VII MULLUSGA — THE SHELL 57 



whole race. The following are a few cases in which the utility of 

 the reduction of the shell in adaptation to special conditions is to 

 some extent evident: (1) In free-swimming marine forms, where the 

 shell is too heavy and increases friction ; (2) in Tcdacella and allied 

 forms, which prey upon earthworms, where a large shell would prevent 

 them from following their prey into narrow holes and passages ; 

 (3) in Gastropods, which browse among thick tangles of Corals, 

 Bryozoa, Hydroida, or Algte (e.g. many Nudibranchia). 



The loss of the shell is generally followed by compensatory 

 adaptations for protection, such as great capacity for regeneration, 

 especially of the easily detachable appendages, voluntary amputation of 

 portions of the body, stinging cells, and colouring which may be 

 protective in various ways. 



The carnivorous Cejihulopods are protected (1) by their extraordinary 

 swimming powers, which are in keeping with their highly developed 

 organisation; (2) by their well-developed sight ; (3) by great muscular 

 strength ; (4) by strong jaws ; (5) by the discharge of the secretion 

 of the ink-bag ; (6) by their partly mimetic changes of colour, etc. 



Certain peculiarities of organisation, which can only be under- 

 stood as remains of a shelled condition (e.g. the lateral position of the 

 genital and renal apertures and also to some extent of the anus in the 

 Nudibranchia), always persist after the shell has disappeared. 



Chemical Composition of the Shell. 



The shell of the MoUusca consists principally of carbonate of lime, with traces 

 of phosphate of lime and of an organic substance related to chitin, — conohyolin. 

 Besides tliese, various colouring matters may occur. 



Structure of the Shell. 



The shell of the Lamellihranchia consists of three layers, the innermost layer 

 being applied to the surface of the mantle. The shell is to be regarded as a 

 cuticular structure. 



The outer layer (shell-integument, epidermis, cuticle, periostracum) is, so far as 

 its physical constitution is concerned, horny and wanting in lime. It generally 

 disappears off the older portions of the shell. 



The middle layer (columnar, prismatic, or porcelanous layer) consists of slender 

 prisms of carbonate of lime, usually perpendicular to the surface of the shell and 

 closely crowded together. 



The inner (nacreous) layer has a finely lamellated structure. The very delicate 

 transparent laminse of which it is composed are thrown into slight waves ; these 

 cause the wavy lines on that surface of the shell which lies on the mantle, which, 

 by interference, produce the characteristic nacreous lustre. The pearls of the pearl 

 oyster are formed of the same substance as this layer. 



The constitution of these three layers varies greatly in details both in the Lamclli- 

 braiich.ia and in other MoUusca. The outer and middle layers are formed at the free 

 margin of the mantle, the inner layer is yielded by the epithelium of its whole outer 

 surface. 



The shell in the Gastropoda and Geplialopoda consists principally of the middle 



