442 GROWTH AND STRUCTURE OF T^NIA SAGINATA. 



deferens, but bending downwards (Fig. 250) in a somewhat sharp 

 curve, ends in the middle line, half-way between the end of the uterus 

 and the posterior border of the joint. 



The Vagina, like the cirrhus and vas deferens, is lined by a con- 

 tinuation of the cuticle, but its walls possess considerable power of 

 resistance, and are thicker than those of the vas deferens, so that the 

 lumen measures scarcely more than O'OIS mm. It is plain, therefore, 

 that the vagina could not possibly receive and transmit the eggs, which, 

 even after the loss of the external envelope, are double the above- 

 mentioned diameter. We have, however, already noted that the eggs 

 of the Tffiniadae only gain the exterior when the body-wall bursts — 

 usually at the anterior border. 



The Receptaculu7)i Seminis, which is recognisable by its contents, 

 may be seen, just where the vagina reaches the posterior end of the 

 uterus, as a small swelling (O'l mm. long by 0'07 mm. broad). It may 

 be considered as occurring in the course of the vagina, although its 

 connection with the latter, both anteriorly and posteriorly, exhibits 

 several peculiarities. While the anterior portion of the vagina is 

 narrow and lined with chitin, perhaps to serve as a channel for the 

 transmission of the spermatozoa, the continuation posteriorly is wide 

 and thin- walled — so different from the vagina proper, that we are 

 almost justified in regarding it as a special organ. Since it conducts 

 the spermatozoa to the ova, we have previously (p. 314) suggested 

 the designation " fertilising-canal" ('' Befruchtungs-canal ") as not 

 inappropriate for it. 



The Shell-Gland. — The canal just mentioned leads in its wide 

 course first to a spherical body about 0'2 mm. in diameter, which lies 

 some distance behind the uterus, in fact just where the vagina ends. 



Even Mehlis^ and Platner noticed this peculiar body, but had 

 only very imperfect knowledge of its nature and connections. Platner 

 took it for the ovary. As a matter of fact, the body in question 

 consists of closely compressed nucleated cells 0"02 mm. in size ; these 

 are, however, by no means eggs, but glandular cells, provided with 

 small thin ducts opening into the narrow internal cavity of the organ, 

 which measures 0'03 mm. The regular disposition of the cells and ducts 

 often gives the wall a radiate appearance. Since it is inside this 

 structure that the ovarian eggs acquire their outer envelopes of yolk 

 and shell, the shell-substance is probably the secretion of these 

 glandular cells. Since finding a body almost identical in structure 

 in the Distomidse,^ I have called it the "shell-gland," a name often 

 used since by other observers. Sommer calls it " Mehlis' body." 



1 Oken's his, p. 70, 1831. 



' Mrst^Xlermau edition-of this wmk. p. 483. 



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