148 MR. R. OWEN ON THE ANATOMY OF THE BRACHIOPODA. 
margin, are formed the large trunks above described. The size of these vessels at 
once suggests their subserviency to other purposes than that of merely returning the 
blood necessary for the nutrition of the mantle. The four vessels of the perforated 
lobe of the mantle form two trunks near the visceral mass, which pass exterior to the 
muscular disks, and joining those of the opposite lobe, enter the two hearts, or dilated 
sinuses, which are situated exterior to the liver, and, in Ter. Chilensis and Ter. Sowerbii, 
just within the origins of the internal calcareous loop. Corresponding to the large 
branchial veins there appear under the microscope much smaller vessels, which I 
regard as the branchial arteries ; these run parallel with the middle of the branchial 
veins, and terminate in the margins of the mantle from which the veins commence. 
These margins present the following appearances when viewed with a high magnifying 
power :—they are puckered at regular distances, the puckerings being apparently 
caused by the insertions of delicate cilia, which pass as far within the mantle as they 
project beyond it: in the interspaces of the cilia the margin of the mantle is minutely 
fringed: and within this fringe is a canal, which extends along the whole circumference 
of the lobe, and from which the branchial veins appear to take their origin: the 
marginal canal is contracted where the cilia are inserted into it, which gives it a 
sacculated appearance, like that of the canal of Petit in the human eye. 
The uniform results of repeated observations on all the specimens of Terebratula 
which I had at my disposal, convinced me that the vascular mantle was the chief, if 
not the sole respiratory organ ; and the utility of the marginal cilia in reference to this 
function can now be readily appreciated, in consequence of the discovery of the remark- 
able property which cilia possess of exciting determinate currents in the surrounding 
water,—a discovery for which the scientific world is indebted to the observations of 
Dr. Grant!, Dr. Sharpey?, and M. Raspail*. 
The imperforate valve in many species of Terebratula is characterized, as is well 
known, by a peculiar, complex, and extremely delicate testaceous apparatus, attached 
to its internal surface: now, as in those recent specimens with the soft parts which I 
have examined, and in which this structure existed, it was found to give attachment 
to the arms, it becomes necessary to describe it before speaking of those parts. 
The principal part of this internal skeleton, as it may be termed, consists of a 
slender, flattened, calcareous loop, the extremities of which are attached to the lateral 
elevated ridges of the hinge; the crura of the loop diverge, but again approximate to 
each other as they advance for a greater or less distance towards the opposite margin 
of the valve; the loop then suddenly turns towards the perforate valve, and is bent 
back upon itself for a greater or less extent in different species. When the loop is very 
short and narrow, as in Ter. vitrea, Brug., there is but a small tendency towards a re- 
flected portion ; but where the loop is of great length and width, as in Ter. Chilensis, 
1 Edinburgh Phil. Journal, vol. xv. p. 150. Brewster’s Journal, vol, vii. p. 121. 
* Edinburgh Journ. of Nat. and Geogr. Science, vol. ii. p. 334. ° Chimie Organique, p. 247. 
