200 



GASTROPODA. 



D. The Pedal Glands. 



In the larvae of various Gastropods, e.g., Nassa (Figs. Gl D and E, 63), 

 Vermetus, Murex, Firoloida (Fig. 65), etc., a deep tubular or sac-like ectodermal 

 depression has been described in the foot; this shows great agreement in 

 position with the pedal gland found by Kowalevsky in the embryos of Chiton. 

 Such a rudiment is perhaps also present in DentaHuin. In Nassa, this gland 

 forms a rather long tube, and in Murex it has a similar form (Bobretzky, 

 No. 11) ; in Firoloida, it is said to be much shorter and bilobed (Fol, No. 31, 

 Fig. 65, fd.). Salensky describes, in Vermetus, the formation of two ectodermal 

 invaginations in the foot, the one lying at the anterior and the other at the 

 posterior end. The canals lengthen inwards and fork, so as to yield the 

 glandular portion. Various glands are knowji in adult Gastropods also lying 

 one behind the other in the sole of the foot (Carriere, No. 21). The connec- 

 tion of the rudiments we have just described with these glands does not as 

 yet appear to be clearly demonstrated. It is well known that various 

 glands also occur at the anterior end and in the sole of the foot in Lamelli- 

 branchs which have'been homologised with the anterior and posterior pedal 

 glands of the Prosobranchia (Barrois, No. 3), but whether such a homology is 

 correct still seems doubtful.* 



E. The Alimentary Canal. 



The Stomodaeum first appears as an ectodermal depression in 

 which can soon be recognised a ventral outgrowth, the radular sac 

 (Figs. 53, p. 127, 78, p. 177 and other figures). This sac sometimes 

 appears even before the stomodaeum is completely invaginated and 

 consequently lies near the aperture of invagination, as in Helix 

 (Figs. SI and 82, p. 184). When the radular sac lengthens, it 

 undergoes dorso-ventral flattening. Its lateral margins then bend 

 upward so that it assumes the form of a channel, the dorsallv directed 

 cavity of which is filled with a mass of connective tissue. The wall 

 of the channel is formed of the upper and the lower epithelium, the 

 latter taking the principal part in the formation of the radula. 

 The first indication of this organ is found early in the form of a thin 

 cuticle in the radular sac. The formation of the radula, an organ 

 which has been studied in the adult by Rossler (No. 95), and Ruckeb 

 (No. 96), and others, takes place in the following way: The teeth 

 themselves are secreted by the cells which lie ventrally at the blind 

 end, while the basal membrane upon which the teeth are borne is 



*[The pedal glands may attain enormous development in the Pulmonata; 

 this is especially noticeable in Natalina, where the gland takes the form of a 

 very large tube bent on itself and extending along the greater part of the foot. 

 The gland either opens between the head and foot, as in Heli.r, or on the 

 postero-dorsal surface of the latter, as in Helicarion. — Ed.] 



