MESSRS. HANCOCK AND EMBLETON ON THE ANATOMY OF DORIS, 
229 
the auricle by a single trank ; there it is mixed with the imperfectly aerated blood 
from the skin, and propelled again by the ventricle along the arteries as before. 
Thus then we find here a systemic circulation divided into two portions, one 
general, the other partial ; the latter is combined with a portal circulation. It is from 
this hepatic course, in which the blood is most completely deteriorated, that it is sent 
to the branchije, and being returned thence to the heart, joins the current of the 
general portion of the circulation in the auricle. This part of the circulation seems 
not to have been hitherto noticed in the Nudibranchiata, and in 1845 we first pointed 
out the fact of the blood being returned to the heart, both from the skin and the 
branchiae in Doris. 
From what has been here said, it will be observed that we have assumed the skin 
in Doris to be to some extent an agent in respiration. That it is so, seems to be a 
fair inference from these facts following ; that a large quantity of blood traverses it 
on its way to the heart continually ; that although in most species the skin is stiffened 
by spicula, it is nevertheless sufficiently delicate to admit of the necessary changes 
taking place through it; and that in some, as in D. pilosa, this membrane is very 
soft, and clotlied with numerous soft and delicate papillae, which, whilst they ma- 
terially increase the extent of surface, are well adapted to the above purpose; that in 
this last, as in the majority of the species, even the most spiculose, the whole of the 
mantle, and even the foot itself is covered with vibratile cilia. Moreover, if the skin 
be not a respiratory organ, the whole of the blood which supplies the viscera, with 
the exception of that sent to the liver-mass, must be returned to the heart unaerated, 
which is not likely. 
In Eolis the skin in part performs the function of respiration, and \n Limapontia 
entirely. Again, in Terebratula Professor Owen has proved that the mantle alone is 
a respiratory organ ; and in Lingula, he has shown that there is the first appearance 
of a portion of that membrane becoming specialized as a gill; and as we ascend in 
the scale of organization in the Mollusca, it is evident that this relationship between 
the gill and the mantle is always maintained, or in other words, the gill is a develop- 
ment of the mantle, and not of any of the internal membranes. 
Thus, therefore, it is quite in accordance with what might have been expected, to 
find the blood partly aerated in the skin, and partly in the branchiae ; and that such 
a state of things is no anomaly we learn from M. Milne-Edwards, who states that 
in the Great Triton of the Mediterranean, in Haliotis, Patella and Pinna, the blood 
is returned to the heart in a mixed condition, part of it coming from the mantle, and 
part from the gill ; and from Mr. Garner, who several years ago pointed out the 
same condition of parts and functions in the LamelUhranchiata. And when the 
whole subject shall have been fully investigated, it will probably be found that the 
above condition of the circulatory and respiratory organs predominates in the Mol- 
luscan type. If this should be so, then the conditions of the circulation in the 
Mollusca and the higher Crustacea will be found to approximate more closely than 
has generally been thought; inasmuch as, on the authority of John Hunter, and 
