OF THE NEMATOIDS, PABASITIC AND FKEE. 627 



both classes — presents an intermediate bond of union strengthening the alliance between 

 them, and making the division Annuloida a more coherent assemblage than it formerly 

 appeared*. It seems to me, moreover, that the Nematoids approach the Scolecids prin- 

 cipally through their affinities to the order Acanthocephala. For after a comparison 

 of the vaiious descriptions given of the anatomy of the Acantliocephalce, their deep 

 integumental layer with its four cord-like developments, its system of vessels, and com- 

 municating lemnisci, seem to me to find almost an exact parallel in the corresponding 

 structures of the Nematoids; whilst their nervous system, instead of exhibiting the 

 ring-like form met with in the Nematoids, consists of a single ganglion from which 

 l)eripheral branches are sent off, thus conforming pretty closely with what must be 

 considered as the typical form of this system in the class Scolecida. And, again, whilst 

 the Acanthocephalce agree with the Nematoids in being dioecious, they approximate to 

 the Taniadce by the absence in both of all vestiges of the organs of digestion. 



All attempts hitherto to subdivide the order Nematoidea into families and subfami- 

 lies have been more or less unsatisfactory, and at present our knowledge concerning the 

 anatomy of the representatives of so many genera is in such a defective condition, that 

 it would be quite useless to attempt to produce anything more satisfactory. As a pro- 

 visional classification, I think that of Dujakdin is the best; and I further desire it to be 

 understood that I look upon the present location of the free Nematoids in a distinct 

 family as a mere temporary arrangement, which will ultimately have to give place to a 

 more philosophical rearrangement of all the genera composing the order. Any such 

 classification of the genera must, I think, be based principally upon the presence or 

 absence of the ventral excretoiy system, its various modifications in those genera in 

 which it is found, and the aiTangement of the integumental pores where it is absent. 

 As characters of secondary and tertiary importance, I should regard the arrangement 

 of the organs of generation, and the variations in the nature of the pharynx and 

 oesophagus. 



EXPLANATION OF THE PLATES f. 



Parasitic Nematoids. 

 PLATE XXII. 

 Fig. 1. Anterior extremity of a IHs])haragiis from the gizzard of Colymhus septentrionalis, 

 dorsal aspect : — a, a, lateral head-lobes ; b, elongated pharynx ; c, oesophagus ; 

 d, oesophageal ring of doubtful nature ; e, e, lateral integumental processes. 

 Fig. 2. Posterior extremity of same, ventral aspect: — a, a, lateral caudal porea; 

 h, h, lateral bands in profile. 



* If , as seems probable, further investigations should make it desirable to place the orders Nematoidea and 

 Gordiacea apart, in a distinct class, I would propose for this the name Pseudhelminiuia as a suitable designation. 



t Through a mistake on the part of the artist, onlj- discovered when it was too late to be altered, the arrange- 

 ment of the figures in these Plates is quite different from what I had intended. Some of the defects will b© 

 pointed out in footnotes, so as to rectify the faulty arrangement as much as possible. 



4r2 



