119] COMPARATIVE STUDIES ON NEMATODES— BETH ERINGTON 15 



pendently in front by an ectodermal invagination; other workers believe 

 that the definitive mouth arises from an incomplete closure of the blasto- 

 pore giving here the ventral location of the mouth which shifts at an early 

 stage to the terminal position. The anus, however, is ventral and poste- 

 rior, which too is typical of present-day forms barring, for example, a few 

 highly modified individuals such as the adult female of Heterodera schaclii 

 with a dorsal anus; Trichosomoides crassicauda Bellingham, members of 

 the genera Trichuris Roederer, Eustrongylides Jagerskiold, and Hystrichis 

 Dujardin, in which the anus is terminal. In free living forms the anus is 

 always posteriorly ventral and a tail is present through the tip of which 

 three caudal glands pour out their secretions. These glands fabricate a 

 cement-like substance which hardens in the presence of water and serves 

 to hold the individual to the substrate of its habitat. The lack of a tail 

 and the presence of a terminal anus as existing in the groups just men- 

 tioned do not seem to fit into the conception of the primitive nematode as 

 will appear later in this discussion but they may be of significance in the 

 conception of the ancestor of the primitive nematode, a discussion of which 

 will follow in the course of this paper. 



The openings of the reproductive systems of existing forms allow the 

 products of the gonads to reach the exterior differently in the two sexes: 

 by way of the rectum and anus in the male nematode and by way of the 

 vulva in the female worm, an opening quite separate, generally on the 

 ventral surface in the mid-line. It is believed by Steiner that the primitive 

 nematode, male and female alike, possessed only one ventral orifice which 

 was a common opening for the discharge of the reproductive elements and 

 alimentary waste, as well as serving for the outlet of the excretory system. 

 Such a primitive worm possessed a cloaca, which is present now in no 

 known forms; indeed these three systems — alimentary, excretory, and 

 reproductive — terminate in a great variety of positions in extant forms. 



Contrary to the hypothetical condition, the excretory system with few 

 exceptions opens mid-ventrally far anteriorly in the neighborhood of the 

 nerve ring. The vulvar opening may be found posterior and terminal in 

 the parasitic nematodes belonging to the genera Trichuris, Heterodera, 

 Eustrongylides and Hystrichis, but more often it is near the middle of the 

 worm in free living and parasitic forms alike. In Syphacia and some 

 Oxyurids it lies far forward in the anterior half of the body — even close 

 to the nerve ring. In the male organisms the gonads open by their ducts 

 into the rectum in connection with the spicular apparatus. Beside these 

 points in the foregoing paragraphs, the primitive nematode has a simple 

 digestive tract, paired gonads, and paired excretory canals. These with 

 all the other elements of the ancestral form are arranged in such a manner 

 that the body is wholly bilaterally symmetrical. 



