SENSES — SIGHT. 



187 



direction, and situated at the entry of the mouth, between 

 the internal labial processes. These laminae are twenty 

 in number, and are from one to two lines in breadth, and 

 from four to five in length, but diminish in this respect 

 towards the sides. They are supplied by nerves from the 

 small ganglions which are connected to the ventral extremi- 

 ties of the anterior sub-cesophageal ganglions, "-f- 



IV. SIGHT. 



That the tunicated and bivalved mollusca are destitute 

 of eyes has been long an axiom with conchologists, J but 

 recently some exceptions, of a somewhat doubtful character 

 it must be confessed, have been pointed out. Placed be- 

 tween the oral tentacula, as also between the filaments of 

 the vent, of certain Ascidia, there is a series of eight and 

 six little scarlet globular points, which are so like the organs 

 that Ehrenberg affirms to be eyes in the medusae and star- 

 fish, that it is impossible to doubt the sameness of their 

 functions. So also in a few bivalves, of which the Pectens 

 and Sphondyli are the most eminent examples, there are 

 many green metallic lustrous beads placed, at stated inter- 

 vals, on the margins of the cloak among the tentacular 

 filaments (Fig. 33). The use of these beautiful organs re- 



Fig. 33 



mained unguessed, until Poli gave it as his opinion that 

 they were subservient to vision ; whence he named the ani- 

 mal of the Pecten, after Juno's watchman, the Argus, to 

 whose mantle you may suppose the hundred eyes of the 

 fabled son of Aristor had been transferred. How far Poli's 



* Ann. and Mag. N. Hist. ser. 2. iii. 194. 



t Memoir, p. 41. 



X " No organs having the most distant relation to the sense of vision, have 

 ever been observed in any of the acephalous or bivalve mollusca." — Roget's 

 Bridgew. Treut. ii. 481. 



