8o 



demonstrative. In many cases I can obtain no proof that the vessel in question supplies any 

 organs other than the gonads. In the sections of C. levinseni shewn in I'l. X, however, there 

 is evidence that the vessel is prolonged as far as the dorsal side of the dorsal diverticulum 

 of the ]jharynx at least (fig. 119). In several sections through the collar-region I find spaces 

 similar to the triangular space at the dorsal root of the collar-mesentery in figs. 114, 115, or 

 in other positions between the central nervous system and the limiting membrane of the collar. 

 Some of these spaces have very much the appearance of being artefacts (and indeed I have 

 not represented them in all the figures), but it is not impossible that they may be normal 

 structures. In Masterman's original account oi the vascular system (97, 2, figs. 7, 8) the main 

 vessel of the collar is represented as occupying the entire height of the mesentery, and in fact 

 as being a space between the two collar-cavities, whose limiting membranes do not actually meet 

 at any point. In most cases I can obtain no certain evidence that the collar- mesentery has 

 this double character. 



In several of the sections of C. levinseni I find evidence of the existence of a finer 

 membrane internally to the principal limiting membrane of the collar (figs. 116 — 118). The 

 latter is thick and is in immediate contact with the epidermis, by which it has presumably 

 been secreted. The inner mcmlirane is much thinner, and there seems reason to believe that 

 it constitutes that part of the limiting membrane ("Grenzmembran" in Spengel's sense) which 

 is derived from the coelomic epithelium. It is by no means impossible that the spaces between 

 these two layers of the limiting membrane are vascular in character. 



The ]jeritoneum is often not in contact with tlie outer surface of the epithelium of the 

 alimentary canal. Some of the spaces thus constituted are almost certainly due to defective 

 preservation, but I regard it as highly j^robable that splanchnic sinuses do occur, externally to 

 the alimentary epithelium. Perhaps the most constant of these spaces is a large oral sinus (?) 

 which I find jjassing from the sides to the ventral aspect of the mouth in C. levinseni (PI. X, 

 figs. 121 — 124, oi\ s.). 



Although I fully admit the unsatisfactory nature of the evidence with regard to the 

 peripheral vessels of CepJialodiscus^ it may be worth while to attemjjt to torm some idea of 

 the probable nature of the circulation in this animal. 



The cardinal fact with which it appears to me justifiable to start is the large size of 

 the main dorsal vessel which connects the stomach with the reproductive organs. It is highly 

 probable that this is a nutritive arrangement, in which case it may be inferred that the blood 

 is collected from sinuses which lie on the surface of the stomach (as appears to be indicated 

 by PI. \', figs. 57, 58) and that it then jjasses forwards through the dorsal vessel to suppl\' 

 the ovaries. I have no evidence with regard to the efferent vessels of the ovaries; but it is 

 conceivable that these might pass to tiie skin along the oviducts. 



If Masterman is correct in his account of the vessels of the "central complex'", it is 

 probable that that porlicjii of the blood which dcjes not enter the ovaries continues its course 

 along the dorsal side of the alimentary canal and is carried 1)\- the sinus in the dorsal collar 

 mesentery, as far as the heart. Here the circulation is ])resumably reinforced by the contractions 



