400 
Tetrarli ynclms erinaceus 
becomes the general parenchyma. This pai'enchyma secretes a homo¬ 
geneous membrane round the various organs and ducts; and while the 
greater part of it must be used up, so to speak, in this process, the 
nuclei of some of the parenchymal cells remain and become applied to 
the structureless walls of the ducts and glands, where they probably 
exercise some nutritive function with regard to the latter. 
The vas deferens. Vasa efferentia proceeding from the individual 
testes unite together to form vessels of the second and higher orders 
and these finally join to form a fairly wide duct which is the vas 
deferens. The vas deferens runs backwards from the anterior part of 
the proglottis, pursuing a highly convoluted course and finally opens 
into the genital pit. The structure of the wall is difficult to make out: 
it consists of an internal homogeneous membrane outside which are 
some fibrous elements and muscle tissue. The muscle fibres run for the 
most part longitudinally. There are numerous nuclei on the external 
surface of this wall and some of these evidently are of the same nature 
as those which are present on the walls of the vasa efferentia, but some 
of them may belong to the muscle fibres. 
The vesictda seminalis. As it approaches the cirrus sac the vas 
deferens expands to form an ovoid body which is usually described as 
a vesicula seminalis: evidently it is only a dilatation of the vas 
deferens. The whole of the latter tract acts as a sperm reservoir, being 
usually densely packed with spermatozoa. The wall of the vesicula is 
much more muscular than that of the adjacent vessel, and perhaps 
the structure may function in some way with regard to the emission of 
the semen. From the vesicula a shoi’t, greatly narrowed tube pierces 
the wall of the cirrus sac. 
The cirrus. The male duct, after it leaves the vesicula, is known 
as the cirrus, or intromittent organ. This part of the duct lies coiled 
up within the cirrus sac in three or four convolutions. Its wall differs 
in some respects from that of the vas deferens: the muscle fibres are 
more strongly developed and the longitudinal series is more pronounced 
than the transverse one. The cuticula covering the general surface of 
the body becomes reflected at the genital aperture and lines the 
internal surface of the cirrus, being thick near the terminal part, but 
becoming thinner and vacuolised as the cirrus passes into the vesicula 
seminalis. This cuticular lining is continuous with the homogeneous 
“basement membrane” of the vas deferens. 
The cirrus sac. This structure (Text-fig. 9) is ovoid in shape and 
surrounds the cirrus and the terminal part of the vagina. Its wall is 
