30 PAUL PELSENEER. 



This arrangement exists in — 



(1) Various Tectibranchs (e.g. Lobiger, Pelta). 



(2) Pleurobranchidse, Tylodina, and Nudibranchia 



(except Elysioidea). 



(3) Siphonaria. 



(4) Cardium oblongum. 



It must be noticed here that the female acini open into the 

 male acini, and that the acini of the same sex tend to group 

 themselves together, as may already be observed in various 

 Nudibranchs (where the female acini are the most " eccentric," 

 i. e. superficial). This arrangement is clearly marked, and 

 furnishes in Onchidiopsis and Pneumonoderma (vide 

 supra) a transition to the next condition. 



3. Gland with separate male and female regions 

 (with a common duct). — The type which now presents itself is 

 that in which the acini of the same sex are all united together 

 in such a way as to constitute in the hermaphrodite gland a 

 male and a female part distinct from one another, these two 

 parts having, nevertheless, a single genital aperture and a 

 common duct. 



This conformation is characteristic of — 



(1) The hermaphrodite species of the genus Pecten, 



where the male and female regions are contiguous 

 and form an undivided hermaphrodite gland 

 (fig. 10). 



(2) The Cycladidse, where it is already observable that 

 the two regions are fairly separated, and only connected by 

 their duct (fig. 12), which forms a transition to the next type. 



4. Male and female glands in the same individual 

 entirely distinct from one another, and with special 

 ducts. — The acini of each sex can form a region absolutely 

 separate from that constituted by the acini of the other sex, 

 these two regions each having their special duct, and so form- 

 ing veritable testis and ovary. 



At the same time the vas deferens and the oviduct may 

 open into a common orifice (Poromyidae), or there may be no 

 common orifice for the two glands. This last condition, which 



