THE ANATOMY OP PENTASTOMUM TERETIUSCULUM. 57 



of the dilator rod- sac into the tube leading to the external 

 opening. Each sac extends backwards to the level of the vas 

 deferens (fig. 8), and has in transverse section a somewhat 

 concentric shape, with a cavity much compressed laterally, 

 owing possibly to the action of reagents (figs. 13 — 15, C. S.). 

 The walls are thick, and composed internally of a layer of very 

 large columnar cells, the internal surfaces of which are lined 

 by a thin cuticle. External to the cells is a strongly developed 

 coat of muscle-fibres which run in a general longitudinal 

 direction (fig. 48). 



The cirrus has the form of a long chitinous tube, with 

 walls which are continuous, as before said, with the chitin lining 

 of the cirrus bulb (fig. 67). The cirrus appears to vary in 

 development, though possibly this may be due to the fact that 

 it only reaches its greatest length at a very late period of 

 development. In the specimen, otherwise perfectly mature, 

 of which figs. 64 — 66 represent sections, it only extended for 

 a very short distance beyond the end of the cirrus tube ; whilst 

 in other specimens (figs. 14 and 15, C.) it was of great length, 

 and lay coiled up within the sac, as represented by Leuckart 

 and Stiles. In P. protelis Hoyle was unable to detect any 

 cirrus, owing doubtless to the immaturity of the forms which 

 he examined. In very young males of P. teretiusculum 

 (7 mm. in length) there is no trace of a cirrus. When the 

 latter is fully developed it is of great length, and the terminal 

 part is somewhat larger than the rest, and has the property of 

 absorbing stain. In all mature specimens examined the long 

 chitinous process previously referred to in connection with 

 the dilator rod-sac passes for a considerable distance down 

 towards the genital opening. During copulation these two 

 rod-like processes are probably first of all ejected by means 

 of the strong muscles within the sac and those connected exter- 

 nally with its walls. They open up the vagina, each exerting 

 in consequence of the crossing of the muscles close to their 

 insertion a lateral pressure. The result of the action of these 

 muscles must be (1) that the rod-like process is pressed against 

 the outer wall of the male opening, and (2) that it is similarly 



