86 ESSENTIALS OF ZOOLOGY 



the thirteenth segment, and, like the testes, they occur ventrally 

 on each side of the nerve cord. The ova as they are matured 

 are passed into the thirteenth segment and led into the opening 

 of the oviduct, which is situated on the posterior wall of the 

 same segment. Piercing this septum the oviduct gains the 

 fourteenth segment, and it opens to the exterior on the wall of 

 that segment. Both the vasa deferentia and the oviducts are 

 developed as outgrowths of the coelom and are therefore 

 coelomiducts. During the process of egg-laying an exudation 

 is passed out from the clitellum. This forms an annular band 

 around the body, and into this the eggs are deposited. As 

 it passes forwards over the worm the latter retreating all 

 the time a supply of sperms is liberated by the spermathecae. 

 The mucous band is then passed over the head of the worm 

 and narrows anteriorly in front of the worm ; the worm 

 retreating completely from it, the posterior end in turn con- 

 tracts. The exudate hardens and a capsule containing a few 

 eggs is formed, and this is termed a cocoon. Only one of the 

 eggs succeeds in developing. 



The development of Lumbricus has been described by 

 E. B. Wilson (1887, 1889, 1891, ' Jour, of Morph.' vols. 1, 3, 

 and 6) and by Hoffmann (1899, ' Zeit. wiss. Zool.' Bd. 66), and 

 of the parasitic Bdellodrilus by G. W. Tannreuther (1915, 

 ' Jour, of Morph.' vol. 26). The early stages are very similar 

 in Annelids, Mollusca, and Eotifera, and resemble those of the 

 Turbellaria. The segmentation is unequal, and the fate of 

 the primary cells, A, B, C, E>, similar in all. The gastrula is 

 formed by the ectoderm cells, which are the smaller, growing 

 over the endoderm and the mesoderm, which latter is early 

 separated. This process is known as epiboly, but in Lumbricus 

 it is accompanied by invagination. The blastopore narrows 

 by the fusion of the ectoderm cells. In Lumbricus it forms 

 the mouth. In Bdellodrilus it closes, but the point marks 

 the place of origin of the stomodeum and mouth. The 

 proctodeum appears later, after the embryo has made con- 

 siderable progress in length. The ectoderm cells give rise to 

 the ectoderm, nervous system, and nephridia (even evidently 

 the nephrostomes), the endoderm cells to the endoderm, and 

 the mesoderm cells to the mesoderm. The whole of the 



