98 SURVEY OF THE REPRODUCTIVE PROCESS 



from the preceding by a distinct gemmation ; yet, from the 

 gemmae remaining adherent to the stock, the process has so 

 much the character of a continuous growth, that only two 

 alternating forms are generally recognised — the Cystic and 

 the Cestoid — and even these owe their distinction as much 

 to the disappearance of the caudal vesicle of the former as 

 to any formative act. Hence the Tapeworm is often de- 

 scribed as a tsenioid entozoon which has lost its original 

 cystic appendage, and developed a long cestoid one in its 

 stead. But in no proper sense can the caudal cyst be 

 termed an appendage of the Tsenia-head — the head is 

 rather an appendage of the cyst, as developed from it. The 

 T^enia-head is indeed the one common feature that unites 

 the two forms, but it stands in a very different relation to 

 the one and to the other ; it is the offspring of the cyst — 

 the parent of the cestoid body. A sort of inversion occurs 

 in the direction of the gemmation during the course of the 

 genetic cycle. It is forwards till the head is formed, when 

 it is reversed, and goes on subsequently in a backward di- 

 rection, so that, had the cyst not been thrown off in the 

 meantime, w^e should expect to find the whole length of the 

 jointed body interposed between it and the head. Some- 

 thing of this kind would seem actually to occur in certain 

 species,* and such is the normal arrangement in the class of 

 Annelida, i" 



In the third order of these Entozoa, the Nematoid worms, 

 alternation is certainly not the general rule, though there 

 are grounds for admitting its occurrence in some species. 

 Thus of late reasons of weight have been brought forward 

 to induce us to look for the progenitor of the formidable 

 Guinea worm in a microscopical inhabitant of tropical pools 

 — the Tankworm — of such tenuity as to be capable of 



* Especially iu Tetrarhynclms, according to Van Beneden. 

 t Carter, Annals of Nat, Hist., 3d Ser., I., 299 ; IV., 33-99. 



