INTEENAL ANATOMY OF BDELLA. 511 



arrangements extruded; hut I believe that hy the action of its exterior constrictor 

 muscles the inner membranous parts of the penial canal can be evaginated and extruded, 

 and that then the above-mentioned memhi'anous tube would form its distal termination, 

 and that the organ {pe) would in some way function as a penis— either forming a true 

 intromittent organ, in spite of its hj aline nature, by passing between the large and 

 widely open external labia of the female genital aperture, or else depositing the spermatic 

 elements on the exterior of that opening, — and that the two hooks on the membranous 

 terminal flaps wovild serve to hold it temporarily in position. 



The outer wall of the penial canal is, except at th^ lower edge, detached fro:n the 

 inner membrane ; it is mu.ch thicker, and consists of a tunic not showing any distinct 

 cellulation, but thickly strewn with small very darkly staining dots, which apparently 

 are not nuclei. 



The Azi/(jous Accessory Glainl (figs. 17, 18, 19, 34, 41, (/cm) is unquestionably the 

 homologue of Henkin's accessory sac (Anhangschlauch) in Trombidlnm fullglnosum, 

 which organ it greatly resembles, altliough the form in B. Basterl is somewhat more 

 complicated. Pagenstecher had seen and drawn it previously, but having mistaken the 

 male for the female, he described it as a spermatheca : Henkiu correctly suggests that 

 it is in the nature of a prostate gland ; it certainly is an accessory gland of some sort. 

 The form in Henkin's species is that of a long, thinnish, convoluted tube with an almost 

 globular enlargement at the distal end, and usually a slight swelling, of varying form and 

 position, somewhere near the middle of the tube. In B. Bastcri (figs. 17, 19) it divides 

 into two regions : the whole organ is compressed laterally, so that it appears much 

 narrower -when looked at from above or below than when seen from the side ; viewed in 

 the latter direction its distal end is expanded and approaches a discoidal form, but is of 

 much smaller proportion than the globular ending of Trombidmm fuliginosum ; it then 

 becomes a flat bar with almost parallel edges of considerably less width than the disc, 

 and continues so for about twice the diameter of the disc ; it then suddenly turns almost 

 at a right angle and again enlarges, forming a shortly pyriform, or almost discoidal 

 swelUng, much larger than the first ; this completes one region, the whole of which is 

 practically colourless. At this point the organ is suddenly constricted ; it quickly 

 enlarges again, but not to its former size ; it then gradually diminishes once more 

 until it becomes quite a small tube, in which condition it enters the anterior side of the 

 penial canal near the exterior termination of that organ, or at all events neai-er to the 

 exterior genital opening than the entrance of the duct from any other gland into the 

 penial canal is. The whole of this second region is yellow-red or orange in B. Basteri. 

 There is not any long convoluted tube as in Trombidium fuliginosum. The entrance of 

 this duct into the canal is closed l)y two closely approximated lips (figs. 18, 41), which 

 can be separated by muscles (^mdl) arising from the amphioid sclerites {as), as mentioned 

 in the description of the penial canal. 



In Bdella vulgaris the form is very difi'erent; there the thin tube exists, although it 

 is not nearly as long as in Henkin's species, indeed it is quite a short duct of small 

 diameter ; but it swells out at its distal end, forming a great crescentic organ, much 

 larger in proportion than the gland in the other two species named ; in all, however, the 

 form of the organ varies a good deal, although each species maintains its own type. 



69* 



