716 Harorp HEATH, 
firmly held until in the neighborhood of the stomach. The tongue 
is then depressed by muscles of group 18, probably by some fibres 
of No. 7 and by the pressure of the blood that also evaginates the 
semicircular groove to some extent and so carries the subradular 
pharyngeal wall and the radular supports with it. 
The head cavity is bounded posteriorly by a septum or dia- 
phragm as well developed as in the chitons. With the exception of 
the openings of the aorta and pedal sinus it is an imperforate, dense 
and comparatively thick sheet of connective tissue that occupies a 
position between the stomach and radula (Fig. 3). 
In most specimens the pericardial cavity is roughly trihedral 
in form (Fig. 8), the base resting upon the cloacal wall and the 
apex reaching a point anteriorly close to the posterior end of the 
gonad. Its walls appear to consist almost entirely of connective 
tissue fibres of extreme delicacy and are suspended to the body and 
cloacal walls by numerous small radiating muscle and connective 
tissue bundles. The lining epithelium, like that covering the heart, 
is very thin and composed of cells with indistinct cell boundaries 
though of considerable extent judging from the spaces separating 
the pale homogeneous nuclei. The heart in all the specimens is a 
tubular organ composed of heavy muscular walls that show no 
distinct subdivisions. It invariably contains large masses of cells 
which occupy the interstices between the muscles. These may 
function as a blood corpuscle producing organ though it is worthy 
of note that while each of the constituent cells has the general form 
and appearance of a corpuscle it is fully twice the diameter of the 
latter. 
The aorta leaving the front end of the heart passes forward 
holding at first a position between the halves of the gonad about 
equidistant between the dorsal and ventral surfaces, but near the 
middle of the animal it gradually inclines toward the dorsal side 
(Fig. 10). Throughout its entire extent it is a well defined tube 
with walls composed of connective tissue and muscle fibres and in 
several places showing indications of an endothelial lining which 
certainly exists in the section near to the heart. In most if not 
all solenogastres the aorta passes almost at once into the space 
between the dorsal surface of the gonad and the body wall but in 
this species while such a sinus is present its connection with the 
aorta is by means of relatively short and seemingly few vessels 
located here and there throughout the gonad. In a ventral direction 
