434 MOLLUSCA phylum vi 



pellicle secreted by the original shell gland ; it is therefore neither external 

 nor internal. With its subsequent dift'erentiation, and the thickening of the 

 valves by calcification deposited about it, it occupies a depression in the 

 cardinal margin which Bernard has regarded as internal. In a sense it is 

 internal, but its position at this stage is not significant, and there is no 

 fundamental difference between the cases. The differentiation in function 

 and structure which we find in the adult between the ligament, properly 

 so-called, and the " internal ligament " or resilium, is a later development. 



The ligament may be regarded as a 'fundamental character of Pelecypods, 

 and is universally present, though in some cases as a mere degraded rudiment 

 (Pholadacea) ; it may be separated from the valves and functionless (Chlamydo- 

 concha), or present only in the young stages and lost through specialisation 

 due to the sessile habit (Rudistids). 



As the most important factor in the mechanism of the valves, the liga- 

 ment has undoubtedly developed with the evolution of the class, and its 

 chief modifications date from the earliest period in the life-history of the 

 group. The function of the original ligament was that of an external link 

 between the valves having the essential nature of a C-spring. That is, the 

 insertion of the ligament edges on the cardinal margins, or, at a later period, 

 on thickened ridges or nymphae by which these margins are reinforced to bear 

 strains, resulted in the following conditions : — The valves being held together 

 and, in closing, approximated by the contraction of the adductor muscles, the 

 preservation of their precise apposition, marginally, is due to a rotary motion, 

 exerted along the axis of the ligament, which pulls the attached edges of the 

 ligament nearer to each other and exerts a strain on its cylindrical exterior. 

 This operation, with a thin ligament, involves a tensile strain on the Avhole 

 cylinder ; with a thick ligament the external layers are strained and the 

 internal layers compressed, so that to the tensile elasticity of the external 

 layers is added the compressional elasticity of the internal portion. The 

 result of the differing strains to which the several layers of the ligament are 

 subjected brings about a difference of structure, and, whenever the ligament 

 becomes deep-seated, there is a tendency for the respective parts to separate 

 along the line where the two sets of strains approximate. We then have two 

 elastic bodies, operating reciprocally in opposite directions, the outer or liga- 

 ment proper tending to pull the valves open to a certain distance correspond- 

 ing to its range of tensional elasticity ; and the other or resilium (for which 

 the objectionable terms "cartilage" and "internal ligament" have been used) 

 tending to push them open to an extent corresponding to its range of expansion. 



The ligament proper is of a more or less corneous nature, tough and semi- 

 translucent beneath its external surface. When dry it has a vitreous fracture, 

 and often shows hardly any fibrous texture. 



The resilium is distinctly lamellar or composed of horny fibres, which are 

 apt to give a pearly sheen to its broken surface. There is often a more or 

 less extensive intermixture of lime in its substance, which may be diffused, or 

 may be especially concentrated along the median plane. As may be seen by 

 examining the unbroken resilium (as in Madra), this organ in such cases has 

 something of an hour-glass shape ; the ends which fit into the " cartilage pits " 

 or resiliifers being more expanded than the centre between them. The deposit 

 of lime in the form of an accessory shelly piece, usually termed the ossiculum 

 or lithodesma, serves for the reinforcement of the resilium. 



