OVIPOSITION AND CHARACTEB OF THE EGG-CAPSULES AND EGG. 105 



of liul 'Hi //.-• which are oval, measui'e 5 cm. in length and are there- 

 fore larger than the eggs of pigeons. These eggs, in consequence of 

 their firm, smooth shell, closely resemble the eggs of birds, but are 

 distinguished from the latter by the fact that the actual egg (the 

 yolk) is always very small and floats in a great mass of viscid trans- 

 parent matter enclosed within the egg-shell. But although the yolk 

 or the egg-cell, as compared with the size of the egg is almost nil, 

 the mature embryo almost completely tills the shell, having increased 

 in size to this extent at the expense of the surrounding mass of 

 nutrient material. 



Some Gastropods take care of their eggs. Those species of Crepi- 

 dida which are immovably tixed to one spot (C. fornicata, plana, 

 and convexa, McMubbich, No. 70, Coxklix, Xo. IV) retain the egg- 

 capsules, which are attached to the substratum, under cover of 

 the shell. The wall of the capsules thus protected are naturally of 

 delicate nature. Verinetus attaches a few capsules to the inner sur- 

 face of its shell, near the aperture of the latter (Lacaze-Duthiers). 

 In comparatively few Gastropods, the whole development is passed 

 through within the body of the mother. These forms are therefore 

 viviparous. The best known example is Pain/Una ( Vivipants) oivi- 

 para, the eggs of which develop in the oviduct, which functions 

 as a uterus, until the form of the adult is reached. Its course :>f 

 development, however, exactly resembles that of other Prosobranchia. 

 The egg is surrounded by a conspicuous layer of albumen, which 

 again is enclosed in a membrane that runs out into a twisted stalk, 

 so that a kind of cocoon is formed. As a rule, only one egg lies 

 within this envelope, but two are sometimes found in it (Leydig, 

 No. 68), the resemblance to the egg capsules of other Prosobranchia 

 being thus heightened. Similarly, in a few species of Melaitia [in 

 Typhobia and Nass<rpsis\, the embryos develop in the uterus, and 

 are only born when they have attained the adult form. 



In some species of Melania and in Spekia, the embryos develop in 

 a special brood-pouch formed by an ectodermal invagination near 

 the right cephalic tentacle. The viviparous habit appears to be 

 largely confined to fresh-water forms. 



A tew Puhnonata are, like the Prosobranchia above-mentioned, vivi- 

 parous, the development of the embryo here also taking place in 

 the oviduct which is transformed into a uterus. This is the case in 

 a tew species of Clausilia, Pupa, Helix and Vilrina. Nearly related 

 species are often found to differ greatly in their methods of repro- 

 duction, some being oviparous, and others viviparous (No. 102). 



