8O NORTH AMERICAN 



The dorsal contour of the tail continues as a more or less regular extension 

 of that of the body. The ventral contour, however, is elevated at the anus; 

 at any rate, when the spicula are partly extruded. This elevation appears to 

 be largely median, and there is a distinct groove on either side between it and 

 the bursa. When this groove is brought into optical section and the anal 

 elevation is disregarded the tail appears to be asymmetrically conoid. The 

 two equal, brownish, nearly straight spicula are fully twice as long as the 

 tail, or one and one-half times as long as the body diameter, measured oppo- 

 site the proximal ends of the spicula. The spicula in the distal three-fourths 

 taper to a blunt point. At their widest part, considerably in front of the 

 middle, they are one-fourth to one-fifth as wide as the corresponding portion 

 of the body. In the proximal fourths the spicula are rather suddenly nar- 

 rowed and terminate in bulbs or cephala, which have a diameter twice as 

 great as the portion of which they are expansions. Parallel to the spicula 

 there are accessory pieces. These are two-thirds to three-fourths as long as 

 the spicula, which they partially envelop. They are about as strongly chitin- 

 ized as the spicula themselves. The transparent, colorless bursa springs 

 from the submedian lines, nearly opposite the proximal ends of the spicula, 

 and when seen in profile only slightly exceeds the ventral contour in front 

 of the anus, but behind the anus exceeds the ventral contour of the tail 

 proper so much that nearly the entire length of the ribs comes plainly into 

 view. The ribs are arranged in three groups: (i) Two ribs close together 

 opposite the middle of the spicula. (2) Four ribs close together opposite to, 

 and a trifle behind the anus. (3) Four ribs of which three are very close 

 together, and one a little farther forward, the whole group being opposite 

 the middle of the tail. Most of these ribs reach to the margin of the bursa, 

 which is entire and is not distinctly striated. The outer members of the 

 middle and posterior groups do not reach quite to the margin. The ejacu- 

 latory duct is nearly one-half as wide as the body. The single, outstretched 

 testicle has its blind end located about half as far behind the base of the 

 neck as this latter is behind the anterior extremity. At its blind end the 

 testicle is about one-third as wide as the body. It gradually increases in 

 width, so that it occupies four-fifths of the diameter of the middle of the 

 body. The spermatocytes are of large size, one-third to one-half as wide 

 as the body. 



Habitat : About the roots of aquatic plants, Potomac River, Washing- 

 ton, D. C. 



TRILOBUS, Bastian, 1865. 



Fig. 15, Plate VI. 



40 



26. Trilobus longus, (Leidv), Bastian^ * 9 ' 46 ' ^ 1.2 mm. 



2. 2.8 3.8 4.6 2.3 



The thin, transparent layers of the colorless cuticle are traversed by 

 exceedingly fine plain transverse striae, more particularly visible in the 



