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 7erebra, 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 Murer, 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 Voluta, 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 which 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 Aply siomorpka, 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 
