438 THE ANATOMY OF INVERTEBRATED ANIMALS. 



movements of dilatation and contraction. The macromeres 

 form a large mass inclosed within a spheroidal dilatation of 

 the greater part of the bcemal wall of the body, which deserves 

 the name of yelk-sac even better than the structure so named 

 in the Cephalopoda^ inasmuch as it more nearly corresponds, 

 morphologically, with the vitelline sac of vertebrated animals. 

 Between this sac and the foot the small remainder of the 

 haemal wall becomes converted into the mantle. 



The walls of the vitelline sac undergo contractions which 

 sometimes, but not always, alternate with those of the pedal 

 appendage. On each side of it appears the "primitive kid- 

 ney," consisting of a curved elongated series of cells within 

 which concretions are developed, and terminating in a duct 

 which opens on the posterior face of the vitelline sac, close to 

 the mantle. The exact mode of origin of the alimentary 

 canal has not been made out ; but, in any case, only a very 

 small portion of the endodermal cells can take part in its 

 formation, and the archenteron is, at first, a sac which nearly 

 fills the small projection formed by the rudimentary mantle. 

 The oral involution of the ectoderm gives rise to the odon- 

 tophore, and extends across the base of the foot, to open, 

 eventually, into the archenteron. 



The fold of the mantle which overhangs the iqespiratory 

 aperture makes its appearance very early; and, immediately 

 behind it, the intestine is visible as a short tube, which ex- 

 tends from the archenteron to the surface, but does not, at 

 first, open there. 



As development proceeds, a movement of the macromeric 

 part of the vitellus takes place in exactly the opposite direc- 

 tion to that of the food-yelk of the Cephalopoda ; that is to 

 say, from the vitelline sac into the constantly enlarging foot. 

 The alimentary canal accompanies it, the anus alone remain- 

 ing in its primitive position. The constantly lengthening 

 alimentary canal becomes disposed in folds ; between these 

 the macromeric part of the vitellus, which gradually forsakes 

 the diminishing vitelline sac, disposes itself around the coils 

 of the intestine. Eventually, for the mcst part, it becomes 

 converted into the liver. 



The rudimentary shell first makes its appearance in the 

 form of a few subcrystalline calcareous plates, on the inner 

 side of the ectoderm.* 



The development of Helix is similar to that of JLiinax ; 



* Compare Gegrcnbaur, " Zur Entwickeluns^sgescliichte der Laiid-Gastero- 

 poden." {Zeitschrift fiir Wise. Zoologie^ 1852.) 



