34 



The membrane which is commonly spoken of as ' the mantle,' and 

 which may be stripped from the shell by the use of sufficient force 

 to overcome its adhesions, must, I maintain, be considered as really 

 its inner layer only; for I find that an outer layer exists, so intimately 

 incorporated with the shell as not to be separable from it without the 

 removal of its calcareous component by maceration in dilute acid. 

 When thus detached, this outer layer is found to be continuous with 

 the membrane lining the perforations in the shell (fig. 1 b) ; so that 

 their tubular caeca are, in fact, prolongations of the real external 

 surface of the mantle. The adhesion of the inner to the outer 



Fig. 1. 



Diagram of the intra-palleal sinus-system of Terebratula, with its caecal prolonga- 

 tious into the shell ; A, B, section of valve ; a, inner layer of mantle, 6, outer 

 layer in contact with the shell, and giving off caeca ; c, continuity of the two 

 at margin of valve. 



layer (which Professor Owen, not being aware of the existence of an 

 outer layer, interpreted as an adhesion of the mantle to the shell) 

 does not extend to the whole of the contiguous surfaces, but is 

 limited to certain bands or spots, the two layers of membrane, in 

 the intervals between these, being separated by a set of irregular 

 spaces, freely communicating with one another, and with the cavities 

 of the caeca, so as to form a rude network. This arrangement is 

 peculiarly well marked in Terebratula caput-serpentis, as shown in the 

 figure (fig. 2) ; and to those who are familiar with the condition of the 

 circulating apparatus in the inferior Mollusca, it is scarcely possible 

 not to recognize in it a ' sinus-system,' corresponding to that which 

 is formed in the Tunicata by the partial adhesion of the second and 

 third tunics to each other. 



