[43] OLIGOCII^TOLOGIOAL RESEARCHES. 921 



this efferent duct is developed from the interior matrix of the sexual 

 gland explains at once why it has one coating less than the atrium, the 

 exterior coating of the latter beiug the original cortical layer of the 

 primitive gland. 



Thus the eft'erent duct and copulative organs are exteriorly modeled 

 long before their interior parts are in any more advanced state of differ- 

 entiation. In maturer specimens we find the first signs of such a dif- 

 ferentiation to be a bursting of the cortical layer somewhere midway 

 between its base and top (Figs. 18 d : x, 18 g : c, 18 h : ap. ps.), and shortly 

 afterwards a hoof-like line is perceived in th# interior of the inner 

 matrix, the convexity of the line being turned towards the sexual porus 

 (Fig. 18 d : xx). At the same time another fissure is seen to extend from 

 the opening in the cortical layer towards the interior of the inner layer 

 or matrix, without, however, uniting itself with the hoof-like line (Fig. 

 IS d: /), This first-mentioned hoof-like fissure is only the outline of a 

 cavity, which, developing, shortly fills the larger part of the interoir 

 matrij?:, and as yet communicates with the exterior only through the 

 lumen leading to the sexual porus. 



This cavity I consider as the beginning of the future oviduct. That 

 part of the matrix which surrounds the cavity, and which is directly 

 inclosed by the cortical layer, is destined to become the oviduct proper; 

 that part, on the contrary, which is surrounded by the cavity becomes 

 the penis proper and its sheath (Fig. 18 g : ca. = cavity, p. = penis, 

 ps. = penis-sheath, and ovd. = oviduct). 



When this cavity has reached a certain development all connection 

 between the oviduct and the penis proper is severed at the top of the 

 former (Fig. 18 h: in. ap.), or just inside the first circular fissure in the 

 cortical layer. 



Gradually a penis sheath is separated from the main body of the penis 

 bj^ the further extension downwards of the above-mentioned vertical 

 fissure (Fig. 18 g: f ; Fig. 18 h : f). To begin with, this fissure does not 

 reach the cavity of the oviduct, and the primitive penis sheath remains 

 connected with the penis at the exterior apex. Further on, however, 

 this connection is fully severed in species of the subfamily Tuhijicim, 

 but in TelmatodriUni this connection remains even in the highest de- 

 veloped specimens, and forms one of the principal characteristics of this 

 family (Fig. 17 a: ps.). 



The i)euis sheath, when first separated, is very thick and cellular, but 

 is finally partially absorbed and assumes a chitinous consistency j even 

 the oviduct decreases in thickness, at least in the species of Tubificini. 

 In species with a double oviduct, the cortical layer remains separated 

 from the interior oviduct, and forms the exterior sheath o^ the oviduct, 

 but in species where the oviduct is single I believe the cortical layer 

 and the interior oviduct to have grown together to form the final single 

 oviduct. Even the pellucid epithelium, which once surrounded the cor- 

 tical layer, is absorbed, leaving the entrance to the oviduct free (Fig. 18 

 h ; in. ap.). 



