6 COMPARATIVE ANATOMY 



From the shell-gland the conjoined ovarian and vitelline duct 

 is continued forwards as a wide tube which is thrown into 

 many convolutions, and pursues a tortuous course to the 

 generative aperture, where it opens by a small crescentic pore 

 to the left side of the male opening. This tortuous tube is 

 called the uterus, and it nearly always contains eggs, being 

 sometimes greatly distended by their presence. The egg 

 has an ellipsoid chitinous shell, within which is a single 

 ovicell surrounded by a number of vitelline cells, serving as 

 nutriment for the developing embryo. The ovicells are 

 developed in the ovary, and each one, on entering the conjoined 

 ovarian and vitelline duct, receives a coating of vitelline cells 

 derived from the vitelline glands, and is immediately encased 

 in a firm chitinous shell secreted by the shell-gland. 



The Laurer-Stieda canal is a short tube leading from the 

 point of union of the ovarian and vitelline ducts to a pore 

 placed on the mid-dorsal surface. Its function is not fully 

 known, but as it often contains spermatozoa, it is most prob- 

 able that it serves as a copulatory duct, admitting the sperma- 

 tozoa to the ova as they pass down the oviduct. 



The nervous system consists of a nerve-collar surrounding 

 the pharynx, having a single median ganglionic enlargement 

 on the ventral side of the pharynx, and a pair of lateral gan- 

 glionic enlargements, one on either side of the pharynx. Nerves 

 are given off from these ganglia to the anterior region of the 

 body, and from each lateral ganglion a large lateral nerve-cord 

 runs backwards, passing beneath the bifurcation of the alimen- 

 tary tract of its own side and reaching to the hinder end of the 

 body. Nerves are given off from the lateral cords at tolerably 

 regular intervals. (Fig. i, ^.) 



All the organs which have been enumerated are embedded 

 in a peculiar form of tissue generally known as the paren- 

 chyma, which, except for casual lacunar spaces, fills up all the 

 interstices between them. According to the most recent 

 accounts the parenchyma consists of a reticulum of branched 

 cells, the meshes of which are occupied by large clear oval 

 cells with relatively large nuclei situated close against the cell- 

 walls. The parenchyma is traversed by muscle fibres which 

 pass diagonally from the dorsal to the ventral wall of the body. 



External to the parenchyma is a composite sheet of 



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