88 On the Natural Historij of the 



near where these tubes bend towards this cavity, the cord 

 passes around so as to form a circle, about the upper part of 

 the chymiferous system or near the centre of the disk. There 

 is thus a marginal, and (if there be no mistake as to its 

 oervous character) a central circle to the nervous system ; 

 from the latter at a point half-way between two chymiferous 

 tubes, a branch passes off and descends to the buccal or 

 digestive mass below, and other branches go to the inner 

 muscles. Professor Agassiz observes that there is a differ- 

 ence between the cord of the lower margin and the other 

 threads, and he was not fully satisfied of the real nature of 

 the latter. Instead of consisting of distinct cells, they are 

 thin threads in which the cellular appearance is almost gone, 

 excepting where they combine to form a plexus, in which some 

 of the threads have the form of long caudate cells : they 

 differ from muscular fibres as much as they do from the main 

 nervous cord. No contraction was observed in them ; this 

 fact, and also their connexion with the chymiferous tubes 

 and the sensitive bulbs below, the nature and position of the 

 plexuses, as well as their branching to the digestive organ 

 and to the muscles, favour the view taken. The close juxta- 

 position of the chymiferous system — the source of nutriment 

 - — with the nervous system and the sensitive bulbs and eye- 

 specks, is remarked upon as a fact of much interest. 



The muscular system is minutely developed in this memoir, 

 and well brought out in the figures. The muscles consist of 

 contractile cells rather than of fibres. Over the surface of the 

 bell-shaped part of the disk the epithelium consists of irre- 

 gular polygonal cells, of very faint outline, hardly distin- 

 guishable except by a kind of mosaic arrangement seen by 

 means of their granular contents. On the sides and above 

 the disk, as well as on the inner surface of the main cavity, 

 these cells are more regular and distinct. The network of 

 muscles, or rather lines of contractile cells beneath the epi- 

 thelium, extend in two main directions ; the main bundles 

 being vertical, four in number, and alternating with the chymi- 

 ferous tubes ; and the others, transverse circular, in four 

 narrow ranges, but with others smaller and less regular. 

 The fibres are chiefly superficial, though penetrating some- 



