115 



spending fibrous layers, and between them of a reticulated tissue, 

 which makes up the principal thickness of the layer, and in which 

 the nerves and great sinuses are imbedded. 



The trabeculse of this reticulated tissue contain granules and cell- 

 like bodies, and I imagined them at first to represent a fibro-cellular 

 network, the interspaces of which I conceived were very probably 

 sinuses. Sheaths of this tissue were particularly conspicuous along 

 the nerves. On examining the arms, however, I found that the oblique 

 markings, which have given rise to the supposition that they are 

 surrounded by muscular bands, proceeded from trabeculae of a simi- 

 lar structure, which took a curved course from a canal which lies at 

 the base of the cirri (not the great canal of the arms, of course) 

 round the outer convexity of the arm, and terminated by breaking 

 up into a network. These trabeculse, however, were not solid but 

 hollow, and the interspaces between them were solid. The network 

 into which they broke up was formed by distinct canals, and then, 

 after uniting with two or three straight narrow canals which ran 

 along the outer convexity of the arm close to its junction with the 

 interbrachial fold, appeared to become connected with a similar 

 system of reticulated canals which occupied the thickness of that 

 fold. 



It was the examination of the interbrachial fold, in fact, which 

 first convinced me that these reticulated trabeculae were canals ; for 

 it is perfectly clear that vessels or channels of some kind must sup- 

 ply the proportionally enormous mass of the united arms with their 

 nutritive material, and it is so easy to make thin sections of this part, 

 that I can say quite definitely that no other system of canals than 

 these exists in this locality. 



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

 organs of the Terebratulid<e, 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. 



