134 LECTURES ON BIOLOGY 



Nauplius stage ? Can this aimless moulting in the ovum 

 indicate anything else but a tenacious retention of a once vitally 

 important process? 



The following instance will convince even the greatest sceptic. 

 If we take from any lake in Europe or North America a 

 tumblerful of water and pour it through a fine sieve we see a 

 number of minute ' transparent animals, each formed round a 

 minute brown point. By means of a magnifying glass we are able 

 to distinguish them as crustaceans, about 1 centimetre in length, 

 whose scientific name is Leptodora hyalina, a species of the 

 Daphnidse. No better case of adaptation of an animal to its 

 environment is known. The transparent body makes the 

 crustacean practically invisible in the water and thus enables 

 it to escape its enemies, but approach its prey undetected. The 

 tiny brown point, the only part which is visible of this re- 

 markable creature, is the food in the stomach. 



Like the related forms, the Leptodora produces two different 

 kinds of female reproductive cells, described as summer and 

 winter ova. Whilst from the former the growing animals 

 emerge under normal conditions fully developed, having passed 

 the larval stage in the ovum, the winter ovum produces a free- 

 swimming Nauplius. It seems difficult to find a more convincing 

 proof. The cause of this differentiation is probably to be found 

 in the fact that the supply of food furnished to the winter ova, 

 which leave the maternal body prematurely, is insufficient for 

 the complete course of their development. Unless it is therefore 

 to die in the egg from hunger, the young crustacean is compelled 

 to seek its own livelihood. 



The higher crustaceans, the so-called Malacostraca, begin their 

 free life with the more advanced Zosea stage (fig. 44). Forma- 

 tions of a Nauplius larva occur now only rarely ; our common 

 crayfish, indeed, emerges from the egg fully developed, differing 

 from the adult in size but having all organs properly formed. 

 In contrast to it, some of its nearest relations, for instance the 

 beautiful Brazilian Penseus (P. potimirum) has a very complete 

 ontogeny (fig. 43). It comes into existence as a dainty little 

 Nauplius. Gradually the larva increases in size, the body be- 

 comes segmented and the anterior half protected by a shell. 

 Thus cephalothorax and abdomen are formed. Simultaneously 



