PHYLUM ARTHROPODA 



563 



opening on the exterior between the second last pair of legs 



Connected with each uterus where it leaves the ovary are two 



diverticula the rcceptaculum seminis and receptaculwn ovornm. In 



some species one or other of these 



may be absent. 



In the male there are two tubular 



testes, each with a narrow vas effercns 



opening by a funnel-like aperture into 



a vcsicula seminalis ; this is followed 



by a long, narrow, coiled vas defer ens. 



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. novce-zcalandiw 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 (E). 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 invagination 

 cavity being formed of yolk with scattered nuclei. 



In another species P. capensis the segmentation has the 

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

 issures externally, are fused internally. 



o o 2 



FIG. 445. A nephridium of P. 

 Edwardsii. tr, funnel ; sri, 

 looped canal ; eb, terminal vesi- 

 cle. (From Lang's Text Bool:} 



