132 REPRODUCTION. 



cession is an incomplete animal, we cannot better explain 

 their office, than by considering them analogous to the larvae 

 of the Cercaria, that is, as nurses.* 



345. The development of the Medusse is not less instruct- 

 ive. According to the observations of M. Sars, a Norwegian 

 naturalist, the Medusa brings forth living young, which, 

 after having burst the covering of the egg, swim about 

 freely for some time in the body of the mother. When 

 born, these animals have no resemblance whatever to the 

 perfect Medusa. They are little cylindrical bodies (Fig. 

 142, ), much resembling infusoria, and like them covered 

 with fine cilia, by means of which they swim with much 

 activity. 



346. After swimming about freely in the water for some 

 days, the little animal fixes itself by one extremity (Fig. 

 142, e). At the opposite -extremity a depression is gradu- 



* There is a certain analogy between the larvae of the plant-louse (Aphis) 

 and the neuters of the working ants and bees. This analogy has given 

 rise to various speculations, and, among others, to the following theory, 

 which is not without interest. The end and aim of all alternate gene- 

 ration, it is said, is to favor the development of the species in its pro- 

 gress towards the perfect state. Among the plant-lice, as among all 

 the nurses, this end is accomplished unconsciously, by means of the 

 body of the nurse. Now a similar end is accomplished by the working 

 ants and bees, only, instead of being performed as an organic function, it is 

 turned into an outward activity, which makes them instinctively watch 

 over the new generation, and nurse and take care of it. It is no longer the 

 body of the nurse, but its instincts, which become the instrument of the 

 development. This seems to receive confirmation from the fact that the 

 working bees, like the nurses of the plant-lice, are barren females. The at- 

 tributes of their sex, in both, seem to consist only in their solicitude for the 

 welfare of the new generation, of which they are the natural guardians, 

 but not the parents. The task of bringing forth young is confided to other 

 individuals, to the queen among the bees, and to the female of the last 

 generation among the plant-lice. Thus the barrenness of the working bees, 

 which seems an anomaly as long as we consider them complete animals, 

 receives a very natural explanation so soon as we look upon them 

 merely as nurses. 



