VII MOLLUSGA—THE ALIMENTARY CANAL 187 



lingual gland, opening into that part of the pharyngeal cavity which 

 lies between the tongue and jaws. 



The Lamellibranchia, as already mentioned, have neither pharynx, 

 jaws, tongue, nor salivary glands. In the Nuculidce, however, which 

 are rightly considered to be primitive forms, the mouth leads into a 

 widening of the intestine, on each side of which a glandular pouch 

 opens. These pouches perhaps correspond with the oesophageal 

 pouches of the Chitonidce and lihipidoglossa, which will be described 

 later. 



Natica, which bores through the shells of living Lamellibranchs and 

 feeds on their bodies, has a sucker-like organ on its proboscis (Fig. 98, 

 p. 107). The epithelium of the concave side of this organ, which is 

 applied to the shell attacked, forms a gland for secreting acid — prob- 

 ably sulphuric acid — which serves for dissolving the carbonate of 

 lime of the bivalve shell, which is then at once thrown out in the form 

 of powdered sulphate of lime. 



C. The (Esophagus. 



That portion of the intestine which lies between the pharynx (or 

 the mouth in Lamellibranchs) and the stomach is called the oesophagus, 

 the stomach being here used as the name of that widening of the 

 intestine into which the gland of the mid-gut opens. It is always 

 easy to detect the anterior boundary of the oesophagus. In Lamelli- 

 branchs it lies at the mouth, but in the Glossophora at the posterior and 

 upper end of the pharynx. The posterior boundary, however, can 

 often only arbitrarily be defined, as the oesophagus, which is usually 

 narrow and tubular, often widens very gradually into the stomach, 

 the structure of its walls at the same time gradually changing. In 

 other cases, widenings of the alimentary canal occur before the 

 stomach, and it is difficult to decide whether these are anterior 

 divisions of the stomach or posterior widenings of the oesophagus. 



In Lamellibratichia, terrestrial Pidmonata, most Opisthobranchia, and 

 the Cephalopoda Decapoda the oesophagus is a simple ciliated tube 

 running to the stomach, being often provided with longitudinal folds, 

 and therefore extensible ; in other divisions, however, complications 

 occur, which are caused by glandular outgrowths or muscular en- 

 largements. 



In a few Solcnogastres {e.g. Proneomcnia), on the boundary between the short 

 tt-sophagus and the mid-gut, a more or less long blind diverticulum occurs ; this is 

 single, and runs forward dorsally to the pharynx, and may extend over the cerebral 

 ganglion to the end of the head. 



In Chiton there are two lateral glandular sacs (sugar glands) connected with the 

 short oesophagus ; their inner glandular walls project into the lumen in the form of 

 villi, and their secretion changes boiled starch into sugar. 



