102 DOliVMENIA AtlTA. 



bridfiic tlic cavity. In the neighborhood of the seminal receptacle these are 

 numerous hut what their office may l)e it is difficult to conjecture. 



'rhc seminal receptacle is a thin-walled subconical sac provided with very 

 slijihf internal folds and composed of low cubical cells which bear no trace of 

 cilia nor secretory ])roducts. During the time it is filled with sperms, however, 

 the cells become more or less liquified, the nuclei relatively large and pale after 

 treatment with haematoxylin and great numbers of spermatozoa become im- 

 bedded in their substance. It unites by means of a short narrow canal with 

 the coelomoduct at the point wliere the first and second sections meet. 



Tlie second section of the coelomoduct, or shell gland, is a tube of compara- 

 tively large size (Plate 15, figs. 4, 6). In the anterior third its walls are thin, 

 almost devoid of folds and the cells composing it vary from cubical elements to 

 others of low columnar form, if the animal be immature or out of the breeding 

 season. As this last mentioned time approaches the cells become greatly thick- 

 ened and the meagre secretion becomes abundant, filling the cell as a dark, 

 almost black, substance like that of the muciparous gland in many molluscs. 

 This condition of affairs continues along the mid ventral line of the duct for a 

 consideral)le distance posteriorly. The same modifications occur in the suc- 

 ceeding portions of the coelomoduct, but as the time of sexual activity 

 approaches the cells become greatly elongated, are thrown into large transverse 

 folds and are filled with a faintly yellowisli secretion which at other times is 

 scarcely visible. 



In this species, as in P. wehcri, two genital spicula are present and of large 

 size (Plate 6, fig. 4). Each is inserted in a deep sheath, a diverticulum of the 

 wall of the cloacal cavity, which extends forward and slightly upward to a point 

 about level with the base of the seminal receptacle. The cells of the distal 

 extremity, which are probably the spicide-matrix cells, are very slender elements 

 (Plate 15, fig. 10), with dense spindle-shaped nuclei imbedded in an almost 

 homogeneous cytoplasm having somewhat the appearance of the odontoblasts 

 of various molluscs. Throughout the remainder of the sheath, especially on 

 its inner half, the cells are considerably smaller and their distal portion appears 

 to be more or less cuticularized (Plate 15, fig. 5). 



Two powerful sets of muscles, the retractors and protractors, attach to 

 the sheath (Plate 9, fig. 2). The first named consists of a large number of minor 

 bands inserted in the distal end, and on the other hand to the body wall, after 

 having spread out fan-like, a short distance anterior to the seminal receptacle. 

 The protractors are more numerous and attach at various points within a narrow^ 



