GASTEROPODA. 309 



others for their symmetry and beauty. Here are displayed the nida- 

 mental sacs of the frail Janthina*\ they are of a flattened pyriform 

 shape, composed of a delicate reticulate film of albumen, and are at- 

 tached by one extremity to a float, formed likewise by a secretion of 

 albuminous matter, dilated into a group of cells filled with air. To 

 this float the parent Janthina commits her little progeny, and having 

 securely fastened their several cradles or nursery cells, she detaches 

 the float, which bears the ova to the surface, and sustains them Avhere 

 they may best receive the full influence of solar light and heat. These 

 nidamental capsules of the Pyrula rapa-f are attached in regular 

 linear series to portions of decayed wood ; they are of a flattened 

 sub-conical figure, adhere by their apex, and have their base emar- 

 ginate. The nidamental capsules of the whelk j: are common objects 

 on our sea-shore ; they are aggregated in large irregular masses, 

 often attached to portions of oyster-shell ; each capsule presents a 

 depressed ovoid figure, with one side convex, the other flat or concave. 

 The small nidamental cells of the Cowrey (^Ci/prea) are aggregated 

 in a flattened group. In the Turbinella^ the cells are of a flattened 

 sub-pentagonal form, and adhere together, superimposed one upon 

 the other, forming what is termed a camerated nidus. Each chamber 

 contains between twenty and thirty embryos. In this preparation 

 you may observe that the rudimental shell is completely calcified and 

 fitted to defend the little Gasteropod before it emerges from the tem- 

 porary shelter provided for it by the parent. Numerous other modi- 

 fications of these secreted nests of the Gasteropodous Mollusca might 

 be enumerated. 



The general course of development of the Mollusca of the present 

 class has been observed in the Plcmorbis by M. Jacquemin, and in 

 the LimncBUS by M. Dumortier. The transparency of the mucous 

 capsules of the ova and of the ova themselves in these fresh-water 

 Gasteropods renders them favourable subjects for such observations. 

 The development of marine Gasteropods has been traced by M. Sars 

 in the Tritonia, Doris, and Aplysia, and has been likewise studied 

 in the latter genus, by Dr. Van Beneden. 



In all these Mollusca the first steps in the formation of the embryo, 

 after the disappearance or metamorphosis of the germinal vesicle, 

 are the multiplication of the cells of the vitellus producing the 

 twofold, fourfold, and more numerous subdivisions, like those which 

 have been observed in the ova of so many of the foregoing classes. 

 The leading differences in the Gasteropods at this stage of develop- 



* See Preps. 2945, 2946. f See Prep. 2947. A. 



\ Preps. 2948, 2949. § Prep. 2950. 



X 3 



