116 HISTORY OF CRUSTACEA. CHAP. XL 



different may be the course of this process in other 

 respects. 



In general, as already stated, it will be advanta- 

 geous to the young to commence the struggle for exist- 

 ence in the form of their parents and furnished with all 

 their advantages in general, but not without excep- 

 tions. It is perfectly clear that a brood capable of 

 locomotion is almost indispensable to attached animals, 

 and that the larvse of sluggish Mollusca, or of worms 

 burrowing in the ground, &c., by swarming briskly 

 through the sea perform essential services by dispers- 

 ing the species over wider spaces. In other cases a 

 metamorphosis is rendered indispensable by the cir- 

 cumstance that a division of labour has been set up 

 between the various periods of life ; for example, that 

 the larvae have exclusively taken upon themselves the 

 business of nourishment. A further circumstance to 

 be taken into consideration is the size of the eggs, a 

 simpler structure may be produced with less material 

 than a more compound one, the more imperfect the 

 larva, the smaller may the egg be, and the larger is 

 the number of these that the mother can furnish with 

 the same expenditure of material. As a rule, I believe 

 indeed, this advantage of a more numerous brood will 

 not by any means outweigh that of a more perfect 

 brood, but it will do so in those cases in which the 

 chief difficulty of the young animals consists in finding 

 a suitable place for their development, and in which, 

 therefore, it is of importance to disperse the greatest 

 possible number of germs, as in many parasites. 



