208 



Royal Society. 



always csecal, notwithstanding that tt/ey are sometimes distributed 

 as minutely as blood-vessels*. 



Ffe. 2. 







l.. v., 



Sinus-system of Terebralula caput-serpentis (as shown by the grinding away of 

 the shell, without detaching the mantle), being a network of canals formed by 

 the adhesion of the two layers of the mantle at certain spots, leaving passages 

 around them. 



On this interpretation, the cells which are found within the ca>ca, 

 and in the spaces between the contiguous surfaces of the two layers 

 of the mantle, are to be regarded as blood- corpuscles, and they cor- 

 respond in size and appearance (so far as can be determined by spe- 

 cimens preserved in spirits) with the blood-corpuscles of Ascidian 

 and Lamellibranchiate Mollusks. 



The sinus-system from which this collection of ca?ca proceeds, 

 appears to be altogether distinct from the vascular apparatus of the 

 (so-called) ' mantle,' (that is, according to my interpretation, of the 

 inner layer of the mantle) which has been described by Professor 

 Owen ; but it probably communicates with the ' common sinus ' at 

 the back part of the visceral chamber, which is stated by Professor 

 Owen to receive the blood, not only from the palleal sinuses of the 

 dorsal and ventral valves, but also from " other sinuses that there 

 fill, line, and seem to form, the visceral or peritoneal cavity f." 



It cannot be deemed improbable, then, that the apparatus in ques- 

 tion is branchial in its nature ; and that it is designed to provide for 

 certain tribes a more special means of aerating the blood, than is 

 afforded by that distribution of blood to the general surface of the 

 mantle, which is common to the entire group. This view of its re- 

 spiratory office is confirmed by an observation communicated to me 

 by Professor Quekett ; viz. that the discoidal opercula which cover 

 the external orifices of the caeca, and which, though adherent to the 

 periostracum, are not structurally continuous with it, present ap- 

 pearances in young shells, which seem indicative of the existence of 



* Ann. des Sci. Nat., 3 e ser., Zool., torn, xviii. p. 307. 



f See Mr. Davidson's Monograph on the " British Fossil Brachiopoda," 

 published by the Palreontographical Society, vol. i. p. 15. 



