INTERNAL ANATOMY OF BDELLA. 515 



bodies, which gradually become more elongated ; those in and neai' to the testicular 

 bridge have become more or less filamentous, and have a tendency to a slight head, which 

 stains more deeply than the rest of the spermatozoon. These spermatozoa in and near 

 the testicular bridge always seem to nic to be somewhat larger than those anywhere else, 

 even in the penial canal. 



The Course of the Spermatozoa from the Testes to the Exterior and the Functions of the 

 different Parts of the System. — The course of tlie spermatozoa is difficidt to trace satis- 

 factorily : to summarize the facts bearing upon it, the two testes on each side have very 

 short vasa effereutia Avhich unite to form a very short common duct ; the two common 

 ducts, one on each side of the body, are joined by the tubular and very elastic testicular 

 bridge ; this bridge and the common ducts are usually crowded with spermatozoa in their 

 largest and ripest condition. Tlie bridge curves round the back of the head of the penial 

 canal, and we should naturally expect that there would be a connection between the two ; 

 if there were, the course of the spermatozoa would be simple enough ; but I have 

 never been able to trace any such communication, although I have sought for it 

 assiduously in dissection and in sections cut in all directions of several species of Bdella ; 

 the two organs, although closely approximated, alw'ays seem entirely separate and 

 without communication. How then do the spermatozoa escape ? Apparently there is 

 one mode and one only ; and that is a very small tubular passage leading from the point 

 where the common duct joins the bridge, on each side of the body, to the distal end of 

 the great mucous gland on that side ; here it is firmly closed by a constrictor muscle ; 

 on the other side of the constriction is the lumen of the mucous gland. Spermatozoa 

 swarm on the testes and bridge side up to the constriction, but I have never been 

 able to detect one in the mucous gland on the other side of the constriction. The 

 mucous gland discharges into the glandular antechamber, and that discharges into the 

 head of the penial canal. If the spermatozoa come through the mucous glands and 

 glandular antechambers into the penial canal the course is easily understood ; but although 

 they occur in great quantities in the bridge and common ducts, and up to the entrance to 

 the mucous glands, and are also abundant in the jienial canal, yet I never could detect one 

 in the mucous glands or glandular antechambers. How comes this if they be the 

 channels, and the only channels, of communication ? One possible explanation is that 

 I have never obtained Bdella at the moment of coition and that the spermatozoa 

 pass rapidly through into the penial canal ; but the chambers of the mucous glands, 

 when the cells have opened, seem well fitted to retain them. Another possible 

 explanation is that the secretion of the mucous glands does not stain, wdiich is a fact, 

 and that the spermatozoa become so enveloped in it that they are protected from staining 

 while in the mucous glands and antechambers ; but one would have expected in that 

 case that the secretion would have still covered them when they were passed into the 

 penial canal, but they stain there freely enough. If the spermatozoa pass through the 

 mucous glands and antechambers then these are the vasa deferentia ; and this view is 

 rendered very probable by the fact that in Trombklinm the vasa deferentia do actually 

 discharge into the head of the penial canal in the precise placa where the antechambers 

 of Bdella enter. On the other hand, the vasa deferentia in Acari are usually small 



