ver^- equalh' developed, 



m the figure, also to the dorsal vessel (c), the intestine (6), and the nervous cord (n). 

 The tracheaB and a nervous lilameut are also sent into the legs and to the wings. 

 The tracheae are also distributed to the dorsal vessel and intestine by numerous 

 branches which serve to hold them in place. — Orir/inal. 



Fig. 5. Young Terebella, soon alter leavnig the egg. — From A. Agassiz. 



Fig. 6 represents the embryo of a worm (Autolytus comutus) at a Inter stagv 

 of growth, a is the middle tentairle of the head ; e, one of the posterior tentacles ; 

 h, the two eye-spots at the base of the hinder pair of feelers ; c is one of a row of 

 our-lilie organs (cirri) at the base of which are inserted the locomotive bristle.s, 



