CESTODA. 105 



itself is in some cases less than an individual. This, for example, 

 occurs in Tcenia coenurus and T. echinococcus, where the single 

 larva produces a number of tapeworms. Ordinarily, however, only 

 a single tapeworm is produced, in which case it may be regarded 

 as the mature individual. 



Every cestode passes through several distinct phases during its 

 life-history. In the ordinary colonial or tapeworm condition it has 

 been termed the s^roS-iZa (Van Beneden). The separate joints of 

 which the strobile is composed are denominated proglottides or 

 zooids. The anterior segment forms the head, and remains barren, 

 those of the neck and front part of the body being sexually imma- 

 ture during the process of strobile-formation. The mature pro- 

 glottides at the caudal end are capable of realizing an independent 

 existence,- and the eggs which they contain develop the six-hooked 

 embryos, or proscolices (Yan Beneden), in their interior. These 

 latter become metamorphosed into scoUces or nurses, representing 

 the well-known cysticercal state which, in its sterile or aborted 

 condition, forms the common hydatid. 



Habits. — During the greater part of their life-period the cestodes 

 are parasitic animals, the mature proglottides and eggs being free 

 only during a certain interval of time. The cestode entozoa, like 

 the trematodes, frequently change their residence either in an 

 active or passive manner ; their transmigrations being absolutely 

 necessary for the propagation and continuance of the species. The 

 cestodes are bisexual, and all of them exhibit the phenomena of 

 alternate generation. Details of these genetic changes and 

 wanderings will be more fully described hereafter. 



Distribution. — Mammalia.— li ^e except their feeble prevalence 

 among the reptiles, the cestode parasites are pretty uniformly dis- 

 tributed throughout the vertebrate division of the animal kingdom. 

 In their larval or cysticercal state they are especially abundant in 

 mammals, whilst the mature tapeworm-forms are scarcely less 

 frequent in occurrence. The human body alone is liable to be 

 invaded by ten species of tapeworm, but, save in the cysticercal 



p 



