92 THE GASTROPODA 
calcareous spicules of the animals taken as food. In the aspido- 
branchiate Streptoneura and in many other Gastropods the salivary 
glands are racemose in structure, but in more specialised genera 
they have the form of more or less elongated tubes (Janthina) or of 
sacs (Dolium). In the Aspidobranchs, Ampullaria, and the Actaeo- 
nidae (Fig. 57) the salivary ducts are very short and open behind 
the perioesophageal nerve-collar, but the glands traverse the nerve- 
collar, and their ducts are long and open in front of it in the 
majority of Gastropods. This is the case in all the Euthyneura— 
the salivary glands being situated very far back in the Pleuro- 
brancheae—and in the Taenioglossa, with the exception of Natica, 
certain species of Calyptraea, etc., in which the ducts are too short 
to traverse the nerve-collar. Finally, in the Stenoglossa and 
Heteropoda the salivary glands open in front of, but do not 
traverse the perioesophageal nerve-collar, their ducts, if they are 
sufficiently long to reach it, passing outside the structure. In 
certain forms, ¢g. Mulgur, Conus, many Terebra, Umbrella, several 
Pulmonates, ete., the two salivary glands appear to be fused, but 
retain their individuality. In some siphonate probosciferous 
Taenioglossa, such as Doliwm, Cassis, Triton, Voluta, and also in 
Pleurobranchaea, the salivary ducts bear a dilatation near their 
extremities. The two glands exhibit a certain degree of asymmetry 
in Strombus, Xenophorus, and some species of Atlanta. In several 
cases there is more than one pair of salivary glands ; the Docoglossa 
possess two pairs, with distinct and separate ducts. In Janthina 
and Scalaria there are two pairs of glands, lying close together and 
appearing to be formed by the bifurcation of a single pair. There 
are also two separate pairs—the second pair being ventral and 
anterior to the normal pair—in various Rachiglossa, Purpura, 
Trophon, Voluta, Cancellariidae, and Haliidae; with the exception 
of the Muricidae this second pair is anterior to the perioesophageal 
nerve-collar, and its ducts are often fused in the median line. 
Many probosciferous Opisthobranchs also have more than two 
salivary glands: in the porostomatous Doridomorpha (Doridopsis, 
Phyllidiidae) the second pair is ventral and anterior, with a single 
duct ; in Pleurobranchaca and Pleurobranchus there is a third dorsal 
and median gland. 
The buccal cavity is followed by an oesophagus, with plicated 
walls. This oesophagus is generally long, and often presents dila- 
tations on its course, which may be described under one or any 
other of the following headings :—(1) A sort of simple pouch with 
thin walls, as in the Heteropods (Figs. 141 and 142, m) and certain 
Opisthobranchs and Pulmonates, or sometimes a muscular swelling, 
as in Murex, Amphibola, Doris, ete. (2) In the majority of the 
Aspidobranchs there are, as in the Chitonidae, paired anterior 
glandular oesophageal pouches, with papillated internal walls. 
