470 PEor. ^v. a. haswell ox austealasian poltclads. 



comparatively short and broad cells and a thick muscular wall. Not far behind its 

 orio-in it receives on the ventral side a median duct formed by the union of the right 

 and left uterine ducts. In the lumen of these there are many spermatozoa. 



The two receptacula seminis (fig. 1) are of great size, and when distended with sperms 

 become very conspicuous structures. In a mounted specimen 30 mm. in length one of 

 them is 4 mm. long. In the distended condition the stretched wall is very thin and its 

 structure is difficult of determination. But in the collapsed state the wall appears 

 relatively thick, and is found to have essentially the same structure as that of the 

 unpaired receptacle of Leptoplana aitstraUs. It has a thick muscular investment, and an 

 epithelium of large cells, each of which has a prominent rounded process at its inner 

 end, having the appearance of an exuding droplet of secretion. 



Each lateral vas deferens is dilated to form an extensive elongated seminal vesicle, but 

 this does not appear specially thickened — at least in the distended condition it is in in the 

 specimen sectioned. Erom the two lateral vesiculae a pair of narrow ducts run inwards 

 and at the base of the penis unite to form the ejaculatory duct. 



The penis is enclosed within an elongated sheath. The penis itself consists of a 

 very thin-walled chitinous tube enclosed in a thick layer of circularly-arranged fibres. 

 The tube or stylet is 5 or 6 mm. in length, tapering to a sharp point at its free end, 

 slio'htly dilated and funnel-shaped proximally. The layer of circular fibres is continued, 

 somewhat reduced in thickness, over the portion of the penis which is protruded in the 

 specimen sectioned : its fuaction must be to bring about peristaltic contractions of the 

 thin-walled chitinous tube and so of enclosed ducts. Within the tube is a layer of 

 lono-itudinal muscular fibres : internally the lumen is occupied by a core of parenchyma 

 in which rim ejaculatory and prostatic ducts, the former towards the centre, the latter 

 towards the dorsal side. 



The prostate is a median structure which extends forwards as far as a point a little in 

 front of the point of union of the oviducts. Its duct, at first narrow, widens, then 

 becomes narrower and sinuous, then expands into a channel devoid of epithelial lining, 

 which acts as a reservoir. This rims back for some distance parallel with the ventral 

 limb of the vagina and on its ventral side. This sinus or reservoir passes behind into a 

 narrow cylindrical duct with well-defined walls, which runs to the base of the penis and 

 traverses that organ throughout its length, running within the hollow stylet, parallel 

 with and dorsal to the ejaculatory duct, but remaining separate from the latter. 



The following are the chief features which distinguish this genus : — There is a pair of 

 nuchal tentacles with groups of tentacular eyes : no marginal eyes. Reproductive 

 apertures closely approximated. Vagina long and narrow throughout, without bursa 

 copulatrix. A pair of large receptacula seminis. Duct of prostate separate from 

 ejaculatory duct throughout its entire length. A pair of vesiculae seminales in the 

 form of large dilatations of the right and left vasa deferentia. An elongated 



penial stylet. 



So far as I am aware, only two Polyclads are known to possess paired receptacula, 

 viz. Discocelis tigrina, Lang ("horseshoe-shaped gland"), and Leptojplcma subviridis, 

 Piehn (Laidlaw). This character, together with the complete separation of prostatic 



