CRUSTACEA AND MOLLUSCS 247 



crustaceans. In fresh water, also, there are crustaceans 

 that have assumed the most curious shapes owing to 

 parasitism, such as the Achtheres percarum, a parasite 

 of the perch. 



The Crustacea are particularly striking illustrations of 

 the biogenetic law that we mentioned in the fifth chapter. 

 The crustacean reproduces in its ontogeny its develop- 

 ment from the ovum the course of its phylogeny, or 

 ancestral development in past ages. 



In the above-mentioned parasites we can understand 

 why they cannot give up the free-living stage of their 

 ancestors in their development. How could they reach 

 their host how could the animal, for instance, that 

 comes from the sacculina egg reach its crab, if it could 

 not swim freely about at the beginning of its career, and 

 then settle like a plant on its victim ? 



There are only a few animals in which it was 

 necessary to retain the ancestral stages. In many 

 cases it will be an advantage to the animal to be as 

 fully developed as possible when it leaves the egg, so 

 as to reach sexual maturity quickly and provide off- 

 spring. The whole development has, therefore, to take 

 place in the egg, as is the case with the birds, and 

 individuals of this kind are protected by the shell of 

 the egg or within the mother's womb. But a develop- 

 ment of the animal within the egg implies a certain size 

 in the egg, because, whilst the free-swimming larva can 

 obtain from without the necessary nourishment for its 

 growth, the animal in the egg is restricted to the supply 

 of food inside the shell, or to the substance that we 

 call the yolk. Eggs with much yolk can, in view of 



