232 COMPARATIVE ANATOMY CHAP. 



but has shifted far forward along the right side of the neck, probably in con- 

 nection with the development of the copulatory apparatus. The position of this 

 aperture is thus not necessarily affected by any further displacement of the pallial 

 complex, or indeed of the whole visceral dome, which explains the fact that, in Ddude- 

 bardia and Testacella, the common genital aperture, and in Oncidium, the male 

 aperture, lies far forward on the right side of the body, although the pallial complex 

 has shifted completely to the posterior end of the body. 



In the Opisthobranchia also, the single or (secondarily) double genital aperture 

 lies to the right in front of the anus and even in front of the kidney. This position 

 seems inexplicable except by the supposition of a shifting back of the pallial com- 

 plex in which the genital aperture, emancipated from the complex, took no part, 

 thus coming to lie in front of the shifted anal and renal apertures. 



Monotocardia. Unlike the Diotocardia, which, with the exception of the Neritidce, 

 have no copulatory organs, the Monotocardia possess a penis, which, however, does 

 not lie in the mantle cavity where the genital aperture originally lay. It w r ould be 

 unable to function in this position, and is therefore placed on the right side of the 

 head or neck (Fig 71, p. 73), and forms a freely projecting, extensible, muscular 

 appendage, which often attains a considerable size. The male genital aperture, 

 however, in very many, perhaps in most, Monotocardia, remains in its original posi- 

 tion in the mantle cavity, to the right, near the rectum. In such cases, a ciliated 

 furrow runs forward on the floor of the respiratory cavity, along the right side of the 

 neck, to the base of the penis, to the tip of which it is continued as a deep groove. 

 This furrow conducts the semen to the penis from the genital aperture. In some 

 cases the furrow closes, and forms a canal ; the penis then becomes tubular, and the 

 seminal duct enters into it. The genital aperture is thus shifted far forward from 

 its original position. The seminal duct, which arises from the testis, usually forms 

 coils as it runs along the columellar side of the shell. The vas deferens has no 

 special appendages, although it may widen into a vesicle at some point in its course. 



In the female, the genital aperture remains in the mantle cavity, lying to the 

 right near the rectum, behind the anus. The duct remains, as a rule, more or less 

 simple ; it is divided into the following consecutive sections : (1) an oviduct, rising 

 from the ovary, which may bulge out to form one or more receptacula seminis ; 

 (2) the uterus, a wider section with thick glandular walls, in which the eggs are 

 provided with albumen and a shell : (3) a muscular sheath, the vagina, which leads 

 to the outer genital aperture. In Paludina, there is a special albuminous gland 

 opening into the oviduct. 



In hermaphrodite Prosobranchia (Valvata, a few Marseniadce, e.g. Marsenina, 

 Onchidiopsis) a hermaphrodite gland is found. This gland gives rise either to one 

 duct, which divides later into a vas deferens and an oviduct, or to a vas deferens and 

 an oviduct which are from the first distinct. The vas deferens runs to the penis as 

 in the males of dioecious Prosobraiichiates ; the oviduct runs to the female genital 

 aperture. Both these ducts are, owing to the occurrence of accessory glands, etc., 

 more complicated than in other Prosobraiichiates. 



Opisthobranchia and Pulmonata. The ducts in these orders are extremely 

 complicated, both by division into many consecutive sections and by the develop- 

 ment of various accessory organs. 



In the following descriptions of several types of genital ducts only the most 

 important points can be mentioned. We give first the type of duct commonly 

 found in the Cephalaspidce (Tectibranchia}. 



1st Type. The hermaphrodite gland has a single undivided efferent duct, 

 opening out through a single genital aperture. From this aperture the fertilised 

 eggs pass out direct, but the spermatozoa pass into a ciliated seminal furrow which 

 runs along in the mantle cavity, and by which they are conducted to the penis. 



