GASTEROPODA. 311 



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

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

 double movement of the embryo is more conspicuous in the Limnceus 

 than in the Planorbis. 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 egg ; 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, the 



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. Fi^r. 124, exhibits one of these yolklets 



/'^ (flu P"o*" to the commencement of the fissiparous 



''w tJ|/\ action, by which subdivision of the mass is pro- 



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



that action and of the assimilative powers 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, 



^/^^^\\ ^"^^^''^t^' °"^' t^<^ s^at of the future branchial 



^'" ill ^--^^^^Jl o^'g'i"^' the other that of the muscular mass. 



\J I'ltl "^^^^Jl ^^^^ ciliated epithelium, with which the 



^*^*^ *-— '^^^ vitellus is now almost entirely covered, oc- 



Apiysia. 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 mass (^^. 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 



X 4 



