196 Natural History of Entozoa. 



as well as redice (or nurses), are developed within the redias, 

 this variation of nurse-contents probably depending on the 

 season.-" " When the immature contents of both nurse-forms 

 (i. e., of sporocysts and rediee) are accidentally set free, and 

 are situated within the organs of nutrition of a living host, 

 then they appear prepared to develope themselves anew into 

 nurse- forms. . . . Some nurses are likewise capable of multi- 

 plication by division and budding." 



These larval forms only exist in a few hosts, and most of 

 them dwell in only one species of animal, but a migration is 

 indispensable to the creature's development. When it finds 

 itself in the right place sexual organs are developed, and the 

 cycle of reproduction starts again with the appearance of true 

 fecundated eggs. 



Zoologically speaking, a fresh individual is the result of the 

 development of a fecundated egg. Each genuine individual 

 thus starts from two parents, but all the offspring of the bud 

 sort merely constitute one individual in a divided state. Thus 

 we see it is zoologically possible for the same individual to be 

 in several places at the same time. 



It is remarkable that these entozoa could not reach their 

 full development and perpetuate their race, without the 

 existence of more than one creature of higher organization than 

 themselves on whom they can prey. Let us, to illustrate this, look 

 to the history of one of the common tape-worms {Tcenia solium). 

 A long string, or colony, of tape-worms is called a strobila, 

 and the separate joints proglottides, or zooids. " The anterior 

 segments form the head and remain barren ; those at the tail 

 end become mature, are capable of living independently of the 

 colony, and develope eggs containing embryos or proscolices, 

 furnished with hooks. These latter become metamorphosed 

 into scolices, or nurses, representing the well-known cysticercal 

 state which, in its sterile or aborted condition, forms the 

 common hydatid." 



Cestoid parasites are not common amongst reptiles, but 

 they abound in the vertebrates, ten kinds devoting their 

 attentions to man. Birds, fishes, cephalopods, and jelly- 

 fish are likewise known to act as hosts. The mature joints of 

 the common tape-worm are voided by its host, and the embryos 

 get dispersed. If a pig comes in the way of such an embryo 

 and swallows it with his food, it bores its way into his flesh, 

 producing the " measles " known to pork, and when this 

 objectionable article is eaten raw or underdone by a man, the 

 foundation of a tape-worm colony in his interior is at once 

 successfully laid. Should a resting larva (cysticercus) get into 

 the brain a fatal result ensues. 



The eggs of these creatures, being abundant and small, are 



