NO. 17 POLYCLAD WORMS HYMAN J 



sinuous ejaculatory duct. The ducts of the accessory vesicles were 

 not seen. At the distal end of the evert is the penis sheath ; through 

 its lumen extends the curved stylet typical of the genus, protruding 

 from the penis end for about one-third of its length. The sperm form 

 a club-shaped sharply defined mass in the lumen of the seminal vesicle 

 and from this mass a stream of sperm can be followed along the 

 ejaculatory duct and stylet and out of the tip of the stylet. The mode 

 of entrance of the vasa deferentia into the seminal vesicle is shown 

 in the figure. 



Not much can be made out of the female apparatus in the whole 

 mount ; genital pore appears to lead into a cement pouch which re- 

 ceives the encircling cement glands (fig. 15). 



The e version of the male apparatus in this specimen is of great 

 interest. It is undoubtedly normal since the same eversion was seen 

 in a Bermuda Prosthiostomum, P. cy clops (Verrill) where, however, 

 the specimen, through long preservation, was so dark and opaque that 

 nothing could be discerned in the protruded apparatus. This Galapagos 

 specimen indicates that in Prosthiostomum the entire male apparatus 

 everts in copulation. This means that the male antrum must turn 

 inside out, so that its lining becomes the surface epithelium of the 

 evert, and the whole male copulatory complex, including the seminal 

 vesicle and the 2 accessory vesicles, becomes pulled into the evert. It 

 is further seen from figure 15 that the so-called penis sheath actually 

 serves as penis papilla while the penis papilla remains inside and ap- 

 pears to function merely to anchor the stylet. One begins to wonder 

 if these structures have not been misnamed and if the penis sheath 

 should not be regarded as the penis papilla, and the projection at 

 present considered the penis papilla should not receive some other 

 name. Of course, many polyclads do not have a penis sheath. Whether 

 it occurs only in those species with a stylet deserves investigation. 



Remarks. — The species of the genus Prosthiostomum resemble each 

 other so closely in external characters and details of the copulatory 

 complex that separation into species is often difficult. As pointed out 

 by Bock (1913) one must depend chiefly on the coloration and the 

 eye arrangement. Some differences also appear to exist as regards 

 the length and shape of the male antrum, shape of the seminal vesicle, 

 and points of entrance of the vasa deferentia into the seminal vesicle. 

 On the whole it is not advisable to omit the study of serial sections 

 in determining the species of this genus. In the Galapagos specimen, 

 the damage to the marginal set of eyes, the lack of data on the colora- 

 tion in life, and the eversion of the male apparatus, preventing com- 



