SPECIALITIES OF THESE RELATIONS. 489 



ming ciliated embryo, and any one of these, which finds its 

 way into a water-snail, becomes a sporocyst a bag, presently 

 occupied exclusively by masses of cells : each mass by and 

 by becoming a Redia, which makes its way out. Like all 

 its fellows which develop in succession, this, with the excep- 

 tion of a small space occupied by the stomach, devotes the. 

 whole of its interior partly to the formation of other RedicB 

 (which presently escape and become similarly transformed), 

 and partly to the development of Cercarice, into which the 

 internal substance of all the Redice is eventually transformed : 

 Cercarice which, escaping from the host, become agents for 

 infecting other creatures. So that each ovum thus gives 

 rise to a number of forms which severally subserve multi- 

 plication in different ways. Of the other division of Platy- 

 helminthes referred to as carrying on its multiplication by 

 production of ova only, the commonest of the Cestoidea 

 furnishes the best example. Immersed as a Tape-worm is in 

 nutritive liquid, which it absorbs through its integument, it 

 requires no digestive apparatus. The room which one would 

 occupy, and the materials it would use up, are therefore avail- 

 able for germ-producing organs, which nearly fill each seg- 

 ment : each segment, sexually complete in itself, is little else 

 than an enormous reproductive system, with just enough of 

 other structures to bind it together. Remembering that the 

 Tape-worm, retaining its hold, continues to bud-out such 

 segments as fast as the fully-developed ones are cast off, and 

 goes on doing this as long as the infested individual lives; 

 we see that here, where there is no expenditure, where the 

 cost of individuation is reduced to the greatest extent while 

 the nutrition is the highest possible, the degree of fertility 

 reaches its extreme. These Entozoa yield us further 



interesting evidence. Of their various species, most if not 

 all undergo passive migration from animal to animal before 

 they become mature. Usually, the form assumed in the body 

 of the first host is devoid of all that part in which the repro- 

 ductive structures take their rise; and this part grows and 



