138 THE GASTROPODA 
As regards the formation of the internal organs of the adult, 
the stomach, the liver, and nearly the whole of the intestine arise 
from the endoderm. The liver lobes are formed before the 
absorption of the nutritive sacs borne on the posterior part of 
the larval stomach. The proctodaeal invagination, placing the 
intestine in communication with the exterior, is always of small 
importance, but, on the other hand, the buccal bulb and oesophagus, 
with their numerous accessory organs, are formed from an im- 
portant stomodaeal ectodermic invagination, which always corre- 
sponds in position with the extreme anterior end of the blastopore, 
whether the latter is closed or remains open. The remaining organs 
are formed in the same manner as in other Mollusca, as has been de- 
scribed in the first chapter, and the ontogeny of the Gastropoda does 
Fie. 119. 
Embryos and larvae of various Molluses, after the ventral flexure and before the torsion (for 
A), ventralaspect. 4A, Gastropod (Limnaea, after Fol) ; B, Dentaliwm (after Lacaze) ; C, Lamelli- 
branch (Dreissensia, after Meisenheimer); D, Cephalopod (Oigopsid, after Grenacher). «i, 
anus ; ar, arms ; e, eye; f, foot ; fu, funnel; g, gill; m, mouth ; ot, otocyst; pa, mantle ; pa.o, 
posterior pallial orifice ; pe.g, pedal ganglion ; sh, shell ; ve, velum ; vi, vitellus. 
not exhibit any special features other than certain post-larval meta- 
morphoses and the torsion produced during development (Fig. 51). 
Up to the trochosphere stage the larva is strictly symmetrical 
(Fig. 14, B), but afterwards the torsion sets in, as a result of which 
the asymmetry characteristic of adult Gastropoda is established. 
At first the aperture of the pallial cavity and the anus are always 
posterior (Fig. 118), as is the case in all symmetrical Molluscs (Fig. 
22, A, B, D, E); then they are carried forward by a ventral flexure 
(Fig. 119) in the same manner as in the Cephalopoda, Scaphopoda, 
and many Lamellibranchia. But in the Gastropods a lateral torsion 
is superadded to this primitive flexure, causing the pallial aperture 
to pass from the postero-ventral surface (Fig. 51, A) over to the right 
side (Fig. 51, B), and thence to the antero-dorsal surface (Fig. 51, C). 
If the animal be supposed to have the mouth turned towards the 
observer, this torsion may be seen to follow the movements of the 
hands of a watch (Fig. 52). 
