334 CONTRIBUTIONS TO THE ANATOMY OF THE BRACHIOPODA 



The facts, then, with regard to the real or supposed circulatory 

 organs of the TerebraUdidcE, are simply these : — 



1. There are two or four organs (hearts), composed each of a free 

 funnel-shaped portion with plaited walls, opening widely into the 

 visceral cavity at one end, and at the other connected by a constricted 

 neck, with narrower, oval or bent, flattened cavities, engaged in the 

 substance of the parietes. The existence of muscular fibres in either 

 of these is very doubtful. It is certain that no arteries are derived 

 from the apex of the so-called ventricle, but whether this naturally 

 opens externally or not is a point yet to be decided. 



2. There is a system of ramified peripheral vessels. 



3. There are one or more pyriform vesicles. 



4. There are the large ' sinuses' of the mantle, and the 'visceral 

 cavity ' into which they open. 



To determine in what way these parts are connected and what 

 functions should be ascribed to each, it appears to me that much 

 further research is required. 



Nervous System of Terebratitlidce. — Professor Owen describes and 

 figures the central part of this system as a ring surrounding the oral- 

 aperture, its inferior portion being constituted by a mere commis- 

 sural band. 



M. Gratiolet, however, states with justice that the inferior side of 

 this collar is the thicker, and I find both in Rhynchonella and in 

 Waldheimia that it constitutes, in fact, a distinct oblong ganglion,, 

 of a brownish colour by reflected light. F'rom its extremities com- 

 missural branches pass round the mouth, while other cords are 

 distributed to the arms, to the superior and inferior pallial lobes,, 

 and to the so-called hearts. The nerves are marked by fine and 

 distinct longitudinal striations, and can be traced to the margins of 

 the pallial lobes, where they become lost among the muscular fibres, 

 of the free edges of the mantle. 



Structure of the Arms. — I have not been able to convince myself 

 of the existence of that spiral arrangement of the muscular fibres of 

 the arms which has been described in Rhynchonella and Waldheimia. 

 I have found the wall of the hollow cylinder of the arm to be con- 

 stituted (i) externally, by an epithelium, within which lie (2) the 

 reticulated canals, which have been already described ; (3) by a deli- 

 cate layer of longitudinal or more oblique and transverse fibres,, 

 which are probably muscular, and (4) internally by a granular 

 epithelial layer. 



In Rhynchonella the bases of the arms are terminated by two con- 

 siderable sacs, which project upwards into the visceral cavity. Have- 



