720 A. T. MASTERMAN. 



lieart now proves to be a pericardial sac, containing the true 

 heart^ within it. The dorsal sinus can be seen running along 

 immediately under the ectoderm and above the dorsal collar 

 mesentery (figs. 1 — 4). Anteriorly it terminates against the 

 posterior wall of the pericardial sac (which in a large number 

 of specimens is ruptured). Here it is also joined on each 

 side by a branchial vessel coming from the branchial plumes 

 (fig. 7). Further, the anterior end of the dorsal siniis is 

 continued into the cavity of the heart by paired lateral 

 canals, the relationships of which are not easy to find nor to 

 describe. If we could pull the apex of the subneural gland 

 backwards from the mouth of the heart it is clear that the 

 dorsal sinus would communicate directly with the heart. In 

 the normal condition, however, this wide aperture of the 

 heart is almost completely plugged up by the apex of the 

 subneural gland. Dorsally and ventrally (figs. 7 and 8) this 

 organ rests closely up against the pericardial wall, but 

 laterally a small canal remains running downwards from 

 dorsal sinus to heart (fig. 4). This canal is bounded pos- 

 teriorly and laterally by the wall of the collar cavity, and 

 anteriorly by the wall of the pre- oral canal (pre-oral cavity). 

 It is doubtless through this paired canal that the blood finds 

 its way from the dorsal sinus to the heart. 



Below the subneural gland is a well-defined ventral sinus, 

 which passes backwards to the level of the mouth and round 

 it on either side. It is wide and large posteriorly, but passes 

 forwards, getting narrower and narrower till it is lost in the 

 glomerulus (figs. 7 and 8). Ventrally it is bounded by the wall 

 of the pre-oral cavity, which also extends ventrally, laterally, 

 and anteriorly to the pericardial sac. Various parts of this 

 wall (pro-oral cavity) are thrown out into ca3cal prolongations 

 into tlie cavity, with thickened protoplasmic walls. The 

 cavities of these casca are in direct communication with the 

 blood-sinuses. They produce an appearance closely similar 

 to that of the glomerulus or pericardial gland of Balano- 

 glossus, with the exception that the walls are simple and 

 ' The " ))rc-oral s:ic " of niv previous work. 



