170 GASTROPODA. 



that of the Opisthobranchia, a fact which is specially evident in 

 the forms that have a coiled shell (Figs 75 A, and 72, p. 162). The 

 posterior part of the foot here also usually carries an operculum 

 which, in the Lima tin I doe, is retained throughout life, and in the 

 Cymbuliidae, is thrown oft' after the shell has been lost ; but in those 

 Pteropods that have straight shells an operculum is not found. A 

 well-developed primitive kidney is not known to occur in the Ptero- 

 poda ; they may, in this respect, resemble the Opisthobranchia, a 

 comparison which would be supported by their internal organisation. 

 We have here, for instance, as in the Opisthobranchs, the two sacs 

 rilled with food-yolk as appendages of the enteron. In the formation 

 of the alimentary canal, the entoderm becomes differentiated in such 

 a way that the median (ventral and dorsal) parts become the epi- 

 thelium of the archenteron, while the lateral parts which appear 

 composed of large cells rich in yolk, by growing out into caeca, be- 

 come the nutritive diverticula. These caeca have been supposed to 

 yield the liver, but this organ, according to Fol's statements, forms 

 independently of them as an outgrowth of the archenteron. A pos- 

 terior tubular diverticulum of the archenteron runs out towards the 

 ventral surface and fuses with the ectoderm at the spot where the 

 anal cells lie to form the anus. This lies either in the middle line 

 behind the foot, shifting secondarily to the left side (Gavoliniidae) or 

 else it lies from the first on the right side of the body {Cymbuliidae, 

 Gymnosomata). There are also other indications of asymmetry, such, 

 for instance, as the lateral position of the mantle-cavity. This indi- 

 cates that the Pteropoda which, as adults, are somewhat symmetrical 

 in structure, are derived from asymmetrical forms. 



As the fins increase in size, the velum gradually degenerates. The 

 mouth takes up its final position between the fins. The disappear- 

 ance of the velar area leads to the great redaction of the large section 

 of the larcal body which lies in front <>f the foot. At a later stage, 

 the two tentacles bud out in this region, carrying the eyes. This 

 reduction of the anterior part of the body as compared with the 

 massive foot, which has shifted far forward, is specially characteristic 

 of the Thecosomata. In Tiedemnnnia, however, the oral region be- 

 comes raised up to form the proboscis which bends backward. After 

 the growing shell has reached the base of the foot, a slit-like invagina- 

 tion appears in the Caoolini.idae (according to Fol) on the right side 

 between the base of the foot and that of the velum, extending then 

 dorsally and ventrally. The mantle-cavity thus formed finally 

 encircles the body (visceral dome) on three sides, so that the latter 



