Sf)4 ORGANS OF DIGESTION 



without prehensile or masticating organs, they depend on 

 the respiratory currents for their supply of food. The 

 two respiratory apertures reach to a variable extent from 

 the opening of the valves according to the habits of the 

 species, and are generally provided with tentacular filaments, 

 and sometimes with minute organs of vision, as in some of 

 the tunicata. The buccal orifice of the alimentary canal is 

 situate at the bottom of a large respiratory cavity, in which 

 the branchial folds are suspended, and is provided with two 

 pairs of long lateral lamelliform tentacula, which are exten- 

 sions of the upper and lower lips. The mouth is unprovided 

 with mandibles, or maxillae, or any form of solid dental 

 apparatus, and opens by a short and wide oesophagus into a 

 capacious gastric cavity, perforated, as in the tunicata, with 

 the numerous openings of the biliary ducts. The stomach is 

 generally a soft membranous or muscular cavity, destitute of 

 teeth, of an elongated form, and surrounded by the lobes of 

 a large and conglomerate liver. On opening the stomach, 

 several perforations are seen near its pyloric extremity, which 

 lead by short wide ducts to the ramifications and follicles 

 composing all the lobules of the liver, the liver consisting 

 here, as in other molluscous classes, of an aggregate of 

 minute follicles or coeca, the ducts of which unite into larger 

 trunks and terminate in the pyloric portion of the stomach. 

 The intestine is generally long and wide, corresponding to the 

 simple and inferior nature of the food brought to these 

 animals in small parts by the respiratory currents. On leaving 

 the stomach, the intestine forms a few convolutions in the 

 cavity of the abdomen, closely surrounded by the lobes of the 

 liver, then proceeding along the convex dorsal part above the 

 ovary and the thoracic cavity, it commonly perforates the 

 cavity of the muscular ventricle, and terminates near the vent 

 for the exit of the respiratory currents, as in the naked 

 acephala. 



The mouth in the common oyster ostrea edudis (Fig. 120. 

 A. b.) is concealed, as usual, at the back part of the cavity 

 of the mantle, near the hinge of the valves, and is furnished 

 on each side with two long tapering fleshy tentacular folds 

 (A. a.) which are striated like gills on their inner surface and 

 smooth on their exterior. The buccal aperture leads almost 



