132 THE NATURE AND VARIETIES OF 



Tapeworm. In Echinococcus, several successive gemmations 

 of simple cysts may occur before Tsenia-heads are formed 

 at all ; and it is by its cystic growth, that the parasite 

 becomes formidable, the resulting Tseniae being minute three- 

 jointed worms, of no importance on their own acconut. 



Of the two kinds of alternation thus exhibited by the 

 Cestoidea, it is obviously only the protomorphic or cystic 

 that corresponds to the alternation of the Trematoda 

 a point which seems to have escaped the notice of Professor 

 Van Beneden, as he applies the term scolex alike to the 

 protomorphic Redia or Sporocyst of the Distoma, and the 

 orthomorphic Tsenia-head of the Cestoid worm ; and that of 

 proglottis to the Cercaria, or larval stage of the typical 

 Distoma, and to the ovigerous segments budded off in the 

 gamomorphic phase of the Tapeworm. As this application 

 of the nomenclature confounds forms which I cannot but 

 consider of different significance, I have thought it best to 

 avoid using these terms, though fully sensible of the 

 advantage of having short names to express corresponding 

 stages in the cycle of development and reproduction in 

 different species.* 



The association of these two kinds of alternation may 

 be traced in a more latent form in the Polyzoa, as will 

 be more fully noticed afterwards. Such co-existence, 

 however, is, on the whole, exceptional, for it would appear 

 that organisms, which are propagated by protomorphic 

 gemmation, do not ordinarily throw off sexual zooids, and 

 that species, in which the latter phenomenon occurs, do 



* In the use of the terra Strobila there seems to be an equal ambiguity, 

 as it is sometimes applied to the typical form (as of a polype), from 

 which sexual zooids are eventually to be budded off, sometimes to the 

 aggregate of these zooids, already formed, but still adhering in a chain 

 or pile to the typical stock. Its derivation (strobilus, a fir cone) is, of 

 course, suggestive of the latter meaning, and it was to an animal in this 

 transition stage that the name was first applied by Sars. Steenstrup on 

 Alternation, p. 19. 



