THE CLASS OF INSECTS. 5 



but all becoming respectively more complicated. For example, 

 in the Earth-worm (Lumbricus) , each ring is distinguishable into 

 an upper and under side, and in addition to these a well- 

 marked side-area, to which, as for example in marine worms (e. g. 

 Nereis) , oar-like organs are attached. In most worms eye-spots 

 appear on the front rings, and slender tentacles grow out, and 

 a pair of nerve-knots (ganglia) are apportioned to each ring. 



In the Crustaceans, such as the fresh-water Crawfish (Asto- 

 cus), as shown by the German naturalist Rathke ; and also in 

 the earliest stages of the Insect, the body at once assumes a 

 worm-like form, thus beginning its embryonic life from the goal 

 reached by the adult worm. 



The young of all Crustaceans (Fig. 7) first begin life in the 

 egg as oblong flattened worm-like bodies, each end of the body 

 being alike. The young of the lower Crustaceans, such as the 

 Barnacles, and some marine forms like the Jcera and some 

 lowly organized parasitic species inhabiting the gills of 

 fishes, are hatched as microscopic embryos which would readily 

 be mistaken for young mites (Acarina). In the higher Crus- 

 taceans, such as the fresh-water Crawfish, the 

 young, when hatched, does not greatly differ 

 from the parent, as it has passed through the 

 worm-like stage within the egg. 



Fig. 7 represents the }*oung of the fresh- 

 water Lobster (Crawfish) before leaving the 

 egg. The body is divided into rings, ending 

 in lobes on the sides, which are the rudiments 

 of the limbs, b is the rudiment of the eye- Fig. 7. 

 stalk, at the end of which is the eye ; a is the fore antennae ; 

 c is the hind antennae ; d is one of the maxilla-feet ; e is the 

 first pair of true feet destined in the adult to form the large 

 1 'claw." Thus the eye-stalks, antennae, claws, and legs are 

 moulded upon a common form, and at first are scarcely distin- 



with the cirri serving as swimming and locomotive organs ; rf, the caudal styles, or 

 tail-feelers. In this figure we see how slight are the differences between the 

 feelers of the head, the oar-like swimming organs, and the caudal filaments ; we 

 can easily see that they are but modifications of a common form, and all arise 

 from the common limb-bearing region of the body. The alimentary canal, with 

 the proventriculus, or anterior division of the stomach, occupies the middle of the 

 body; while the mouth opens on the under side of the head. From A. Agatsiz. 

 FIG. 7. Embryo of the Crawfish. From Rathke. 



1* 



