XI 



PHYLUM ARTHROPOD A 



563 



opening on the exterior between the second last pair of 

 Connected with each uterus where it leaves the ovary arc- two 

 diverticula the receptaculum seminis and receptaculum ovorum. In 

 some species one or other of these 

 may be absent. 



In the male there are two tubular 

 t<:*tes, each with a narrow ms cffercns 

 opening by a funnel-like aperture into 

 a vesicula seminalis ; this is followed 

 by a long, narrow, coiled vas defercns. 

 The two vasa deferentia unite together 

 to form a median tube the ductus 

 ejaculatorius opening on the exterior, 

 like the vagina of the female, between 

 the second last pair of legs. The wall 

 of the proximal part of the ejaculatory 

 duct is glandular and secretes a sub- 

 stance forming complicated cases which 

 enclose masses of sperms to form 

 spermatophores. 



Development. - - The differences 

 between the different species of Peri- 

 patus as regards the segmentation and 

 the formation of the germinal layers, 

 as described by various observers, are 

 very considerable. All the species 

 are viviparous, but in some the egg, before the completion of 

 embryonic development, is enclosed in a well-formed shell, and 

 in certain cases the eggs may pass out to the exterior before the 

 emergence of the embryo. In some species the egg encloses a con- 

 siderable amount of food-yolk, in others the quantity of food-yolk 

 is small, and nutriment is obtained from the parent. 



In P. noi'ce-zecdandiou there is a superficial segmentation. The 

 first segmentation-nucleus is itself superficial, and segmentation 

 results in the development of a number of nuclei, each with its 

 island of protoplasm, which arrange themselves on what is destined 

 to become the dorsal side (Fig. 446 A}, opposite the site of the 

 future blastopore, while some pass inwards to the central part of 

 the ovum. The peripheral nuclei multiply rapidly and grow round 

 the yolk so as to completely enclose it except on a small space 

 (blastopore) in the middle of the ventral side (B}. There a thicken- 

 ing takes place, and an involution of the lips of the blastopore 

 results in a sort of invagination, the floor of the imagination 

 cavity being formed of yolk with scattered nuclei. 



In another species P. c<~<2>c-nsis the segmentation has the 

 appearance of being total; but the cells, though separated by 

 fissures externally, are fused internally. 



o o -2 



Fin. 44.0. A iiepliridium c.f P 

 Edwardsii. //-. funnel ; ./, 

 looped canal ; t '/, terminal vesi- 

 cle. (From Lang's 7V..Y Hook.) 



