THE ANATOMY OF PENTASTOMUM TERETIUSCULUM. 55 



containing a duct {S'.). The outer cavity opens into the rod- 

 sac to be described later; the inner duct leads at one end 

 into the vas deferens^ and at the other into the cirrus-sac. 

 There are thus three definite layers of cells bounding the 

 two cavities. 



The meaning of the outer space is difficult to determine, 

 and in the adult it becomes obliterated, with the result that 

 the central duct remains surrounded by three layers of cells. 



The whole structure passes forwards, wedged in between the 

 dilator rod-sac externally and the cirrus-sac internally. The 

 chitin which lines the duct is continued directly into the cirrus 

 of which it forms the internal layer, whilst the outer layer of 

 the latter may probably be regarded as continuous with that 

 lining the external cavity above referred to, though in the 

 young stage, in which this is distinct, there is no trace what- 

 ever of a cirrus. 



(f) Dilator Rod-sac ( = Chitinzapfen of Leuckart and 

 Stiles). — This, in important details, agrees more closely with 

 the structure described by Stiles in P. proboscideum than 

 with that of other species. It is enclosed in a cavity with 

 strongly muscular walls, which are directly continuous with 

 the tube leading to the common genital opening, and into the 

 inner side of which opens the cirrus-sac (fig. 62, D. R. S.). 



The sac and its contents form a somewhat complicated 

 structure, and on its inner and dorsal side is fused with the 

 cirrus bulb. The whole consists, in the main, of a muscular 

 mass attached to the outer and posterior walls of the cavity, 

 and on the inner side to the wall of the cirrus bulb. Its free 

 end has a deep concavity, in which lies a tongue-shaped struc- 

 ture {To.) which projects slightly beyond the mouth of the 

 former. A hollow space, lined by cuticle, passes inwards from 

 the posterior end of the concavity just external to the tongue, 

 and from its walls the muscles radiate outwards. The inner 

 lining of the concavity is continued downwards into a strong 

 chitinuous process flattened out into a somewhat shovel-shaped 

 structure when seen laterally [D.R.). This is also continuous 

 with the strong chitinous lining on the inner aspect of the 



