ALTERNATE AND EQUIVOCAL REPRODUCTION. 163 



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 Medusa? is not less instruc- 

 tive. According to the observations of 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 minute cilia, by means of which they swim with much 

 activity. 



V 



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 or 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 

 generation, it is said, is to favor the development of the species in its 

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

 the nurses, this end is accomplished 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 gen- 

 eration, nurse and take care of it. It is no longer the body of the nurse, 

 but its own instincts, which become the instrument of the development. 

 This seems to receive confirmation from the fact that the working bees, 

 like the plant-lice, are barren females. The attributes 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. 



