SPECIES OF THE GENUS AULODRILUS. 957 



epithelium is thus replaced by a mass of secretion, which has 

 apparently originated in the prostate, Vejdov'sky (16), who 

 studied the development of the atrium and the prostate has 

 stated that the latter is formed by the proliferation of the cells of 

 the lining epithelium of the atrium at a point, where the 

 muscular and peritoneal layers are interrupted, so that the 

 prostate cells and those of the atrial epithelium are intimately 

 connected with one another ; this agrees very well with what 

 I find in sexually mature specimens. Though I am not in a 

 position to say anything at present as to the origin of the 

 prostate, its connection with the atrium certainly suggests its 

 origin from the latter. It is, however, clear beyond doubt that 

 the prostate cells become disorganized when full of secretion, 

 especially in their long inner ductule-like portions, and the 

 secretion is discharged mechanically as a stream of inwardly 

 moving fluid running through the centre of the glandular mass 

 into the atrial cavity at the point of communication. 



Muscular or ccelomic sac. — The atrium is followed by a narrow 

 duct of about the same diameter as the vas deferens, which after 

 a short length of about 2 2 /a undergoes a few irregular windings 

 and may be called the proximal portion of the atrial duct (or the 

 middle portion of the atrium, if it is regarded as a part of the 

 atrium itself). It is continued into a longer and much wider 

 terminal part, which also undergoes several windings, fairly 

 regularly arranged one below the other, till it passes vertically 

 downwards to open into the spermiducal chamber. The convo- 

 luted middle and distal regions of the atrium which constitute the 

 atrial duct are bound up together and enclosed by a thick coat of 

 muscle fibres so to form an ovoid structure, somewhat similar to 

 the ccelomic sac of Branchiura and Kaivamuria ; the muscle fibres 

 forming its walls are I'lot compactly arranged, but somewhat 

 loosely connected, so that there are left some narrow spaces here 

 and there in between the strands of fibres, which have a few 

 nuclei at certain places. The muscular sac extends ventrally as 

 far as the spermiducal chamber ; the muscle fibres forming the 

 wall are continued in places as strands above and below into the 

 musculature of the adjacent body-wall, where they pass through 

 the layer of longitudinal muscle fibres to reach the layer of 

 circular muscle fibres, and thus attach the sac to the body-wall. 

 The height of the sac vai'ies ordinarily from 182ju to 240 yu; in a 

 specimen of smaller size it was 125"5/x ; its breadth varies from 

 115-182/2. The narrow proximal portion of the atrial duct is 

 14-17 /x, in diameter, having a lumen circular in transverse section 

 with a diameter of about 5 /z ; it occupies the topmost portion of 

 the muscular sac, in front of which it leads out from the atrium. 

 It is lined by cubical cells of size 1 by 3 //, with conspicuous nuclei, 

 but no definite outlines ; outside it there is a thin layer of 

 circular muscle fibres, which is continuous in front with the thick 

 muscular wail of the atrium. 



The epithelial cells of the wider distal portion of the atrial 



