THE GASTROPODA 137 



accumulated. In the Opisthobranchs they are closed pouches ; in 

 Paludina, Bithynia, and the Pulmonates they are canals opening to 

 the exterior. In the last-named group these organs consist of tubes 

 of ectodermic origin, each of which bears an ampulla on the middle 

 of its course, and its internal portion is formed of perforated cells 

 and ends internally in a closed flame-cell. In the Basommatophora 

 (Fig. 118, re) these cells of the larval kidney are four in number, 

 three being perforated and one a flame-cell. These organs are 

 absent in Vaginula. 



The nerve-centres and organs of special sense originate from the 

 ectoderm, almost always from an ectodermic thickening on the buccal 

 side of the apical organ of the larva (Crepidula), but in Vermetus and 

 the Cavoliniidae an ectodermic invagination has been described as 

 taking part in the formation of each cerebral centre. Again, in the 



FIG. 118. 



Embryo of Limnaea stagnalis, viewed from the right side, a, anus ; c.g, cerebral ganglion ; 

 /, foot; in, intestine; m, mouth; o.r, aperture of the embryonic kidney; pa, mantle; ra, 

 nulula ; re, embryonic kidney ; sh, shell ; st, stomach ; vi, vitellus. (After Erlanger.) 



Pulmonates, even if the principal part of the cerebral centres is formed 

 from an epithelial thickening, the posterior accessory lobe origin- 

 ates from a subsequent ectodermic invagination (Fig. 117, o.ca.c) 

 the " cerebral tube " the cavity of which generally disappears 

 after the animal is hatched out, but persists in the adult Planorbis 

 and Limnaea. The eyes arise in the velar field, near the cerebral 

 centres, and are formed by invagination in the Aspidobranchia, 

 Paludina, the Stylommatophora, etc., but from thickenings in 

 Vermetus, and frequently after the veliger larva is hatched, as, for 

 example, in many Nudibranchia, with the exception of the Tergi- 

 pedinae (Fig. 6 1, e). The otocysts are always formed at an early stage, 

 on the antero-lateral aspects of the foot, by invagination in the 

 majority of marine Gastropods, but from ectodermic thickenings in 

 many Pulmonata. At first they invariably contain a single otolith, 

 even in the species which in the adult state possess multiple 

 otoconia. In pelagic larvae the otocysts may often be seen to be 

 asymmetrical, and sometimes the tentacles share this peculiarity. 



