BY W. A. HASWELL. 331 



pair of legs. On the dorsal surface is a very conspicuous median longitudinal 

 white band with irreg-ularly lobed edges, marking the position of the excretory 

 organ. 'I'his bifurcates in front and bifurcates also though less distinctly, be- 

 hind. Towards the posterior end of the ventral surface is a narrow longitudinal 

 slit bounded by a pair of chitinous plates set on edge — the reproductive aperture 

 (PI. xxxvi., fig. 1, g.a.; PI. xxxvii., flg. 18). On either side of this extends a row of 

 four "genital suckers" (PI. xxxvii., fig. 18) each about .02 mm. in diameter, the 

 row curved with the concavity inwards and set in a semi-lunar area which is ap- 

 parently a thickening of the cuticle. A little distance behind the genital slit is 

 the small, sometimes slit-like, sometimes rounded excretory aperture (PI. xxxvi, 

 fig. 1, ex.a.; PL xxxvii., flg. 18) with a slight raised rim. In addition to the 

 mouth, to be described with the capitulum, the only other external apertures are 

 those of the ducts of the coxal glands — a pair of minute pores .01 mm. in dia- 

 meter on the coxae of the fourth pair of legs. There are no stigmata. 



4. Capitulum; appendages. 



In the larger specimens the capitulum (PI. xxxvi., fig. 1, cp; flg. 2), is situated 

 well behind the anterior margin, so as to be completely concealed from view 

 when the animal is looked at from above. Its length is about one-fourteenth to 

 about one-eighth of that of the body. 'Though it is freely movable, it is not 

 definitely articulated with the body. Distally it is divided by a slight median 

 notch, on either side of which are attached the bases of the pedipalpi. The 

 mouth is situated on the ventral aspect of the distal end of the capitulum. It is 

 a nearly circular aperture, bounded by a thick chitinous ring, continuous in front 

 with the edges of a median slit between the bases of the pedipalpi. The area 

 of the ventral surface of the capitulum behind the mouth is perforated by numer- 

 ous extremely fine pores which, as sections show, perforate the cuticle. 



The chelicerae, instead of lying in a groove on the upper surface as in the 

 Ixodidae and Bdellidae and a few other Acarida, are roofed over by a continuation 

 of the integument of the capitulum, as in Trombidium and the Hydrachnids, the 

 entire basal joint being enclosed within the latter and the distal podomere alone 

 being thrust out through the mouth. 



Each ehelicera (PI. xxxvi., fig. 2, chi, ch2; fig. 3) is composed, as in the 

 majority of the Acarids, of two joints or podomeres, proximal and distal. The 

 distal podomere is a sharp stylet, strongly curved, with the concavity dorsal and 

 anterior. At a little distance from the point is an oblique flange strengthened 

 by a sharp spine, the end projecting like a tooth; this must play the part of the 

 oarb of a hook in hindering withdrawal when a gill fllament lias been pierced. A 

 second smaller oblique spine appears nearer the apex. Close to the point is a 

 minutely serrated ridge of somewhat variable extent. Along the inner side runs 

 a longitudinal flange extending from the base to near the apex. In the normal 

 relations of the parts, with pie two chelicerae in close apposition, their two 

 longitudinal flanges combine to complete a longitudinal canal for the passage of 

 the blood of the crayfish to the mouth of the parasite. 'ITae base of the distal 

 joint lies within the mouth and is incapable of protrusion. It is expanded and 

 divided into two condyles for articulation with the proximal joint and for the 

 insertion of the muscles arising within the latter. Sections sLow that it is hollow, 

 being pierced by a canal which opens into the cavity of the proximal joint by a 

 fine aperture at the base between the two condyles. The canal contains a core 

 of tissue, which, though tho structure is not clear in any of my sections, is 

 probably a duet opening near the distal end. The gland of which this would 



