566 LECTURE XXTI. 



calcified and fitted to defend the little Gastropod before it emerges 

 from the temporary shelter provided for it by the parent. Numerous 

 other modifications of these secreted nests of the Gastropodous 

 Mollusca might be enumerated. 



The development of the Gastropods has hitherto been traced in a 

 few apneustal, nudibranchiate, tectibranchiate, pulmonate, and pecti- 

 nibranchiate species ; and the results show considerable modifications 

 in its course and phenomena. Most Gastropods are oviparous ; some 

 species of Litorina are ovo-viviparous : Paludina and Clausilia 

 ventricosa are viviparous. 



With the singular exception observed in Buccinum*, in which the 

 segmentar or granular germ-mass from which the embryo is deve- 

 loped results from a confluence of numerous, perhaps fifty, previously 

 distinct minute germ-masses f in the nidamental capsule, that germ- 

 mass is due to progressive segmentation of the yolk, the result of the 

 usual multiplication of germ-cells which clothe themselves with the 

 so-subdivided, but coherent, yolk substance. One cell in Pulmonata 

 and Aplysia, indeed, is from the beginning conspicuously larger than 

 the rest, and has been called the directive cell (richtungs blaschen. 

 Jig. 210, a). In all Gastropods the germ-mass takes the form of a 

 long round embryo, one end of which becomes indented and clothed 

 by ciliated epithelium, by which it rotates on its axis in the albumen 

 of the egg. Sars J, who noted the oviposition of the Tritonia Ascaiiii 

 on the 1st February, 1840, traced the segmentation of tjie yolk to the 

 8-fold division on the 4th day, and the completion of the germ-mass 

 on the 8th day. The indentation producing the bilobed end of the 

 embryo took place on the 15th day; after which the ciliated lobes 

 extended outwards, and assumed the form of wings or " vela." Dr. 

 Grant § had long before discovered the corresponding ciliated vela in 

 the embryos of Purpura^ TrocJius, Nerita, Doris, and jEoUs. 



On the 20th day the rudiment of the foot appeared beneath the 

 bases of the vela, and on the 22nd a transparent shell was developed 

 which covered all the body save the ciliated vela and foot. " I could 

 scarce believe my eyes," writes Sars, " when I made this discovery." 

 As the body and shell enlarge and elongate, the cilia become stronger 

 on the lobes, and the active movements of the embryos, crossing each 

 other like midges in the clear albumen of the nidamental band, offer 

 a most singular spectacle : the vela now move by muscular contrac- 



* CCCLII. 



f Called " ova " by the Authors of CCCLII., whose observations were, 

 however, made on " ova " in the nidamental capsule, not in the oviduct of the 

 female whelk, where the first effects of impregnation are to be traced. 



X CCCLXVII. § CCCLXXIV. p. 121. 



