, 1809.] On the genus Onchidium. !)J 



Digestive organs and their appendages. 



The food first enters through the mouth which, as already stated, 

 is surrounded by thickened, soft and grooved lips, with the oesophagus 

 (oe), a large muscular sack of an oval shape, closed posteriorly. 

 This sack encloses two cartilaginous plates, which are situated in a 

 strongly muscular mass, attached to the posterior and inferior sides of 

 the oesophagus. Sometimes, as in this particular species, these plates 

 resemble a bivalve shell, being convex externally and concave inter- 

 nally ; they are white, connected by a membrane below and open 

 above. Their microspcopical structure distinctly shews the formation 

 of a cartilaginous tissue, many of the cells being of irregular shape, 

 others granular and hardened. Externally they are covered by the 

 tongue membrane, or radula, which is provided in its entire extent 

 with very numerous teeth. 



This radula is thus very differently formed from the narrow and long 

 lingual ribbon of the Prosobranchia. Fig. 4 on plate xiv, represents 

 the relative position of these organs. The cartilaginous plates (cp) ac- 

 tually only give support to the radula (ra), which is by the muscular 

 action of the former pushed out of the mouth, scraping the organic 

 substance in the usual way from below upwards; the food then 

 passes in the cavity behind the plates where the salivary glands 

 (sg) enter. At the beginning of the alimentary canal, immediately 

 behind the catilaginous plates, there is a small fleshy tubercle (to) 

 which appears to act as a tongue, pressing the food down the canal every 

 time that the oesophagus contracts. Each of the salivary glands (sg) 

 is represented by a small, whitish, dendritic organ, connected with 

 each other by a thin string, and by numerous threads with the 

 hepatic mass, enveloping the anterior part of the intestines. The 

 alimentary canal issues at the upper part of the oesophagus, lying 

 in a special muscular cavity of the tissue of the body, it bends down- 

 wards, then passes through the hole of the principal central ganglion 

 ring (ng) to the stomach. This consists of two, almost quite se- 

 parate divisions. The first portion (pst) has the form of a double com', 

 pointed on either end and widened in the middle ; it is soft and com 

 posed of numerous folds or partitions. On this anterior portion follows a 

 second one, which is more elongated, consisting of three sub-divisions, 

 being in the middle surrounded and partially divided by a very strong 

 muscular tissue (mst). The extreme end (m) is capped by a separate 



