GASTEROPODA. 311 



In the Physa, the nidamental mass is short and ovate : in the 

 LimiKBus it is oblong, and not striated, as in the Planorbis. The 

 double movement of the embryo is more conspicuous in the LimncBus 

 than in the Plajiorbis. The first movement of the yolk is one of 

 rotation upon its axis ; but, as development proceeds and the ciliary 

 vibrations are strengthened, the embryo begins to travel in an ellip- 

 tical course around the interior of the e^^ ; its two movements (to 

 compare small things with great) resembling those of the planets in 

 the solar system. 



The ova of the Aplysia are excluded in a long string, enveloped 

 by a transparent flexible mucus, the ova being aggregated in several 

 irregular series in its centre. When examined at this period, tlie 

 yolk has apparently divided itself into six, seven, or more numerous 

 globules ; or_, in other words, as many germinal vesicles included in 

 the same mass of albumen and in a common chorionic coat, have 

 given origin to as many aggregations of vitelline cells ; these, therefore, 

 may be regarded as so many independent yolks^ in each of which the 

 same progressive fissiparous multiplications have been observed, as 

 in the single vitellus of the ovum of the Planorbis, and of animals in 

 124 125 general. Fig, 124. exhibits one of these yolklets 



C^^ prior to the commencement of the fissiparous 

 \jj\ action, by which subdivision of the mass is pro- 

 Apiysia. " duced. Fig. 125. shows the quadrifid product of 

 that action and of the assimilative powders of the resulting divisions 

 A small clear vesicle, probably the seat of further subdivisions, is 

 specially indicated by M. Van Beneden at a. 



In Jig. 126. the multiplication of the globules has increased, and 

 126 127 two of them, of larger size than the rest, 



indicate, one, the seat of the future branchial 

 organs, the other that of the muscular mass. 

 The ciliated epithelium, with which the 

 vitellus is now almost entirely covered, oc- 

 casions the usual rotations of that body. 

 The progress of transformation of this monad-like embryo to the Gas- 

 teropodous form, resembles closely that which has been described in 

 the Planorbis. The remains of the vitelline msiss (Jig. 127. a), not 

 yet metamorphosed into special organs, indicates the expanded 

 alimentary sac; b is the apex of the rudimental foot, and c the 

 coarsely ciliated surface, which constitutes the now external branchia. 

 These parts protrude from a rudimental, thin, pellucid and flexible 

 shell, which covers all the rest of the surface of the body. The 

 arrows indicate the direction of the rotatory movements of the embryo, 

 which now likewise describes its elliptical revolutions in the chorionic 



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