SPECIALITIES OF THESE RELATIONS. 465 



Entozoa yield us many examples of this causal relatiorij 

 raised to a still higher degree. The Oordias, or Hair- worm, 

 is a creature which, finding its way when young into the 

 body of an insect, there grows rapidly, and afterwards emerg- 

 ing to breed, lays as many as 8,000,000 eggs in less than a day. 

 Similarly with the larger types that infest the higher 

 animals. It has been calculated by Dr. Eschricht, as quoted 

 by Professor Owen, that there are " 64,000,000 of ova in the 

 mature female Ascaris Lumhricoides. " Even a still greater 

 fertility occurs among the cestoid Entozoa. 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 available for germ-producing organs, which 

 nearly fill each segment: 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, con- 

 tinues 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 ex- 

 penditure, 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 nature. Usually, the 

 form assumed in the body of the first host, is devoid of all 

 that part in which the reproductive structures take their rise ; 

 and this part grows and develops reproductive structures, 

 only in some predatory animal to which its first host falls a 

 sacrif ce. Occasionally, however, the Qg^ gives origin to the 

 sexua. form in the animal that originally swallowed it, but 

 the development remains incomplete — there is no sexual 

 genesis, no formation of eggs in the rudimentary segments. 

 That these may become fertile, it is needful, as before, for the 



