Royal Society. 543 



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 csecal structure (as was 

 first suggested to me by my friend Mr. T. H. Huxley) bears ar 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-system of Tere- 

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

 csecal 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 

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

 always caecal, notwithstanding that they are sometimes distributed 

 as minutely as blood-vessels*. 



Fig. 2. 



II 



Sinus-system of Terehratula 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 caeca, 

 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. 



ITie sinus-system from which this collection of caeca proceeds, 



* Ann. des Sci. Nat., 3« ser., Zool., tom. xviii. p. 307. 



