GASTROPODA. 



381 



this cavity is unknown; it appears to break up, at any rate it 

 has not been traced. In some other forms (Paludina, Bithynia) 

 v. Erlanger* has shown that it gives rise to the pericardium, 

 kidney, and gonad. 



Ms 



The same observer also asserts that in*I}aludina 



FIG. 299. Some stages in the embryonic development of PlanorUs (after C. Rabl). a, optical 

 section through a segmenting ovum, with 24 segments, b, stage with four mesoderm cells, 

 viewed from the lower pole, c, oblique optical section through b. d, older embryo, with 

 shell gland (trochosphere stage). D alimentary canal ; EG ectoderm ; En endoderm ; FJi 

 segmentation cavity ; Ms mesoderm ; N primitive kidney (pronephros) ; mouth ; Oc eyes ; 

 Ot otocyst ; R rudiment of radula ; Rk polar bodies ; S shell ; Stir shell-gland : Sp apical 

 plate (thickening of ectoderm of preoral lobe) ; Ve velum. 



it is developed as a bilobed outgrowth of the primitive gut of the 

 embryo in the neighbourhood of the blastopore.f In the land 

 Pulmonates the young are born with the adult form, and there are 

 but very slight traces of a velum in the development, but in most 

 Gastropoda the young leave the egg-membranes as a trochosphere 

 larva (Fig. 299 d), which very soon 

 acquires, if it does not already 

 possess them, a shell-gland as an 

 ectodermal invagination of the 

 extra-velar dorsal surface, and a 

 rudiment of the foot as a pro- 

 jection of the ventral surface 

 (Fig. 299 d). The shell-gland may 

 or may not secrete a chitinous 

 plug, but in the veliger stage 

 (Fig. 300), which follows the tro- 



Vel 



FIG. 300. Young veliger larva 01 a Gas- 

 tropod (after Gegenbaur). S shell ; 

 P foot ; Op operculum ; Vel velum ; 

 T tentacles. 



* R. v. Erlanger, "Zur Entwick. d. Falndina vivipara," Pt. 1 and 2. Morph. 

 Jahrb., Bd. 17, 1891, and "Zur. Ent. v. Bithynia tentaculata," Mith. ZooL Stat. 

 NeapeL, Bd. 10, 1892. 



f This is denied by Tormiges, who asserts that the mesoderm in Paludina 

 is derived from the ventral ectoderm along the line of the blastopore. Vide 

 Z.f. w. Z.,G1, 1896. 



