THE ALIMENTARY CANAL OF STYLE‐BEARING PROSOBRANCHS: 5. On the Structure of the Alimentary Canal of Style‐bearing Prosobranchs
Authors: Graham Alastair 
Published: August 2010

In the primitive prosobranch mollusc the esophagus may be divided into three regions:‐

(1)Anteriora?sophagus.–lying immediately behind the point where the radula sac separates from the gut. Histologically it resembles the buccal cavity. On its roof runs a pair of folds enclosing a groove the ciliated dorsal food‐channel.

(2)Mid‐mophagus.‐Lying behind the anterior esophagus. bears lateral glandular pouches in which digestive enzymes are secreted, and has a continuation of the dorsal food‐channel on its roof in which food and mucus from the anterior esophagus is mixed with the secretion of the pouches and the mixture carried back.Thisregion is involved in the torsion of the visceral mass.

(3)Posteriorœsophagus.‐Lying behind the mid‐esophagus. glandular pouches; its wall bears numerous longitudinal folds, and it carries the mixture from the food‐channel to the stomach.

The structure of this region of the alimentary canal has been examined in eighteen genera of prosobranch gastropods in which a crystalline style indubitably occurs, and compared with the published descriptions of twenty other genera and with the thecasomatous Pteropods. In all cases (with the single exception ofAckorbisthe pouches have lost their original secreting epithelium, and have been reduced in size or have completely disappeared.

In the Rissoacea (text‐fig. 6), Strombacea, and Paludominae, the original croplike expansion of the gut persists; in more fully adapted molluscs‐Cerithiama (test‐fig. 7), Calyptraacea‐a11 trace of expansion has disappeared, and the anterior esophagus appears to pass directly into the posterior. InCrepeilulano torsion of the alimentary canal is visible.

The structure and the ciliary currents of the stomachs of seven animals described and shown to be very similar (text‐figs.6,6,9,10).In all provision made for (1) the mixing of food particles with the style substance; (2) the sorting of small particles which can be ingested by the cells of the digestive diverticula from larger ones which cannot; (3) the removal of the latter and waste from the digestive diverticula into the intestine;(4)the rotation and paasage into the stomach of the crystalline style. The reason for the loss of the cesophageal pouches is bound up with those characteristics of style‐bearing animals first expressed by Yonge(1930, 1932)that no free protease can exist in their gut, and(2)that they are continuous feeders. Prosobranchs which live in habitats where continuous feedii.; is possible can have a style; those which, like littoralorterrestrial forms, have rhythm in their feeding,orwhich, like carnivores, require a free protease in their gut, have cesophageal pouches.

The style appears to be found in only one archsogastropod and in only six superfamilies of mesogastropods, some of which are closely allied. This suggests that it has been evolved independently in the gastropods and the lamellibranchs, and that the ability to produce it is limited among the gastropods to a few stocks, probably inter‐related, within that group.

No style occura inDiodora, Theodom, Septaria, AmpullarimorPila, genera in which it was once thought to be found.Diodorafeeds on sponges.