THE ANATOMY OF PENTASTOMUM TEUETIUSCULUM. 47 



The walls of each consist internally of cells more cubical in 

 form than those of the main ducts, and differing slightly in 

 appearance from these. They have, if any at all, only an 

 extremely thin cuticular layer, and the cavity is elongate from 

 above downwards. They lie embedded in the special muscles 

 which pass backwards from the common portion of the oviduct 

 on to the dorsal wall of the vagina, and are curved in such 

 a manner (fig. 54, Ace. F.) that a somewhat oblique section 

 may pass at the same time through the blind end and the lips 

 of the opening into the main tube. Possibly the cells serve 

 to secrete some fluid which is passed into the latter, but, as 

 Leuckart pointed out, their function is uncertain. They are 

 relatively very small in size, and vary in this respect in different 

 forms. They are most conspicuous in P. subuliferum, 

 smaller in P. tsenioides, and smaller still in P. tereti- 

 u senium (fig. 40, Ace. F.). They must be considered as 

 connected with the spermathecal ducts. Possibly they are 

 only active just when spermatozoa are either passing into or 

 out of the spermathecfe, for in the specimens examined their 

 contained cavities were quite empty. It is interesting to note 

 that in the younger forms they arise from the spermathecal 

 ducts somewhat farther away from the median line than they 

 do in the adult, thus showing their undoubted origin from 

 these ducts. In this respect the young P. teretiusculum 

 resembles the adult P. tsenioides. 



[d) Vagina ( = Uterus of Adult). — This name may be 

 applied to the tube which leads from the external opening at 

 the posterior end of the body to the junction of the sper- 

 mathecal ducts and the oviducts. It differs very greatly in 

 length in the young and adult form. 



In the young form it has simply the structure of a more or 

 less straight tube, sometimes lying in the mid-ventral line ; 

 sometimes, as described by Hoyle^ in P. protelis, pushed up 

 to one side (fig. 23, U.). In this stage its walls are composed 

 of an internal layer of columnar cells with a very distinct 

 chitinous lining. External to these lies a layer of connective 

 ' Loc. cit., p. 187. 



