18 ILLINOIS BIOLOGICAL MONOGRAPHS [122 



covers the body of Spira parasitifera Bastian while vorticella may attach 

 itself to the tail of the same worm. Such ectoparasites could neither remain 

 attached to the nematode nor stand the wear and tear if the host thrashed 

 about among debris and sand. According to Irwin-Smith, some members 

 of the family Chaetosomatidae, which however are not clearly true nema- 

 todes, hitch along the rocks and vegetation in the manner of measuring 

 worms by means of special adhesive bristles arranged in two rows on the 

 ventral surface near the tail and by other adhesive bristles on the dorsal 

 portion of the cephalic region. Seurat believes these bristles are a special 

 adaptation. Some other free living nematodes according to Cobb's 

 observations move as many rotifers do, in a looping fashion, using the 

 caudal glands and suction created by the muscular esophagus as alternate 

 means of fixation during progression. 



The points reviewed in the foregoing paragraphs seem to indicate 

 rather strikingly that the primitive nematode led a half sessile life, oriented 

 in an upright or nearly upright position, as do many of the free living forms 

 today. Another feature of interest in this connection, the sessOe tendency, 

 is the prevalence of radial symmetry in the anterior regions of great 

 numbers of non- parasitic and parasitic forms. A characteristic of sessile 

 animals like the Coelenterata is their radial symmetry, or like the Echino- 

 dermata their pseudo-radial symmetry, which has become superimposed 

 secondarily upon their primary bilaterality. As a result of the sessile 

 tendency among the free living nematodes, pseudo-radial symmetry would 

 materially develop. 



Definition of the primitive nematode 



Steiner (1919) has defined the primitive nematode in short as a bilater- 

 ally symmetrical, spindle-shaped animal afiixed to its support by the 

 secretions of three adhesive glands at its caudal extremity, possessing a 

 simple digestive tract with no diverticula or convolutions but with a 

 muscular esophagus, having paired gonads in the two sexes lying parallel, 

 one on each side of the intestine, throughout their length, their ducts 

 opening with those of the paired excretory vessels and the intestine into a 

 cloaca discharging by an anus to the exterior in the mid ventral line, 

 slightly anterior to the termination of the tail. Seurat (1920) after a careful 

 consideration of what he believes to be primitive characters still main- 

 tained in some of the present day nematodes, avoiding characters induced 

 by adaptation to environment (parasitic adaptations like complex ovejec- 

 tors, organs of fixation, buccal cavities armed with teeth, or free-living 

 adaptations such as long cephalic bristles, ventral adhesive setae of the 

 Chaetosomatidae, buccal stylets of Xiphinema and Dorylaimus, etc.), 

 defines the primitive nematode as follows: 



"Vermiform organisms of small size living in detritus or decaying 



