52 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



body cavity (fig. 32 A, SlmGld). The following appendages are 

 the legs, varying in number with dififerent species from a minimum 

 of 13 pairs to an average of perhaps 25 or 30 pairs, though some 

 species have 40 or more. Behind the last legs the body tapers to a 

 terminal cone on which is situated the anus. The genital aperture 

 in each sex is a median ventral opening lying either between the legs 

 of the last pair, or behind the last pair present in species having one 

 or two of the posterior pairs of legs absent. 



EARLY STAGES OF DEVELOPMENT 



Were it not for the evidence of annelid relationships shown in the 

 adult structure of the Onychophora, we should have little reason for 

 believing that the onychophorons are descended from Annelida, for 

 in their ontogeny we encounter none of the familiar early phases of 

 development so characteristic of the annelids. Most of the Ony- 

 chophora are viviparous, the embryos developing to maturity in 

 uterine chambers of the oviducts (fig. 32 A, Utrs) ; only a few 

 species are known to be oviparous. Eggs supplied with a large quantity 

 of deutoplasm complete their development from their own store of 

 yolk, but the embryos of viviparous species with small eggs receive 

 nourishment from the uterine walls, and in some cases a placentalike 

 growth of the blastoderm forms a large vesicular trophoblast applied 

 to the walls of the uterus. 



The early stages of onychophoran development are so variable in 

 diflferent species that it is impossible to give any general account of 

 the processes of cleavage and germ-layer formation. Cleavage in 

 some species with small eggs is holoblastic, producing first a solid 

 morula and then a hollow blastula (see Sclater, 1888). Contrary to 

 what we might expect, however, gastrulation in such cases does not 

 take place by invagination. In Peripatus imthurni, as described by 

 Sclater, an internal proliferation of cells proceeds from a definite 

 point on the blastula, and the cells thus produced become dififeren- 

 tiated into endoderm and mesoderm. A similar method of endoderm- 

 mesoderm formation is described by Kennel (1888) in Peripatus 

 edwardsi, there being here a blastoporic depression of the blastoderm 

 from which an internal proliferation gives rise to endoderm and to 

 ventrolateral bands of mesoderm. With eggs having much yolk, 

 meroblastic cleavage is the rule. The &gg nucleus divides within the 

 yolk, and the cleavage nuclei enclosed in small masses of cytoplasm 

 migrate to the surface and form a blastoderm. In Peripatoidcs novae- 

 zealandiae, however, according to Sheldon (1888), the blastoderm 



