292 



ZOOLOGY 



SECT. 



A pair of remarkable excretory organs or nephridia have been found to 

 occur in Echinorhynchus gigas. These consist of a pair of ramified protoplasmic 

 masses situated in the body-cavity at the posterior end near the genital aperture. 

 In the interior is a system of branching canals, the terminal branches of which, 

 each contained in one of the terminal lobes of the tree-like iiephridium, are pro- 

 vided with ciliary flames ; at the end of each lobe are a number of fine perforations 

 placing the contained canal in communication with the body-cavity. The stalk 

 of each iiephridium contains a single main canal ; these unite to form a wide 

 median dorsal channel which opens behind in the female into the unpaired 

 portion of the oviduct and in the male into the ejaeulatory duct. 



The greater part of the body-cavity is occupied by the reproductive organs. 

 The sexes are separate, and the female is larger than the male. In both sexes 

 the goiiads and their ducts are connected with a great suspensory ligament (-s.fr/. ), 

 which extends backwards from the end of the proboscis-sheath. 



In the male there are two ovoidal tixtt* (Fig. 236, Ay.) connected with the 

 suspensory ligament. From each a ra-s tl<f< /( H* (c. <//'.), furnished with several 

 vesicula < minales or sacs for the storage of the spermatic fluid, passes backwards 



and unites with its fellow to form an tjnrti/afory duct, 

 with which are connected about half a dozen cei)i< H/ 

 i/Ifniif* ('.;//.) The ejaeulatory duct opens into the 

 bursa or bell-like copulatory organ (b), and has at its 

 opening a small papilla acting as a penis. 



In the female the ovary is connected with the sus- 

 pensory ligament (Figs. 237 and 238, -s.A/. ). When ripe 

 groups of ova known as the " swimming ovaries " 

 (fi.oi'y.) become detached and swim freely in the 

 body-cavity, where they are impregnated. The ducts 

 are very peculiar. On each side of the bod}- is a 

 muscular 'Uterine hell (b), widely open anteriorly (Fig. 

 238, a-) into the ccelome, and having towards its 

 posterior end a small aperture, also communicating 

 with the ccelome (y). Each bell is connected with an 

 oviduct, and the two oviducts open into a uterus (lit.), 

 which itself opens by the genital aperture at the 

 posterior end of the body. The uterine bells perform 

 rhythmical swallowing movements, and as the eggs 

 containing partly developed embryos float in the 

 ccelome they are swallowed by the bells. The im- 

 mature eggs, which are globular, are passed back into 

 the ccelome through the posterior aperture (y) of the 

 bell ; but the mature eggs, which are spindle-shaped 

 and covered with a chitinous investment, make their 

 way from the bell to the uterus, and so to the vagina. 



The early stages of development take place in the 

 cu'lome. Segmentation is regular, and, according to 

 recent researches, a peculiar form of gastrula is pro- 

 duced, having neither archeiiteron nor blastoccele in 

 other words the ectoderm and endoderm are in close 

 contact with one another, and no central cavity is 

 enclosed by the latter. The ectoderm cells secrete 

 a cuticular membrane investing the embryo, then a 



second membrane is formed within the first, and a 



third within the second ; the embryo thus comes to be 



enclosed in a triple case, which differs from an egg-shell in being formed by the 

 developing ectoderm. At what will become the anterior end chitinoid hooks 

 appear. 



At about this period the embryo is born, and reaching the intestine of the 

 host, is extruded with its f;vces. Its further development depends upon its 



-Uf 



FIG. 238. Female organs of 

 Echinorhynchus. 



b. uterine bell ; .. If/. 

 suspensory ligament ; 

 v.t. uterus ; rri. vagina ; 

 .-. //. apertures nf bell. 

 (After Hertwig.) 



