228 COMPARATIVE ANATOMY CHAP. 



originally paired. The two ducts, i.e. the two seminal ducts in the male and the 

 two ovarial ducts in the female, open into the mantle furrow on each side, somewhat 

 in front of the reual aperture (Fig. 180, p. 217). 



(2) Gastropoda. The gonads of the Prosobranchia offer but few points of 

 interest to the comparative anatomist. In the Pulmo-nata and Opisthobranchia, the 

 germinal gland is a hermaphrodite gland, in which spermatozoa and eggs are 

 produced simultaneously. This gland is much lobed, or else consists of numerous 

 converging diverticula ; the spermatozoa and eggs arise intermingled on the walls, 

 become detached at one of the stages of their development, and then lie free in the 

 cavity of the gonad. The same applies to the large hermaphrodite gland of the 

 Tectibranchia, which varies much in its outer form. It lies in the posterior part of 

 the body, on the digestive gland, penetrating at times between its lobes ; it is itself 

 more or less lobed, its lobes consisting of secondary lobes (vesicles or acini). In 

 all these acini, spermatozoa and eggs are simultaneously produced. It is only in 

 the Pleurobranchcea and allied forms that the parts of the gland which produce 

 spermatozoa and those which produce eggs are localised ; this arrangement resembles 

 that in the Nudibranchia, which will presently be described. The constituent lobes 

 or vesicles are either male or female, the former producing only spermatozoa, the 

 latter only eggs. This is the arrangement found also in some Nudibranchia 

 (Am})horina, Capcllinia), but in most Nudibranclis the male and the female 

 germinal regions become separated in such a way that the terminal acini yield eggs 

 only, but open in groups into lobes of the gland which produce only spermatozoa. 

 Each lobe has its duct ; these ducts, uniting together, finally form the duct of the 

 hermaphrodite gland. This gland thus forms an extensive organ spread out in the 

 larger posterior part of the primary body cavity ; where there is a compact diges- 

 tive gland it covers this organ. Pltyllirhoe has 2 to 6 (usually 3) separate globular 

 acini whose long and thin ducts combine to form a hermaphrodite duct (Fig. 

 195, p. 238). 



The hermaphrodite gland of the Pteropoda ( Tectibranchia natantia] always lies 

 in the upper (dorsal) portion of the visceral dome ; it is sometimes acinose and 

 sometimes consists of converging tubular follicles or of lamina} closely crowded 

 together. The eggs are always produced at the peripheral part of the acini, tubes, 

 or lamella-, while the spermatozoa arise in the central parts, near the ducts. These 

 two parts are generally separated by a membrane, which the eggs have to break 

 through to reach the hermaphrodite duct. The Pteropoda are protandrously 

 hermaphrodite, i.e. the spermatozoa are produced before the eggs, an arrangement 

 found in many hermaphrodite Molluscs. 



(3) Scaphopoda. The gonad (testis, ovary) in these animals is a long spacious 

 sac, provided with lateral diverticula ; it lies above the anus, rising high up into the 

 visceral dome along the posterior side of the body. In the Solenopoda (Siphono- 

 dentalium, etc.) a large part of the gonad stretches into the mantle. In young 

 animals, the gonad is closed on all sides, but in adults its wall appears to fuse with 

 the right kidney, and in the partition wall so formed an aperture arises which 

 establishes communication between the gonad and the right nephridium. 



(4) Lamellibranchia. The gonads are here found in the form of much-branched 

 tubular or lobate masses lying on each side in the primary body cavity, surrounding 

 and partly penetrating between the other internal organs. In some cases (Anomiidce, 

 Mytilidir), the gonad on each side stretches into the mantle. In others (Axinus, 

 Montacuta), it bulges out the body wall in such a way that branched outgrowths, 

 containing the germinal tubes, project from the body into the mantle cavity. 



In most Lamellibrauchia the sexes are separate, but hermaphroditism sometimes 

 occurs. There are (1) whole groups of bivalves which are hermaphrodite ; e.g. the 

 most specialised forms, such as the Anatinacea and Scptibranchia ; (2) families 



