THE NEMATOIDEA. 



547 



tion of the alimentary canal, each of these lateral canals passes 

 inward and toward the mid-ventral line, and, joining with its 

 fellow, opens by a pore on the exterior. In some cases, con- 

 tinuations of the lateral canals extend forward into the head. 

 A ring of fibres and nerve-cells surrounds the gullet, about 



Fig. 156. — Oxyuris^—a, mouth; b, pharynx; c, commencement of intestine, and d, 

 its termination. The intermediate portion is not figured, e, genital aperture ; f, 

 opening of vessels ; g, their receptacle; h, one of the vessels ; i, cellular matter 

 enveloping them. A portion of one of the contractile vessels is represented more 

 highly magnified in the upper Ggure. 



the level of the opening of the water-vascular system, and 

 gives off filaments forward to the head, and backward to the 

 muscles and to the lateral area ; while two cords pass back, 

 along the dorsal and ventral median lines, to the hinder end 

 of the body. In the males of some species, nervous ganglia 

 have been observed in the neighborhood of the sac of the 

 spicula. 1 Organs of sense are not certainly known to exist, 

 unless the pigmented spots on the nervous ring of some free 

 Nematoids have this character. 



The Nematoidea are for the most part dioecious. In the 

 females, the reproductive aperture is usually placed toward 

 the centre of the body ; in the males, it is always situated at 

 or near the posterior extremity. 



The female apparatus (Fig. 155, III.) consists of a vagina, 

 with which is connected a single, or double, elongated, tubu- 

 lar, organ, which tapers to a point at its blind extremity, and 

 is at once ovarium, oviduct, and uterus. The cagcal end is 



1 The question of the structure and disposition of the nervous system in the 

 Nematoidea is, perhaps, not even yet completely decided ; but there is much 

 evidence in favor of what is here stated. See Leuckart, " Die menschlichen 

 Parasiten ; " the monograph of Schneider, cited below ; and especially Butschli, 

 "Beitriige zur Kenntniss des Nervensystems der Nematoden" ("Archivfur 

 Mikr. Anatomie," 1873). 



