THE GASTROPODA 93 



These organs also recur in Littorina. (3) An unpaired folded 

 dilatation near the middle of the oesophagus is found in various 

 carnivorous Taenioglossa, e.g. the Naticidae, Lamellariidae, and 

 Cypraeidae ; in the last named it is well developed and has 

 a lamellate internal wall. In the same position in the Cassididae 

 there is a pouch separated from the oesophagus and opening into 

 it by a slit. (4) In all the Taenioglossa, with the exception of 

 Cancellaria, the Harpidae, and some species of Terebra, an important 

 oesophageal gland, known as the "gland of Leiblein," opens into 

 the middle of the oesophagus. Slightly developed in the Olividae 

 and Fasciolariidae, this organ appears under diverse forms : it is 

 a thick glandular mass in Murex, a long caecum with thin walls 

 in Buccinum, and in Toxiglossa it forms the so-called " poison 

 gland," whose duct traverses the perioesophageal nerve-collar, 

 as in Valuta, and opens into the buccal cavity, giving the whole 

 structure the appearance of a third salivary gland. In Halia 

 and Marginella this organ forms a siphon opening into the oeso- 

 phagus by its two extremities. (5) In Nerita there is also an 

 unpaired oesophageal gland, and among the Opisthobranchs one 

 finds an azygous dorsal pouch in some Bullomorpha, an oesophageal 

 caecum in the Elysiomorpha, and a long glandular appendage in 

 the Lophocercidae. 



The terminal part of the oesophagus sometimes presents modifi- 

 cations Avhich produce an apparent modification of the configuration 

 of the stomach following immediately after. This terminal portion 

 is differentiated to form a gizzard with thick muscular walls and 

 furnished internally with masticatory teeth or plates. The last 

 named are variable in number, and may be simply chitinous 

 and pointed or calcified and flattened. This arrangement is found 

 in a large number of Opisthobranchs, viz. in the majority of Bullo- 

 morpha (Fig. 76, m.p), including the " Pteropoda Thecosomata" 

 (Fig. 60, m.p), in various Aplysiomorpha, and in certain Nudi- 

 branchia Tritonomorpha (Marionia, Scyllaea, Melibe). The muscular 

 girdle of this gizzard is also recognisable in a certain number of 

 basommatophorous Pulmonates, viz. Amphibola, Auricula, and in 

 Limnaea it is differentiated to form two globular and symmetrical 

 muscular projections. In consequence of the proximity of the 

 specialised portions of the terminal part of the oesophagus to the 

 stomach, the latter appears, in certain cases, to be divided into 

 several successive portions separated by constrictions, notably in 

 Aplysia, and also in Amphibola and Limnaea, in which the oesophagus 

 exhibits an ampulliform dilatation in front of the gizzard. 



The stomach proper consists of a simple enlargement of the 

 digestive canal, and its walls are normally and fairly consistently 

 thin, especially in the Streptoneura. The internal wall of the 

 stomach, however, may frequently be lined by a more or less thick 



