76 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



ally the apertures of the lateral ducts come together on the midline 

 of the venter, where they are carried inward at the end of an ecto- 

 dermal invagination that forms a common definitive exit tube, the 

 ejaculatory duct or median oviduct. 



The adult reproductive organs of the Onychophora are strikingly 

 arthropodan in character. In the male, the testes retain the tubular 

 embryonic form (fig. 32 F, Tes) ; each discharges into a seminal 

 vesicle (Vsin) from which proceeds a long tubular vas deferens 

 (Vd), the anterior part of which is thrown into an epididymislike 

 mass of coils (Epdm). The ejaculatory duct (Dej) is usually long 

 and irregularly looped ; its opening is on the region of the penultimate 

 somite. Associated with the gonopore is a pair of tubular accessory 

 glands {AcGld), said to be the reduced coelomic sacs of the last 

 somite (fig. 34 E, Gld). In the female, the tubular ovaries are 

 united at their extremities and lie on the dorsal surface of the ali- 

 mentary canal in the posterior part of the body (fig. 32 A, Ov). The 

 oviducts {Od) proceed first forward from the posterior ends of the 

 ovaries, and then turn backward to unite beneath the rectum (Red) 

 in a very short terminal atrium, or common oviduct, opening in the 

 same position as the gonopore of the male. In viviparous species the 

 intermediate parts of the oviducts are enlarged in a series of uterine 

 chambers (Utrs) containing the embryos. Sperm receptacles usually 

 occur on the lateral oviducts near their ovarian ends. 



VI. THE ARTHROPODA 



The fundamental characters of the arthropods are those of the 

 Onychophora and the Annelida. The three groups have in common 

 the following features: (i) The ventral elongation of the blastopore 

 and the closure of its intermediate part, resulting in the formation 

 of a tubular enteron with a ventral subapical mouth and a terminal 

 anus, and in the conversion of the preblastoporic region of the trunk 

 into a prostomial cephalic lobe; (2) a definitive tripartite alimentary 

 canal composed of the primitive endodermal enteron, and of a secon- 

 dary ectodermal stomodaeum and proctodaeum; (3) the dififeren- 

 tiation of a part of the mesoblast, originally formed in the posterior 

 end of the body, into a specific mesoderm taking the form of ventro- 

 lateral bands that extend forward through the entire length of the 

 body and penetrate into the prostomium; (4) metamerism of the 

 somatic ectoderm and mesoderm, involving a segmental repetition of 

 organs derived from these germ layers; (5) the continuity of the 

 acronal centers of the primary nervous system with the somatic centers 



