6 0/7 vascular and exira-vascular Sulstances. 



borescent pores which are common to all vascular parts, but 

 microscopicaily examined they resemble the exuvial or epider- 

 moid membranes. To tliese may be added, the notorious cir- 

 cumstance of the unchangeableness of the outer surfaces of tes- 

 taceous shells during their growth, and the continued renewal of 

 their other surfaces which admit of contact with the living in- 

 habitant; next, the stains and coloured transudations which 

 they often derive from metallic salts, and other colouring ma- 

 terials placed in their vicinity ; and lastly, that such occurrences 

 do not affect the living animal. The mechanical connexion or 

 contact that subsists between the living animals which occupy 

 the testaceous shells and their extraneous dwellings is in many 

 instances very slender. The common ovster possesses its first 

 pair of valves, consisting of single laminae, before it quits the pa- 

 rental organs ; a muscle passes between the centres of the con- 

 cavity of eacii shell adhering to each, and it acts upon the valves 

 nearly at right angles. The animal has no other continuity with 

 the shell : at the hinge an elastic substance is wedged in, the 

 spring of u'liich is excited by compression ; but it does not possess 

 the property of extension beyond the passive state : when dried 

 this substance cracks into cubes. As the animal grows it aug- 

 ments the margin of its shells, and tliickens them by adding new 

 laminas on their insides ; the muscular adhesion glides forward, 

 still keeping the centre of the valves. The elastic substance at 

 the hinge is augmented along the inner surfaces only, and must 

 have been always deposited during the expanded state of the 

 valves, since the limits of its elastic condition is exactly adapted 

 to that state. As the laminoe of the shells increase, there is a 

 gap at the outside of the binge, filled with soft crumbling and 

 decomposing worn-out elastic ligament. This gap presents two 

 inclined planes which meet in an acute angle ; and that space is 

 kept free from pebbles and hard extraneous bodies by the pre- 

 sence of the decomposing ligament, as such an accident would 

 prove fatal, by preventing the opening of the valves. The growth 

 of all testaceous shells affords remarkable proofs of their extra- 

 vascular formation. The muscular adhesions aie generally the 

 only points of continuity between the animal and its shell, and 

 these are constantly changing with the augmentations of jjulk. 

 In all the conoid univalves which revolve upon spiral axes, the 

 successive parts of the shell are merely spread upon the older 

 parts without any intermixture of their substances, and the epi- 

 dermoid or extraneous bodies arealike involved in the successive 

 folds. In the other classes of animals similar phoenomena oc- 

 cur. The calcareoTis shells of birds' eggs are merely deposited 

 ujjon the membrana putaminis, and the inner portions are regu- 

 lai ly crystallized prisms, the long diameters of which point to 



the 



