36 



The sinus-system from which this collection of caeca 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*." 



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 

 a fringe of cilia round each, designed to produce currents of water 

 over the extremities of the caeca. 



The resemblance which these caecal prolongations of the sinus- 

 system into the shell of the Terebratula bear to the vascular pro- 

 longations of the sinus-system into the test of certain Ascidians, is 

 not without its parallel in another group, which (as pointed out by 

 Mr. Hancock, Ann. of Nat. Hist. vol. v. p. 198) is intimately re- 

 lated to that of Brachiopoda, namely, the Bryozoa. The stony 

 walls of the ' cells ' which invest the soft bodies of many'species of 

 Eschara, Lepralia, &c., are marked, like the shells of Terebratulae, 

 with punctations, which are really the orifices of short passages ex- 

 tending into them from their internal cavity, as sections of these 

 structures demonstrate. These passages I have found to be occupied 

 by prolongations of the visceral sac, which is the only representative 

 of a circulating system among these animals ; and they thus convey 

 the nutrient fluid which this contains, into the substance of the 

 framework formed by the calcified tunics of these animals. 



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

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



