312 INVERTEBRATE MORPHOLOGY. 



areas, the reproductive elements make their way into a com- 

 mou hermaphrodite duct, which presents variations of structure 

 in different forms and receives the secretion of certain acces- 

 sory glands. In its simplest form, as seen for instance in 

 Aplysia, the duct runs forward, pursuing a somewhat tortuous 

 course and becomes surrounded by an albuminiparous gland, 

 from which it receives a viscid secretion, within which the 

 ova become imbedded just in front of the point where the 

 gland opens into the duct. The latter has attached to it a 

 pouch-like structure, the vesicula seminalis, and is continued 

 on as a somewhat wider tube to open to the exterior at the 

 genital pore situated on the right side of the body, shortly 

 before reaching the pore, however, receiving a duct from a 

 globular sac, the spermatheca. From the anterior edge of the 

 pore a groove, the seminal groove, extends along the right 

 side of the body to the neck region, where it ends in a mus- 

 cular evertible penis, situated near the anterior right tentacle. 

 It seems probable that the spermatozoa mature before the 

 ova, and passing to the vesicula are stored up there. During 

 copulation the seminal fluid is transferred through the penis 

 to the spermatheca of another individual (perhaps the trans- 

 ference is a mutual interchange), and when later the ova pass 

 along the duct they are impregnated by the spermatozoa 

 so stored away, a cross-fertilization being thus brought 



about. 



This arrangement of the reproductive duct is found in the 

 more primitive Opisthobranchs, i.e., in those in which the 

 mantle-lobe still persists, and in the group Pteropoda-, in 

 the more highly-modified forms, such as Doris, JEolis, etc., 

 and, among the more simple forms, in Pleurobranchwa the 

 hermaphrodite duct divides into an oviduct and a vas deferens. 

 The former after receiving the spermathecal duct opens into 

 a genital atrium, with which communicate also the albumiuipa- 

 rous gland and a nidamental gland, which manufactures the 

 outer shell-like investment of the ova. The vas defereus, after 

 a more or less tortuous course, enters the muscular saclike 

 penis-sheath which communicates with the genital atrium; the 

 enlarged termination of the vas, the penis, being thus capable 

 of eversion through the pore by which the atrium communicates 



