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, e.g. Fulgur, Conus, many Terebra, Umbrella, several 

 Pulmonates, etc., the two salivary glands appear to be fused, but 

 retain their individuality. In some siphonate probosciferous 

 Taenioglossa, such as Dolium, 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 Stromhus, 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, Valuta, 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 (Dcnidopsis, 

 Phyllidiidae) the second pair is ventral and anterior, with a single 

 duct ; in PleurobrancJiaea 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, etc. (2) In the majority of the 

 Aspidobranchs there are, as in the Chitonidae, paired anterior 

 glandular oesophageal pouches, with papillated internal walls. 



