Royal Society. 



207 



the membrane lining the perforations in the shell (fig. lb); 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- 

 . tions into the shell ; — A, B, section of valve ; a, inner layer of mantle, b, 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. 



Considered under this point of view, the caecal structure (as was 

 first suggested to me by my friend Mr. T. H. Huxley) bears a close 

 resemblance to the vascular prolongations, which, in many Asci- 

 dians, pass from the sinus-system into the substance of the ' test ;' 

 the chief difference lying in this, — that whilst each of the vascular 

 prolongations into the ' test ' of the Ascidians contains both an affe- 

 rent and an efferent canal, — no such distinction ordinarily manifests 

 itself in these prolongations of the intra-palleal sinus-systern of Tere- 

 bratula, although I have met with indications of it in Crania. Their 

 caecal character, however, is by no means opposed to the views I 

 am now giving of their physiological nature ; for it has been shown 

 by M. de Quatrefages, that the prolongations of the 'general cavity 

 of the body,' which pass into the branchiae and other appendages of 

 Annelida, transmitting to them its nutritive fluid for aeration, are 



