XII 



PHYLUM MOLLUSCA 



729 



sometimes there is a single duct undivided throughout ; some- 

 times there is incomplete division. A receptaculum seminis 

 (rec. sem.) is connected with the oviduct, and also a number 

 of narrow accessory oviducal glands (muc. gl.) ; frequently a gland 

 termed prostate is connected with the sperm-duct, and there are 

 an eversible sac the sac of the dart(ds.) containing a crystalline 

 stylet, and a penis (pen.), which is perforated by a canal and is 

 capable of being retracted by a special muscle. In the Pulmonata 

 the first part of the duct (" hermaphrodite duct " proper) is simple, 

 and serves for the passage both of ova and sperms : the middle 

 part is incompletely divided internally into two passages, one 

 serving as oviduct, the other as sperm-duct. In the distal part 

 oviduct and sperm-duct are completely separate. Where the 

 sperm-duct enters the penis, there is given off a long, slender, 

 tapering diverticulum, the flagellum (flag.}, in which the sperms 

 are made up into elon- 

 gated masses or sper- A 

 matophores. 



Development. 

 The limpets (Patella) 

 are exceptional in lay- 

 ing the eggs one by 

 one and unfertilised 

 impregnation tak- 

 ing place in the water 

 after they have been 

 discharged. In almost 

 all the Gastropoda 

 fertilisation is internal, 

 and the eggs are laid in 

 great masses, embedded in jelly, each egg having its own hyaline 

 envelope. Very often the mass of spawn consisting of the jelly-like 

 substance, with the eggs embedded in it, attains a relatively con- 

 siderable size. In form it varies greatly : frequently it is in the shape 

 of long strings which are cylindrical or band-like : sometimes several 

 such strings are twisted together into a cord. Sometimes the 

 spawn is fixed to sea- weed or other objects ; sometimes it is un- 

 attached, and may float about freely. In the Streptoneura (Fig. 

 652), instead of being embedded in a jelly-like mass, the eggs are 

 enclosed in a firm parchment-like capsule, in which is contained, 

 in addition to the eggs, a quantity of an albuminous fluid, serving 

 to nourish the developing embryos. The shape of the capsule 

 varies greatly in the different genera : sometimes it is stalked, 

 sometimes sessile ; in some cases there is a lid or operculum, the 

 opening of which permits the embryos to escape. Very commonly 

 large numbers of these capsules are aggregated together, and usually 

 they are attached to a rock or a sea-weed or similar object. 



FIG. 051. Follicles of the ovotestis of the Gastropoda. A, 

 of Helix hortensis (Pulmonata): B, of the Eoli.Lx 

 a. a, ova ; b, masses of sperms ; c. common efferent duct. 

 (From Gegenbaur.) 



In 



