286 LECTURE XXI. 



any undulation of the water can have reached them. The labial 

 palpi seem well adapted, both bj' structure and position, to exercise 

 the sense of smell ; but of the existence of this sense, or of taste, we 

 have no proof. The mantle is highly susceptible of impressions by 

 contact. 



Between the freely open state of the mantle in the oyster and 

 similar monomyary bivalves (^Ostracea), and its condition in the 

 dimyary bivalve ( Venus^ selected for the demonstration of the general 

 organisation of the Lamellibranchiata, there are intermediate modifi- 

 cations. The common mussel is the type of a family (JMytilaceci) in 

 which the mantle is widely open anteriorly, the margins of the lobes 

 being united together posteriorly, except for a small space forming 

 an outlet for the excrements. In the Cliamacea the margins of the 

 pallial lobes coalesce, leaving a small anterior aperture for the foot, a 

 second smaller one for inhaling the respiratory and nutrient currents, 

 and a third posterior orifice for excretion. The family typified by 

 the Venus has the two latter orifices produced into a siphonic tube, and 

 the anterior or pedial aperture corresponds in width with the supe- 

 rior size of the foot. The modifications of the mantle are essentially 

 the same in the family called Inclusa ; but the narrower and longer 

 figure of the body occasions a greater proportion of the confluent 

 margins of the mantle between the anterior pedial and the posterior 

 siphonic apertures, whereby the mollusc, especially when the foot is 

 small, becomes inclosed in a membranous tube or sheath. The 

 most essential diff'erence is presented by the Pholadomya, which has, 

 besides the pedial and two siphonic apertures, a fourth orifice at the 

 under part of the base of the siphon, leading by a valvular protuberance 

 into the interior of the pallial cavity. This additional aperture co- 

 exists with a second small muscular process or foot, which is bifur- 

 cate at the extremity. 



The bivalve shell of the Palliohranchiata offers, as might be 

 expected, many modifications corresponding with those of the mantle. 

 The shell consists essentially of an organised extravascular com- 

 bination of gelatinous meitibrane and calcareous earth, chiefly car- 

 bonate of lime, arranged in successive layers, the innermost being 

 the largest and latest formed ; and each layer presenting a cellular 

 or fibrous texture, which presents characteristic variations in different 

 families.* The distinction between the internal or nacreous layers, 



* The microscopic structure of shells has formed the subject of the investigations 

 of a committee of the Microscopical Society of London, and of the independent and 

 original observations of Dr. Carpenter. The results of these inquiries will form a 

 valuable addition to the anatomical history of the Mollusca, and to the physiology 

 of the extravascular tissues in general. R. O. 



