THE ANATOMY OF PENTASTOMUM TERETIUSCULUM. 9 



long axis of the oval at right angles to that of the body. 

 Through the opening can be seen a ridge running across its 

 short axis, and corresponding to the ventral edge of the 

 partition between the two tubes leading down to the atrium 

 genitale ; whilst on either side of this ridge, when examined 

 with a lens under strongly focussed light, can be detected 

 a minute rod-like body, which must be the ventral edge of 

 a special chitinous process, which serves, as will be afterwards 

 described, to open up the female aperture prior to the passage 

 of the cirrus itself. 



Sensory Papillae. — The only variation in these, as com- 

 pared with the female, lies in the presence of an additional 

 pair of secondary papillae placed just in front of the genital 

 opening, one on each side (fig. 5, 9). 



3. Structure of the Body-wall. 

 (Figs. 29, 31, 32, 51.) 



Under this head will be dealt with only the cuticle and the 

 cuticle-secreting cells ; the muscles of the body-wall will be 

 dealt with under the special head of muscles. 



The Cuticle.— The whole external surface of the body is 

 covered with a layer of chitinous material of nearly uniform 

 thickness (average "015 mm.). This cuticle is continuous 

 with the layer which lines (1) the stomodoeum, (2) the proc- 

 todoeum, (3) the openings of the reproductive organs, (4) the 

 ducts of the excretory glands, and (5) the invaginations con- 

 taining the hooks. These are described under the sections 

 dealing with the organs with which they are connected. 



Stiles has described and figured two distinct layers in the 

 cuticle, a thinner outer and a thicker inner one. In P. 

 teretiusculum there can be recognised a very thin external 

 layer, which takes stain more deeply than the main portion, 

 which rarely stains at all. Around each segment runs a 

 pointed ridge dividing the cuticle into a larger anterior and a 

 smaller posterior ring, the latter part of the segment doubt- 

 less corresponding to what Hoyle calls the interannulus. 



