92 ELEMENTS OF BIOLOGY. 



nished with masticatory organs adapted for the division of 

 leaves, and the like. The parts of the mouth in the butter- 

 fly, on the other hand, whilst morphologically identical with 

 those of the larva, are so modified that they form a tubular 

 organ, fitted for the suction of fluids, whilst the biting jaws 

 of the caterpillar are aborted. The caterpillar, again, car- 

 ries three pairs of legs in the front part of its body (fig. 23, 

 a), which correspond with, and are ultimately converted 

 into, the three pairs of legs possessed by the adult insect. 

 The caterpillar, however, has an additional series of loco- 

 motive processes developed upon some of the hinder seg- 

 ments of the body (fig. 23, a), which processes are merely 

 of a provisional nature, and are not present in the adult 

 even in a rudimentary form. 



In some cases, however, not only does the young form 

 exhibit provisional structures, but there is what may be 

 called a " provisional larva," out of a portion of which, and 

 only a portion, the adult animal is developed. Thus, in the 

 sea-urchins the egg gives rise to an actively locomotive 

 larva, which is furnished with a mouth and alimentary canal 

 of its own, and leads a completely independent existence. 

 After a while, however, there is formed upon one side of 

 the stomach of the larva a mass of growing material, which 

 appropriates the stomach, and is gradually developed into a 

 young sea-urchin. Only the stomach, however, of the ori- 

 ginal "provisional larva" is thus retained to form part of 

 the adult organism ; and the remainder of this temporary 

 form, having served its purpose, is either absorbed, or is 

 cast off as useless. 



There is one respect, however, in which the adult animal 

 is always the superior of the young form, or at any rate 

 almost always ; and that is in its possession of generative 

 organs, and the power thereby conferred on it of producing 

 fresh individuals by a true sexual process. Cases are not 

 unknown in which young and immature forms can produce 

 fresh beings like themselves, but this is, in the great majority 



