144 THE CESTOIDEA 



cercus or " hydatid." (2) From the wall of this hydatid, or " nurse " 

 (Steenstrup), the scolex is formed by internal gemmation. After being 

 swallowed by the final host the bladder is destroyed, and (3) the scolex 

 proceeds to produce by gemmation a series of sexual individuals the 

 proglottids which produce eggs. 



Recent investigations, both anatomical and embryological, however, 

 tend to overthrow the view that the hydatid is a nurse, and that the 

 scolex is produced from it by gemmation. Belief in this view was, to a 

 great extent, fostered by the general custom of taking the life-history of 

 T. solium as typical, whereas there is little doubt but that the course of 

 events exhibited by various cysticercoid forms is to be regarded as more 

 archaic ; where, that is, the scolex is merely a part of the proscolex, and 

 becomes invaginated into the latter for the purpose of protection, in the 

 same kind of way that the Ainnion of the higher Vertebrates and of 

 certain Arthropods has been brought about by the sinking of the embryo 

 (Fig. XXIX.) ; and possibly, lower than the cysticercoids, will be found some 

 in which this invagination does not occur, as in Caryophyllaeus. It is, 

 moreover, to be noted that the greatly developed "bladder" occurs in 

 those tapeworms which inhabit the highest vertebrata, whereas in fish 

 tapeworms the cysticercoid, under one form or another, occurs. 



Granted that the scolex does not arise by any asexual mode of repro- 

 duction from the proscolex, it remains to be decided whether this asexual 

 method can be allowed in the production of proglottids from the scolex 

 in other words, is the strobila to be regarded as a metamerically segmented 

 individual comparable to an Annelid, or is it a linear colony, of which the 

 proglottids are the sexual individuals produced by budding from an asexual 

 scolex ? The former view is held by Burmeister, Gotte, Glaus, Perrier ; 

 the latter by von Siebold, van Beneden, Leuckart, and some of the older 

 authors. Up till P. J. van Beneden's time the strobila was almost 

 universally regarded as an individual, but his researches and his careful 

 comparison of a proglottid with a Trematode, followed as it was by the 

 observations of others, impressed naturalists with the idea that the strobila 

 is a colony a view which has been adopted generally up till about ten 

 years ago, as it seemed to illustrate Steenstrup's theory so excellently. If 

 we regard the tapeworm as a colony, it is comparable to the " strobila " 

 of Aurelia and other Scyphomedusae, each ephyra, budded off from the 

 original hydro-polyp or scyphistoma, being comparable to a proglottid, 

 and the Scyphistoma itself, developed from the egg cell, being equivalent 

 to the scolex. If this be the case, the strobilation of Cestodes must be a 

 secondarily acquired phase in their life-history, which must have been 

 originally a metamorphosis ; but owing to the individualisation of certain 

 products of growth, it has given rise to alternation of generation (Glaus, 6). 



But many anatomical facts appear to be against the " colony " theory ; 

 the continuity of the muscular, nervous, and excretory system, not only 

 throughout the entire adult worm and the scolex, but even in the cysti- 

 cercoid phase, where they are absolutely continuous in the bladder and 

 scolex. The excretory pore is at the posterior end of the proscolex, and 

 in many adults it is at the posterior end of the chain throughout life, viz. 

 in those forms which never drop proglottids, and though the development 



