Zoological Society. 137 



like, horny tongue, and at its base are two simple salivary follicles. 

 The oesophagus expands into a small stomach, imbedded in a follicu- 

 lar liver. The intestinal canal is more complicated than in CalyptrcBa 

 or Calypeopsis ; it bends towards the left side, and there forms a 

 small mass of double spiral coils, live or six in number, from which 

 the rectum is continued along the floor of the branchial chamber, in 

 the interspace of the gills, to the outlet of that chamber on the right 

 side of the neck. 



/ '* The nervous system is chiefly distinguished from that of the Ca- 

 lypeopsis by the larger relative size and closer approximation of the 

 supra-oesophageal ganglions, which here equal the inferior masses. 

 Besides the chords connecting the upper with the lower ganglions, 

 the upper ganglions give oiF each three nerves : the largest runs 

 forward in a zigzag course to the clavate mouth ; the second supplies 

 the substance of the tentacle ; the third, a slender nerve, goes to the 

 minute eye- speck on the outer side of the base of the tentacle. The 

 wavy disposition of these nerves, especially of the rostral pair, clearly 

 indicates a provision for considerable elongation of the parts which 

 they supply. 



^'" Thus the genus Lithedaphus diff'ers from the other known forms 

 of the CalyptrceidcB in the smaller development of its locomotive and 

 respiratory organs, and in the greater development of the organs for 

 the prehension and assimilation of food. 



'^Probably no oyster, cemented to its native rock, is more fettered 

 in its movements than this highly developed gastropod, to which, 

 however, a voluntary detachment of the foot from the gastric plate 

 may be possible. M. Dufo however testifies that the only move- 

 ment he was able to recognise in his Calyptraa Roissii was an ele- 

 vation of the anterior part of the shell, and a corresponding separa- 

 tion of it from the supporting plate beneath. 



'* The circumstances under which Mr. Cuming discovered his spe- 

 cimens would hardly be consistent with a greater extent of motion. 

 The foot, therefore, whose normal functions as an instrument for 

 traversing space must be restricted to the early age of the Litheda- 

 phus, may well ofl'er diminished proportions when the animal has 

 chosen a site for the deposition of its ventral plate and has taken up 

 a fixed abode. Muscular action being thenceforward much restricted, 

 the necessity for extensive respiration is in the same degree abolished. 

 The compensation for this abrogation of the power of moving about 

 in quest of foodjas obviously the great development of the proboscidi- 

 form head, wmcji, when outstretched in the living mollusk, must 

 appear like some worm moving to and fro from between the valves 

 of the shell. The tactile organs of sense are co-extended with the 

 prehensile organ ; but the eyes, so useful to the young wandering 

 mollusk, have much shrunk in the sedentary aged ; and the complete 

 elaboration of whatever nutriment may be introduced into the system 

 has been provided for by the long and convoluted alimentary canal. 



" These facts in the anatomy of the Lithedaphus, and their harmo- 

 nious adjustment to its peculiar condition as a sessile gastropod in- 

 closed in a bivalve shell, leave scarcely any doubt as to this state. 



