188 



COMPARATIVE ANATOMY 



CHAP. VII 



Similar glands, which communicate with the anterior part of the oesophagus, are 

 found in the Rhipidoglossa (e.g. Haliotis, Fissurella, Turbo). The glandular epithe- 

 lium in these also projects in the form of villi or folds into the lumen. 



The so-called crop of the Docoglossa (Patella) no doubt corresponds with the two 

 lateral cesophageal sacs in the Chitonidce and Rhipidoglossa. This is a saccular 

 widening of the oesophagus (Fig. 158, m), which, on account of the constitution of 

 its walls, has been compared with the psalterium of a Ruminant. A similar widen- 

 ing of the oesophagus is found in Cyprceidoe and Naticidce, which must be counted 

 among the most primitive of the Monotocardia. 



In those Monotocardia which are provided with a proboscis, the length of the 

 thin oesophagus is in proportion to that of the proboscis. 



The mouth lies at the tip of the proboscis, then follows a short and often rudi- 

 mentary pharynx, and then the long oesophagus, which runs through the whole 

 length of the non-protrusible portion of the proboscis, passes through the cesophageal 



d 6, 



dm 



til 



FIG. 158. Median longitudinal section through Patella (after Ray Lankester). brv, Efferent 

 branchial vessel ; bra, afferent ditto ; asd, duct of salivary gland sd ; go, anus ; no, right nephridial 

 aperture ; sd, salivary gland ; cor, heart ; pe, pericardium ; np, kidney ; d, intestine ; Tip, hepatic 

 gland (liver) ; v, blood vessel ; m (to the right), border of mantle covering the gills ; r, radular 

 sheath ; g, gonads ; m, crop ; ph, pharynx ; rd, radula ; odm, masses of muscle and cartilage of the 

 lingual apparatus ; o, mouth ; k, head or snout. 



ring, and may be even further prolonged posteriorly. When the proboscis is re- 

 tracted, the posterior portion of the oesophagus becomes coiled ; when the proboscis 

 is extended, it lies in the protruded or evaginated basal portion. 



Not infrequently in carnivorous Monotocardia there is a glandular widening in 

 that section of the oesophagus which follows the long proboscidal portion. The 

 oesophagus is most complicated in the Rachiglossa and many Toxoglossa, where this 

 widening, in the form of a large compact accessory gland, can become separated from 

 the intestine (Leiblein's gland, poison gland), and where other glands and widen- 

 ings may occur (Fig. 159). It seems probable that in certain Prosobranchia diges- 

 tion and resorption takes place even in the fore-gut. 



In the Pulmonata and Opisthobranchia, there is sometimes a widening (crop, fore- 

 stomach) anteriorly to the stomach, and in the same way the short oesophagus of 

 the Scaphopoda has a glandular widening, or two lateral glandular diverticula. 



Among the Cephalopoda, the Decapoda have a simple thin tubular oesophagus ; 



