154 MR. R. OWEN ON THE ANATOMY OF THE BRACHIOPODA. 
the fissure in order to avoid disturbing the soft parts, the vascular lobe of the mantle 
with similar free margins is exposed, but the viscera are quite concealed by the dilated 
disk or foot’. 
Each lobe of the mantle can be reflected from before backwards to the extent of five 
lines, and from behind forwards to the extent of half a line, but they adhere too closely 
to the visceral mass to be detached without laceration. When so reflected, the branchial 
vessels may be seen in rich profusion on their inner surface. 
On the lobe? of the mantle which lines the imperforate valve, these vessels are 
seen converging from the respiratory margin to four trunks, which are much shorter 
than the corresponding ones in Terebratula: on the opposite mantle-lobe* the branchial 
vessels form only two such trunks‘. 
The principal trunks in both mantle-lobes unite, and terminate in two sinuses or 
hearts, situated close to two strong tendinous membranes®, which circumscribe the 
visceral mass, and to which the mantle-lobes firmly adhere. The arteries continued 
from the hearts pass obliquely through the membrane, and may be plainly seen distri- 
buting ramuli over the liver and ovary. In one of the specimens I succeeded in in- 
jecting the vessels of one lobe of the mantle from one of the ventricles in the retrograde 
course of the circulation: the solution of carmine which I used pervaded the numerous 
small ramuli given off from the larger branches of the veins, to the extent shown in 
the magnified view (Fig. 11.) of the recent preparation, which is now in the Museum 
of the Royal College of Surgeons. 
In subjecting this injected preparation to high magnifying power, there evidently 
appeared a small uninjected line’, as in the Terebratule, accompanying each of the 
larger branchial veins, running along the centre of every trunk; and these lines I 
conclude to be branchial arteries: if they were retractile muscles of the mantle, they 
might be expected to have a straighter course. At the margins of the lobes, near the 
roots of the cilia, lateral ramulets are given off at right angles, which form a chain or 
circular vessel all round the margin. 
The cilia, besides being longer and more closely set than in Terebratula, are seen 
under a high magnifier to be themselves beset with smaller sete, a structure which 
probably gives them greater power in exciting the respiratory currents’. 
In this profuse distribution of vessels over a plain membranous expansion, we 
perceive the simplest construction of the water-breathing organ, or branchia; and, 
while it proves the close affinity of the Brachiopoda to the Ascidie, it presents, at the 
same time, a beautiful analogy with the elementary forms of the air-breathing organ, 
as it exists, for example, in the pulmoniferous Gasteropods. 
The muscular system of Orbicula differs in some respects from that of Terebratula. 
Eight distinct muscles may be perceived, without including the labial arms. The four 
1 Fig. 6. 2 ¢. Fig. 5. 3 a, Fig. 6. 4n. Figg. 7. & 8. 
5 o. Fig. 11. ® z. z. Figg. 7. 8. 7 n. Fig. 13. * Fig. 13. 
