38 ANIMAL PARASITES. 



sometimes in a less independent form. It is only in a few spe- 

 cies that the eggs, whether singly or enclosed in the segments, 

 must escape outwards in a state in which the formation of the 

 embryo is not yet completed, so that it exists for a time in the 

 external world, as in other Helmintha. 



Some Bothriocephali, which are themselves unarmed, have hook- 

 less embryos. In this case either the formation of the booklets 

 must take place subsequently outside the intestine of the animal 

 infested, or the brood (II, p. 40) is never capable of active migration 

 into another host. Perhaps thsy may pass through their further 

 development behind the villi and in the more sheltered pyloric 

 appendages of the same animal, or in those of lower animals of 

 the sea and land, and find in such situations the repose necessary 

 for this purpose. As a matter of course, when the brood bears 

 the same form of head as the mature cestode worm whilst still in 

 the egg- shell (for example, that of Phyllobothrium tridax, Van 

 Ben., with four bothridia and an unarmed rudimentary bulb 

 between them), the intestine of the same animal would be 

 sufficient for its dwelling-place, and such brood would require no 

 migration for its development. The latter two forms, however, 

 do not come under our consideration amongst the Cestodea of 

 the human subject. The proglottides to be noticed here have, 

 for the most part, a flat, quadrangular form, very similar to the 

 Trematoda, and usually a white, or yellowish, rarely a reddish or 

 brownish colour (caused by the coloration of the eggs) ; they 

 have neither mouth, anus, nor intestine. With regard to the 

 vessels, nerves, muscular system, and skin, the description of the 

 strobila form may be consulted. Of the sexual organs referred 

 to in the same place, some of the parts have undergone a retro- 

 grade metamorphosis, and we can only distinctly find the genital 

 pore situated on the lateral margin in the middle of the segment, 

 with the seminal cord and the male genital opening towards the 

 anterior end of the proglottis, and with the vagina towards its 

 hinder margin, and lastly the uterus, with semen, eggs, and six- 

 hooked embryos. 



As the eggs possess a much greater diameter than the vagina, 

 they cannot pass through this; they cannot, therefore, be depo- 

 sited in this way, but can only escape out of the proglottis when 

 the latter acquires a larger opening in some way, by tearing, &c. 

 The latter takes place sometimes even in the intestine of the 

 first host, and the eggs escape separately into the outer 



