342 MOLLUSCA. 



distinguished from one another by their colour ; the ovary being red, 

 and the testis milk white or yellow. In the Protobranchs the 

 generative glands open into the kidney near the pericardia! opening ; 

 in the Anomiidae and Pectinidae also into the kidney, but nearer 

 the external opening. In Ostrea, Cyclas, and some Lucinidae 

 they open with the renal duct by a common opening. As a rule, 

 however, the two organs (renal and genital) open separately, 

 but close together, on the external side of the visceral commissure 

 (Fig. 257). 



In the hermaphrodite forms the whole gland may be herma- 

 phrodite even to the ultimate acini, which produce ova and 

 spermatozoa simultaneously or alternately (Ostrea edulis and plicata) ; 

 or the male and female follicles may be separate, and open together 

 by a common duct (species of Peden and Cyclas), or by separate 

 ducts leading to separate openings (Poromya, Anatinacea). Herma- 

 phrodite individuals are sometimes met with in the fresh-water 

 mussels Unio and Anodonia. 



Development. The fertilization of the eggs is sometimes effected in the 

 branchial cavity, and the first part of the development often takes place in 

 the mantle cavity, or between the lamellae of the inner (Cyclas) or outer gill 

 (Unio, Anodonta) as in a brood-pouch; sometimes it takes place outside the 

 mother (Pecten, dioecious Ostreac, Mytilus, in all of which artificial fertilization 

 is possible). In Ostrea edulis it is effected in the oviduct. The egg is surrounded 

 by a vitelline membrane with a micropylar aperture at one point. 



The segmentation is unequal (Fig. 268), the formative pole being opposite to 

 the micropyle. The gastrula is generally formed by epibole, rarely by invagina- 

 tion. The blastopore sometimes remains open, e.g. Ostrea, or closes, e g. Cyclas, 

 Teredo, but the mouth is formed almost immediately by an ectodermal invagina- 

 tion at the same point. The stomach, liver, and intestine are formed from the 

 endoderm, and the proctodaeum is established later as an ectodermal invagination 

 after the shell has been formed. 



The embryo, which is partially ciliated and often rotates within the egg- 

 membranes, soon acquires a preoral ring of cilia and a shell-gland (Fig. 268). 

 The latter is on the side opposite to the blastopore, and gives rise to a pellicle 

 which is calcified from two symmetrical points, thus forming the rudiments of 

 the two valves (Fig. 268 c). The cuticular shell remains uncalcified in the middle 

 dorsal line, where it gives rise to the ligament. An ectodermal invagination, 

 giving rise to the byssus gland, is almost always formed at the hind end of the 

 foot, even in forms which are without a byssus in the adult. The anterior ad- 

 ductor appears before the posterior. Among the provisional arrangements the 

 velum, as the preoral ciliated ring is called, is very generally present, and in 

 the free-swimming larvae has the form of a large ciliated ring or collar ; a pair 

 of larval eyes provided with a lens may also exist within the velar area. 



The gills appear as filaments, one by one from behind forwards in the posterior 

 part of the larva between the mantle and the visceral mass. A pair of larval 

 kidneys has been observed in some groups (Fig. 268 d, N). They consist of an 



