22 ILLINOIS BIOLOGICAL MONOGRAPHS [126 



These examples were chosen at random from countless other similar, 

 simple and still other beautifully intricate forms merely to show the 

 variations possible, of which the last specimen {Monochus gerlachei) 

 illustrates symmetry built on two, three, four, six, and twelve radii. Yet 

 this last whole complex arrangement really becomes bilateral because of 

 the unpaired median lateral papillae and a very large dorsal tooth not 

 previously mentioned, situated on the roof of the buccal cavity about 

 midway between the oral aperture and the esophageal region in the mid- 

 dorsal plane. 



Pharyngeal modifications 



Turning now to a closer study of cephalic structure, one finds among 

 some of the marine nematodes head regions remarkably simple from the 

 standpoint of structure whereas from the point of view of their genesis 

 they may not perhaps be termed simple in the sense of meaning primitive; 

 however, this point will be reconsidered in another section. In genera 

 belonging to the order Litinia Cobb 1920, there are forms in which the 

 head is devoid of lips; papiUae are indistinct or minute; no pharynx is 

 present; the esophagus is simple with no bulb; and cephalic bristles may 

 be absent. Litotes minuta Cobb is extremely simple for the mouth opens 

 directly into the esophagus; no lips or bristles are present, but papillae 

 — six in number — exist; the body as a whole is rather simple and the amphids 

 are very indistinct. In Alaimella cinda Cobb, the head possesses a simple 

 mouth surrounded by six papillae and probably six flat amalgamated lips 

 which can scarcely be comparable to lips as defined by an example of As- 

 caris in a previous portion of this paper. There are also four cephalic 

 bristles present in this species. A related species, A. truncata, the type 

 for the genus Ailaimella, has similarly four bristles and six papillae and in 

 the male two testes are present, indicative of a primitive nature. lonema 

 ocellatum Cobb possesses two ocelli with lenses directed anteriad, a simple 

 circular mouth, no pharynx, no lips, and four cephalic setae. Schistodera 

 exilis Cobb and Tycnodora pachydermata Cobb similarly have circular 

 mouths; however, the former bears four minute papillae around the 

 mouth and the latter, two circlets of setae, the first and anteriormost 

 composed of six and the second, of four. Each one of the nematodes just 

 mentioned lives free upon algae and "seagrass" or upon the sand at the 

 bases of this vegetation. 



Still other marine forms possessing no pharynx and a simple mouth 

 circular in outline, devoid of any form of lips, are members of the in- 

 teresting family Chaetosomatidae. These nematodes are not strikingly 

 organized in structure except in the remarkable possession of ventral ad- 

 hesive bristles on the posterior portion of the body and dorsal cephalic 

 adhesive bristles or setae, which according to observations by Irwin-Smith 

 are utilized in creeping. In this direction these forms have specialized to a 



