4 THE CLASS OF INSECTS. 



their young stages, from the time of their exclusion from the egg, 

 until 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 articulates are at first, and how 

 soon they begin to differ, and assume the shape characteristic 

 of their class. 



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 body- walls, 

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

 assumes the appearance of being more or less equally ringed, 

 a 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- 

 e pear and regular locomotive organs, consisting 

 of minute paddles, grow out from each side ; 

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

 mentary eyes) appear on the few front rings 

 of the body, which are grouped by themselves 

 j C 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 

 ^functions ; but only a more perfect system of 

 Fi 6 rings, each relatively very equally developed, 



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

 The tracheae and a nervous filament 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. Original. 



FIG. 5. Young Terebella, soon after leaving the egg. From A. Agassiz. 



FIG. (5 represents the embryo of a worm (Autolytus cornutus) at a later stage 

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

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

 oar-like organs (cirri) at the base of which are inserted the locomotive bristles, 



