THE RAT-FLEA, CBRATOPHTLLUS FASCIATUS BOSC. 455 



"which runs the whole length of the penis, may be called dis- 

 tinctively the urethral duct (d.).* The exact point at which it 

 begins in the axis requires to be determined by sections. 



The urethral duct emerges from the axis of the corkscrew at 

 its apex and there turns and runs outwards round the outer edge 

 of the spiral of the corkscrew, enclosed between two chitinised 

 bars, or more correctly thickenings of the w r all of the duct. 

 These chitinous thickenings are best seen in unstained prepara- 

 tions of the penis (PI. 29). The chitin on the inner side of the 

 duct (i.e. on the side of it turned towards the axis of the spiral) 

 is the thicker and stronger of the two, but is only continued 

 from the base over about three turns of the spiral, while the 

 thinner chitinous bar on the outer side of the duct is continued 

 for nearly a whole turn more. 



The structure of the corkscrew -organ is difficult to make out 

 in full detail without sections, but if a portion of the spiral be 

 carefully examined, the following points can be seen in dissections 

 of the whole apparatus stained and mounted (PI. 30, A). At the 

 extreme outer edge of the spiral is seen the narrow urethral 

 duct (d.) with its chitinous thickenings on the inner and outer 

 side. Running from the axial region, which can also be seen in 

 the unstained preparations to have a chitinous support (ax.), is a 

 superficial layer of radiating striated muscles, which run across 

 from the axis centrally to the duct peripherally; this layer can be 

 focused without difficulty. At a deeper focus, below the radiating 

 muscles, two structures can be made out lying between the axis 

 and the duct ; close to the axis and apparently attached to it, is 

 a spiral muscle (sp.m.) composed also of striated muscular fibres ; 

 and between the spiral muscle and the urethral duct is a cushion 

 of cells which appear to be glandular in appearance, but sections 

 would be necessary to determine their precise histological nature. 

 These various structures can be seen best in the lowest coil of the 

 oorkscrew ; they are depicted in Plate 30, A, but it is difficult to 



* If a dissection of the male reproductive organs be treated with caustic 

 potash, everything up to the base of the corkscrew, that is to say the vasa 

 deferentia, prostates and ductus ejaculatorius, dissolve away, but the 

 urethral duct issuing from the apex of the corkscrew remains very distinct 

 and this is, as a matter of fact, the best way to study its course. It would 

 appear, therefore, as if the urethral duct is distinguished from the other 

 ducts by the possession of a chitinous lining, and therefore represents, pro- 

 bably, an ingrowth of the outer integument in origin. 



