296 THE DEVELOPMENT 



narrower than in the previous stages, and is divided into two 

 regions, namely, a narrow throat, and the stomach proper (st). 

 The latter is as yet closed behind (st 1 ), but is evidently under- 

 going a. change preparatory to its connection with the rest of 

 the intestinal tract which is developing isolately at the extreme 

 posterior end of the body. The latter operation commences 

 by a depression (an) forming immediately under the cap-like 

 mantle (sh), exactly opposite to, and as if in direct continua- 

 tion of, the stomach (st, st 1 ). In process of time the depression 

 becomes elongated into a tube, and meeting the posterior end 

 (st 1 ) of the stomach unites with it and forms a complete intes- 

 tinal canal, which projects in a straight line along the axis of 

 the body. 



In the course of about three days from this time the first 

 traces of a heart appear, as two slowly pulsating globular sacs 

 placed one before the other, in the midst of the mass of little 

 spherical bodies (d) which are so conspicuous in the region of 

 the back during the previous phase of growth. About this time, 

 too, the intestinal canal begins to assume a lateral curvature, and 

 thus becomes the first to depart from the strictly symmetrical 

 relations in which the parts of the organization have hitherto 

 stood, and take on that one-sided character for which the Mol- 

 lusca possessing a spiral shell are so noted. The unequal devel- 

 opment of the other organs soon follows .the example of that 

 of the intestine. The cap-like mantle spreads faster on the right 

 side than on the left, and finally covers the whole back ; the 

 creeping disc, still retaining its symmetry, elongates and flattens 

 into a foot-like form ; the region just above the mouth extends 

 forward and over it beyond the foot, and broadening side- 

 ways, assumes the position and office of a head ; the region on 

 the back behind the head becomes divided into two portions by 

 the formation of a sort of neck which separates the upper space 

 between the head and tail from the great mass which is directly 



diam. A profile view, m, the mouth ; st, the stomach ; st 1 , the posterior end 

 of st ; an, the incipient intestine ; f, the creeping disc ; d, the remains of the 

 yolk mass, on the back; sh, the cap-like mantle. From Lerebnullet. 



