THE ANATOMY OF PENTASTOMUM TEEETIUSCULUM. 59 



(3) an external layer of strongly developed circularly dis- 

 posed muscle-fibres {Mg.). These two tubes run downwards 

 and forwards^ and open into a common atrium genitale 

 (fig. 11, At. gen.). 



On each side just where the two cavities meet there arises 

 from the dorsal wall a short tube (figs. 11, 12, and 25, Ace), 

 evidently homologous with the accessory gland as described 

 by Hoyle in P. protelis. This has the form of a duct 

 running upwards and slightly backwards, and penetrating the 

 substance of the hook-gland in front of the part through 

 which runs the vesicula seminalis, and close to the indefinite 

 point of union of the hook- and head-glands. In this form 

 no connection with the gland or with any special glandular 

 cells can be distinguished. 



The duct-walls are composed of columnar nucleated cells, 

 directly continuous with those of the walls of the tube leading 

 down on either side to the atrium genitale. Certainly close 

 to the mouth, and probably along the whole course, the duct 

 has a thin chitinous lining. Each one runs upwards sur- 

 rounded by the strong muscles (figs. 12 and 25, M^ which 

 pass from the dorsal surface of this portion of the genital 

 apparatus to be inserted into the dorsal body-wall. What 

 function the structure serves it is impossible to say. 



Separating the two tubes leading down into the atrium 

 genitale is a tongue-like structure (fig. 11), the distal end of 

 which has the appearance when the atrium is seen externally 

 of a ridge dividing the latter into two lateral halves (fig. 5). 

 In the upper part of this tongue is placed the nerve-ganglion 

 (fig. 11, N). 



