344 



neighbouring organs and particularly to the heart-vesicle described 

 below, I cannot admit that Mr. Masterman has given any valid rea- 

 sons for disputing the homology of the notochord of Ceph alo discus^ 

 with that of Balanoglossus. 



2. I have re-examined sections of Cephalodiscus , since reading 

 Mr. Masterman's paper; and I am able to assert that this animal 

 possesses a well developed heart- vesicle, which is moreover clearly 

 figured by Mr. Maste rman himself. If this is the case, it is obvious 

 that the median notochord of Ceplialodiscus cannot be the represen- 

 tative of the heart-vesicle of Balanoglossus. 



In PL XXIV. fig. 14, Mr. Masterman figures a sagittal section 

 through the anterior end of Cephalodiscus. A space termed the »sub- 

 neural sinus« is coloured red, since it is supposed to be a part of the 

 vascular system. It lies on the dorsal and anterior side of the noto- 

 chord, its ventral wall together with the notochord limiting a space 

 which is said (p. 352) to be continuous with the anterior body-cavity. 

 This space, uncoloured in the figure, is separated from the »subneural 

 sinus« by a wall which is »thickened and contractile«. In the trans- 

 verse sections shown in PI. XXIII. figs. 2 and 3 the uncoloured space, 

 with its thickened wall, is completely surrounded by the »subneural 

 sinus (f, an arrangement which is readily intelligible from the fact 

 that it projects forwards into that space, and that the transverse sec- 

 tions do not pass through the base of the organ resting on the noto- 

 chord. 



I can quite confirm the accuracy of Mr. Masterman's description 

 in most of the above points, but I believe that his »subneural sinus« 

 is the heart-vesicle, and therefore not a part of the vascular system 

 (following Spengel's account^ of the arrangement of these parts in 

 Balanoglossus)] and that the structure which projects into it (unco- 

 loured in figs. 2, 3 and 14) is the »central blood space« of Spengel, 

 which in Balanoglossus communicates with the dorsal vessel. On Mr. 

 Masterman's own showing, there is the closest similarity between 

 the »subneural sinus« with the layer of muscles on its ventral wall 

 (see his fig. 14) and the arrangement described by Spengel in the 

 heart-vesicle of Balanoglossus. 



This heart-vesicle of Cephalodiscus is a very obvious and well- 

 marked space lying in front of the tip of the notochord. It occurs 

 both in the adults and in the buds, in which it appears at a very early 

 age as a sharply marked vesicle. I cannot find any evidence that it 

 opens into the vascular system (or into the anterior body-cavity 1, nor 



* T. cit. p. 26, 624, and elsewhere. 



