DEVELOPMENT OK THE EXTERNAL FORM — I'ULMONATA. 181 



wide space as the cephalic cavity. Special attention has been directed 

 to this part in consequence chiefly of the pulsating movements which 

 may occur here, a peculiarity also found in the nuchal and the pedal 

 regions of the embryo. 



It has repeatedly been stated that certain regions of the body-covering, 

 those to which a large number of niesoderm-cells became attached, carry on 

 contractions which sometimes follow one another with considerable regularity, 

 this last fact having led to their being called " larval hearts." The circulation 

 of the body-fluid is, in any case, promoted by these contractions, but it seems 

 doubtful whether they should be described as actual pulsations. Sometimes 

 the movements that thus occur are somewhat irregular, and Rabl found that, 

 occasionally, contraction of one part of the body is followed by extension of 

 another part, but we cannot consider this to be regular rhythmical move- 

 ment. The embryo moves in consequence of these contractions. It is well 

 known, however, that Gastropod embryos are able in addition, in consequence 

 of their rich ciliation, to rotate within the egg-envelope. 



Since the embryo, by taking in the albuminous fluid contained within the 

 egg-shell, feeds independently and also has a circulation of its own and special 

 excretory organs, the velum may serve as a respiratory apparatus, this func- 

 tion being also exercised by it ifi addition to its locomotory function in the 

 free-swimming larvae. In the embryos of terrestrial Pulmonates, a special 

 respiratory organ develops, the caudal vesicle (podocyst), which will be further 

 described below. 



The very large apical plate of the embryo has considerably thickened 

 and has become bilobed. According to Rabl, the cerebral ganglion 

 is derived from it, though in other Pulmonates the formation of this 

 ganglion has been thought to arise differently (p. 191). At the 

 posterior end of the "apical plate " the eyes arise as ectodermal pits. 

 Two large superficial prominences, which soon become conical, arise 

 laterally to the optic vesicles and represent the rudiments of the 

 tentacles. Both eyes and tentacles belong to the pre-oral section, 

 whereas the otocysts arise behind the velum (Figs. 79, (SO au, f, ot). 



Up to this point, the embryo is fairly symmetrical in shape, but 

 this symmetry is disturbed chiefly by the further development of 

 the shell which grows towards the right more strongly than towards 

 the left (Fig. 80). The edge of the mantle, which now bulges out 

 more than before, is of course also affected by this unequal growth. 

 The anus is pressed out of its median position to the right. It is 

 evident from this that processes occur in the later development of the 

 Pulmonates similar to those already met with in the metamorphosis 

 of other Gastropods. 



As the mantle extends further, its growth takes place more rapidly 

 on the right than on the left side. In front of the anus an indenta- 



