DEVELOPMENT OF THE TAPEWORMS 297 



In the Acole'ince the vagina is more or less extensively atrophied, 

 and in any case has no external opening. 



A number of genera are distinguished by the duplication of 

 the genitalia in every segment ; the genital apparatus in its entirety, 

 or with the exception of the uterus, is double, or the genital glands 

 and the uterus are single, but the cirrus, vas deferens and vagina 

 are double. 



On comparing the genitalia of the Trematodes and Cestodes the 

 parts will be found to agree, but the vagina of the Cestodes 

 corresponds with the uterus of the Trematodes, and the uterus of the 

 tapeworms to Laurer's canal of the Trematodes, which in most of the 

 Ceslodes has lost its external orifice. 



DEVELOPMENT OF THE TAPEWORMS. 



Copulation. As each proglottis possesses its own genital apparatus, 

 and male as well as female organs are present, the following processes 

 may occur : (i) self- or auto-fecundation (without immissio cirri) ; 

 (2) self- or auto-copulation (with immissio cirri) ; (3) cross-copula- 

 tion between proglottids of the same or different chains (of the same 

 species) ; and (4) cross-copulation in the same proglottis in species 

 with double genital pores. These various modes have actually been 

 observed. 



In those species which lack the vagina (Acoletnce) it appears that 

 the cirri, which are always furnished with hooks, are driven into 

 the tissues and for the most part reach the receptaculum seminis. 



The eggs of all Cestodes are provided w r ith shells, but the shells, 

 like their contents, vary. In genera that possess a uterine pore the 

 mature eggs frequently do not differ from those of the Distomata ; 

 they have a brown or yellow shell of oval form provided with an 

 operculum, and contain a number of yolk cells in addition to the 

 fertilized ovarian cell (fig. 128), but in other genera (with a uterine 

 pore) the lid is absent and the egg-shell is very thin, the eggs of these 

 genera resembling those of Cestodes in which the secretion of the 

 vitellarium is a light albumin-like substance that contains only a 

 few granules, and in which the egg-shell is very delicate and without 

 operculum. 



The eggs of Tceniidce, for example, at first consist of egg-shell 

 (ootype), ovum and yolk cells. The egg-shell is as a rule soft, colour- 

 less and frequently deciduous, and the yolk is scanty in amount and 

 contains few granules. The eggs are, moreover, more complicated 

 than this. They enlarge and change their shape and various envelopes 

 are developed around the embryo. The egg-shell proper often dis- 

 appears, and one or more embryonal envelopes, or protoplasmic 



