494 



FRESH-WATER BIOLOGY 



44 (43) Esophagus with only one well-developed bulb; males without 



bursa 45 



45 (46) Pharynx long and narrow Rhahdolaimus de Man. 



Genus of four known species, three European, one American. 



Representative species Rhahdolaimus minor Cobb 1914. 



c= 



77 » 



....... 



The cuticula appears to be destitute of any but very 

 fine transverse striations, most clearly visible near the 

 head. Careful focussing appears to indicate the 

 presence on the outer margin of the head of almost 

 invisible papilla-like organs which may perhaps be 

 the representatives of cephalic setae. There are no 

 hps. The thin-shelled, smooth eggs are relatively 

 large and elongated and have been seen in the uterus 

 one at a time. They are about four to five times as 

 long as the body is wide and about one-fifth as wide as 

 long. They appear to be deposited before segmenta- 

 tion begins. The eggs are so large in proportion to the 

 size of the ovaries that they push the ovaries first to 

 one side and then the other as they develop, so that 

 both ovaries may occasionally appear to be either in 

 front of or behind the vulva. The specimen figured 

 was so twisted that the head presents the dorso- 

 ventral view, and the tiny amphids (a) therefore 

 appear in profile. The figure shows well the typical 

 distribution of nerve cells, large numbers in front of 

 and behind the nerve-ring, a smaller collection in the 

 cardiac region, and other collections in the anal region. 

 The long, slender spirmeret is characteristic of the 

 genus. 



The figure illustrates the general features of the cen- 

 tral nervous system, which in nematodes consists of a 

 ring of ner\^e fibers encircling the esophagus,'and hav- 

 ing connected with it a number of more or less spherical 

 nerve cells, shown dark in the figure. Under favorable 

 circumstances each one of these cells can be seen to be 

 connected with others, and directly, or indirectly, with 

 the ring of fibers. The whole, therefore, constitutes a 

 rather complicated, coordinated system of nerve cells. 

 In many species the cells, such as those shown at c and 

 h, are arranged in longitudinal groups, and even where 

 the groups are not apparent, as here, the connections 

 among the cells are undoubtedly systematic and cor- 

 respond with the longitudinal grouping that is evident 

 in other genera. From the central nervous system 

 extend forward and backward nerves, — ventral, dor- 

 sal, lateral, and to a lesser extent submedian, connected 

 by commissures. Special collections of nerve cells 

 occur on the ventral side near the cardia, vulva, and 

 anus. The exterior indications of the nerves are found 

 in papillae and setae, usually corresponding in position 

 with the main branches of the ner\'ous system. 



Habitat: Mud, Beach pool, Pine Point, Douglas 

 Lake, Michigan. 



Fig. 797. Rhahdolaimus minor. 



I, lateral view of female; II, head of the same, showing 

 amphid. The head m I is twisted, so that the amphid 

 appears as if ventral, or nearly so. 



a, amphid; b, long, narrow phar>-nx; c, anterior group 

 ot nerve cells; d, nerve-ring; e, cardiac bulb; /, wall of 

 the mtestine; g, fle.Yure in anterior ovarj^; h, posterior 

 group of nerve cells; i, body cavity; j, lumen of intestine; 

 k, ovum; /, blind end of posterior ovary; w, egg; w, flexure 

 in posterior ovary; o, cuticula; /), caudal glands; 9, subcuticula; 

 r, vulva; s, rectum; t. anus; u, nerve cells (?); v, duct of 

 caudal glands; w, spinneret; x, lip region. (After Cobb.) 



