512 LECTURE XXI. 



and certain Pyrolidce : it consists of a small sacculus, with thick 

 transparent walls, attached to the anterior part of the pedial ganglion, 

 and containing a cretaceous nucleus of a crystalline structure, reso- 

 luble into four prismatic pieces, and performing remarkable oscil- 

 latory movements : this sac he regards, with much probability, as a 

 rudimental organ of hearing. It is shown ^ifig. 191, in Mytilus. 



The Pecten has a number of small sub-pedunculated ocelli arranged 

 around the inner margin of the mantle ; they were recognised as 

 rudimental organs of vision, by Poli*, who, thereupon, called the 

 Pecten Argus. But the name is equally merited by many other 

 bivalves. As many as sixty ocelli have been counted on the convex 

 side of the mantle, and ninety on the plane side, in Spondylus gceder- 

 opus. The Pinna has about forty brownish yellow tubercles, at the 

 anterior part of each lobe of the mantle, near the adductor muscle. 

 In the burrowing bivalves, with long siphons, the ocelli are aggregated, 

 as might be anticipated, on the tentacular orifices of the breathing- 

 tubes, usually the only parts of the animal which are exposed. In 

 the cockles (Cardium) each of the numerous protractile tentacles of 

 the short siphons bears an eye of sparkling brilliancy. The Anomia 

 has about forty pallial eyes hidden amongst the marginal tentacles. 

 In the oyster they are more numerous, of a yellowish brown colour, 

 and situated between every second tentacle along about one-third of 

 the mantle border.f 



In the Pecten and Spondylus, the retinal expansion of the branch 

 of the circumpallial ngrve incloses a vitreous body composed of non- 

 nucleated cells, in front of which is a flattened crystalline lens : the 

 pigmental coat consists at the back part of staff-shaped corpuscles, 

 and in front terminates by a circular pupil. In most other bivalves 

 the simple elements for exalting touch to a sense of light, viz., a 

 nerve-mass and pigment mass, are alone found, without any dioptric 

 adjuncts for the recognition of an image. Carlisle J first showed 

 that oysters were sensible of light ; having observed that they closed 

 their valves when the shadow of an approaching boat was thrown 

 forward, so as to cover them before any undulation of the water 

 could have reached them. 



The labial palps seem well adapted, both by structure and position, 

 to exercise the sense of smell ; but of the existence of this sense, or 

 of taste, in the acephalous moUusks, we have no proof. In Nucula 

 the palps are rigid. The mantle is highly susceptible of impressions 

 by contact at its free border ; and the soft, often highly coloured, 

 ciliated processes from that and other parts of the unattached surface 



♦ CCCXII ii. p. 153. t. xxii , fig. 1, 4. 



f XXIV. p. 2C1. cccxvi. X cccxi. 



