ALIMENTARY SYSTEM — SALIVARY GLANDS. 
275 
iu being thrown into a series of distensible longitndinal folds, visible 
through the thin walls and giving the organ a striated appearance. 
In Testacella and probably other species, the crop may acquire 
stronger and more muscular walls and assume the functions of the 
true stomach by canying on the processes of active digestion therein. 
The Gizzards or organs for the mechanical gi’inding of food are 
present in some species and placed on the foregnt between and 
partially beneath the crop and the stomach. In Limncva they con- 
sist of two globular and laterally paired piirple-brown muscnlar 
pouches formed by an excessive local development of the nmsciilar 
investment of the oesophageal tube, and ai’e usually partially filled 
with sand or gravel to assist in the crushing and trituration of the 
food before undergoing the process of digestion within the true 
stomach, as it is only posterior to such muscular enlargements that 
the true digestive ferments are encountered. 
The four cavities, the crop, the paired gizzards and the stomach, 
thus concerned in the storage, tritui’ation and digestion of food really 
form only a single chamber with four recesses, and bears a general 
external resemblance to the Quatrefoil of Gothic architecture or may 
be likened to the nave and transept of a cathedral, the lateral gizzards 
representing the transepts and the crop and stomach the nave of 
the building. 
The Salivary Glands (saliva, spittle), whose presence in the mol- 
Insca is apparently correlated with the 
development of a pharynx, and which do 
not therefore exist in the Pelecypoda, except 
perhaps in such archaic genera as Nuciila, 
etc., in which there are paired glandular 
pouches opening into a vestigial buccal 
cavity, which may represent the early form 
of these organs, that of a pair of simple 
tubules, with the secretory part located at 
the distal end, as in Acta’on and other 
archaic Gastropods. The organs of Semper, 
which I have regarded as more especially 
gustatory in function, are considered by 
many to be homologous with and to repre- 
sent the Buccal or Anterior Salivary Glands present in the Cephalo- 
poda, Amphineura and other gxoups. 
Fig. 546.— Salivary glands of 
Ileli.x hortciisis Mull., x 3, 
showing the character and mode 
of apposition to oesophagus. 
b.b. buccal bulb ; ce, oeso- 
phagus ; s.g. salivary glands 
with their ducts, s.d. 
