BY VERA A. liJWliN -.SMITH. 767 



exceed I'lOmm., and proportionally it is unusually broad, the 

 trunk, in the largest specimens, attaining, at its widest part, a 

 width of 0*092 mm. Dorsal and ventral surfaces are recognis- 

 able, and anterior and posterior ends. Anteriorly, the body is 

 enlarged to form a head-like swelling (PL xliv., hd.) marked off' 

 from the trunk by a well-defined neck-constriction (PI. xliv., nk.). 

 Posteriorly, the trunk terminates in a short, sharply pointed, 

 downwardly directed tail (PL xliv., tl.). The mouth-opening(m.) 

 is antei'ior and terminal, and the anus (a.) is situated on the 

 ventral surface, a short distance in front of the tail (0-06 mm. 

 to 0-09 mm., according to the length of the animal). The sexes 

 are separate, and, in the female, the external genital aperture is 

 situated on the ventral surface, a little in front of the middle of 

 the body (PL xlv., ^, g.p.). When fixed, the animal usually 

 assumes the shape of a query-mark, or a sickle, the anterior half 

 of the body being strongly arched, with the concavity of the 

 arch directed ventrally; while the posterior half is straight, or 

 only very slightly bent in the opposite direction (PL xliv. -xlv.). 

 The female genital aperture is situated close to the beginning of 

 the anterior arch, in the region where the body attains its 

 greatest width. Even in sexually immature worms, the anterior 

 half of the trunk is always slightly wider than the posterior 

 half, which is further distinguished by the presence, along the 

 whole length of its ventral surface, of four longitudinal rows 

 of stout, locomotor rods or setae (PL xliv. -xlv., i.r., o.r.). 'J he 

 whole body is enclosed in a thick, opaque cuticle, which, except at 

 the extreme anterior and posterior ends, is divided up by close- set, 

 transverse grooves, into numerous minute annuli or stria3 (PL 

 xliv. -xlv., cut.). 'V\\Q grooves are broad, and penetrate to nearly 

 the full depth of the cuticle (Text-fig. 12, cut.). For the most part, 

 they encircle the body in parallel lines, but, occasionally, adjoin- 

 ing grooves anastomose. On no part of the head or trunk is 

 any variation in the size and thickness of the striae to be recog- 

 nised. The head-swelling is large in proportion to the size of 

 the trunk, attaining a length of :^th to ith that of the trunk; 

 and a breadth usually slightly greater than the average breadth 

 of the trunk. It is elongated-oval in shape, and is bluntly 



