224 MESSRS. HANCOCK AND EMBLETON ON THE ANATOMY OF DORIS. 
observations have been, the matter requires still more extended investigation before 
any very positive conclusion can be arrived at. 
Organs of Circulation and Respiration . — These consist of central organs of propul- 
sion, arteries, veins and sinuses, and of a plumose branchial organ arranged in a 
more or less complete circle. 
First, we have the systemic heart*, consisting of auricle and ventricle, inclosed 
within a membranous pericardium, of an oval form, best seen when distended, which 
is attached all round to the general peritoneal investment of the viscera, except at 
the entrance of the two lateral venous trunks from the skin and the vein from the 
branchise : there it is continued upon the veins themselves. On removing the heart, 
there may be seen, on each side on the floor, a number of small oblique perforations 
or pores. 
The heart lies upon the upper surface of the posterior part of the liver-mass in 
front of the branchial circle, and the upper surface of the pericardium is in contact 
with the skin of the back. The heart, when distended-f, almost fills the pericardium, 
the auricle being somewhat larger than the ventricle. The latter is very muscular, 
subtriangular in form, provided with numerous and strong carnem columnse, and 
having the auriculo-ventricular opening guarded by a double valve, the edges of 
which come together horizontally and project into the ventricular cavity. There 
is also a valve at the aortic opening. The walls of the auricle are much thinner and 
more delicate than those of the ventricle ; the interior is lined with an irregular opeu 
network of much-attenuated fleshy columns, or rather threads. 
The circulation appears pretty rapid in Doris ; in D. pilosa there are seventy-two 
beats of the heart in a minute ; in D. tuherculata upwards of fifty. 
Secondly, we have lying under the pericardium and opening into it another propel- 
ling organ, which has not hitherto been noticed as such, of considerable interest, 
which shall be more fully described after we have traced the general course of the 
circulation. The general systemic artery:|: comes from the front or apex of the pyri- 
form ventricle, and is almost at once divided into three principal trunks ; of these, two 
are lateral and opposite ; and each results in three branches, which ramify upon the 
upper surface of the sides and posterior part of the common mass of the liver and 
ovarium, dividing and subdividing into ramuscules of extreme tenuity, forming a 
network around the lobules of the ovarium and then plunging into the liver. The 
third or anterior trunk or aorta passes forwards on the right lobe of the liver, to 
which it gives several branches ; it then gives off" on the left side a strong branch, 
the gastric artery ; after this another large tube passes off on the same side, supplying 
copiously a spongy glandular-looking organ, analogous perhaps to some of the vas- 
cular ductless glands of the Vertebrata, overlying the buccal mass. The same artery 
distributes branches to supply the supra-oesophageal ganglia of the nervous system. 
The aorta, soon after, gives off the genital artery from the right side ; this supplies 
* Plate XI. figs. 1, 2 and 4. f Plate XVI. fig. 2. t Plate XII. fig. 1. 
