230 MESSRS. HANCOCK AND EMBLETON ON THE ANATOMY OF DORIS. 
more lately of Professor Owen, it can be shown that a mixed stream of aerated and 
imaerated blood flows into the auricle of the heart. 
Lastly, there remains to be described the special branchial organ*. This is 
situated at the posterior part of the dorsal surface of the body, surrounding the anus 
and the renal orifice. It is composed of a variable number of plumes more or less 
divided or pectinated, and arranged in a more or less complete circle. In D. taher- 
culata they are three or four times pinnate. The plumes in some species, as in the 
last named and others, are retractile within a common cavity, the individual plumes 
being themselves contractile. In D.pilosa, D. hilamellata, &c., the plumes are merely 
contractile. Around the base of the plumes run two concentric canals -i- within the 
skin. The inner contains venous blood poured into it from the front by the hepatico- 
branchial or afferent vein, and communicates with as many channels as there are 
plumes. These channels run up the inner side of the stems of the plumes, and dinde 
to apply themselves to their branches. At the ends of the branches these channels 
communicate with others corresponding to them on the outer side; the outer chan- 
nels converge as they pass down to the stems of the plumes and debouch into the 
outer circle, which therefore receives aerated blood, and then transmits it to the 
branchio-cardiac or afferent vein, which opens into the posterior border of the 
auricle. In passing through these plumes, the blood follows the same course in all 
their subdivisions, running up the inner and down the outer surface ; the main 
trunks of the plumes and all their offsets are clothed with vibratile cilia. 
There is a peculiarity of structure in the branchial plumes of D. pilosa, the pre- 
sence of which is indicated by their well-known white star-like centre. This appear- 
ance is owing to the existence of a double row in each plume stem of irregularly 
globular, hollow bodies:]:, with elastic walls separating the inner and outer channels 
of the stem from each other. The function of this apparatus is somewhat doubtful ; 
but we are inclined to believe that it is for the purpose of giving resilience to the 
breathing organ, and thus enabling the species in which it exists to continue its 
respiration for a time even out of water; and it is worthy of remark, that of all the 
species with which we are acquainted, the last-named enjoys the widest range of 
sea depth, being found from the coralline zone, to more than half-way between low 
and high-water marks. D. tuberculata, which is another species in which this 
apparatus is found, but in a less developed form, is to be taken both below and 
above low-water mark, and is frequently left dry among the crevices of the 
rocks. 
Nervous System . — Of this there are two divisions ; the first is made up of two series 
of ganglia, supra- and infra-oesophageal, interconnected by three commissures or 
collars. The former series is asymmetrical. These ganglia give off about twenty 
pairs of nerves, and four single nerves. 
The second division consists of a complete and extensive network of minute 
ganglia and intercommunicating nerves spread out upon the viscera. 
* Plate XI. fig. 4. f Plate XVI. figs. 2 and 6. + Plate XVI. figs. 6 and 7. 
