BLOOD-VASCULAR SYSTEM OF THE LORICATI 45 
closed in a vascular plate’ and is separated from the current of 
water passing between the gills by a thin membrane. By 
osmosis the carbon dioxide from the blood is exchanged for the 
oxygen held in the pores of the water. Ina like manner the 
pure blood is collected into a pair of dorso-ventral vessels, 
which unite, forming a short efferent filament cross-vessel. 
These vessels in turn form the efferent branchial filament arter- 
zes (Pl. I, fig. 2; E.Fil.A.), which run cephalad along the 
outer margins of the filaments and which in turn empty into 
and form a common trunk, running in the posterior groove of 
each branchial arch, namely, the efferent branchial artery. 
These efferent branchial arterzes (Pls. I and II, figs. 1, 2, 
5, and 12; E.Br.A.) very closely resemble one another. They 
run parallel, but cephalad to the afferent branchial vessels; be- 
ginning ventrally they increase in size dorsally. For the most 
part the efferent filament arteries are poured directly into the 
main efferent branchial trunks, but dorsad and ventrad they 
empty into a branch of that vessel. The ventral branch takes 
its origin from paired vessels, which lie immediately caudad and 
to either side of the large efferent branchial trunk. They re- 
ceive first the most ventral pair of efferent filament arteries ; 
then in their dorsal course take up in succession from either 
side the several following efferent filament arteries, and after 
having received 20 or 30 such vessels, unite, forming a short 
trunk, which empties into the main efferent branchial trunk 
from the rear. In like manner the dorsal branch arises as a 
paired vessel and returns the blood from seveyal of the most 
dorsal fiaments. There is also a gradual variation in the point 
where the various efferent branchial arteries leave their respec- 
tive branchial arches. The first or most anterior efferent 
vessel follows along the posterior edge of the cerato- and epi- 
branchials some little distance beyond the dorsal-cephalic bend ; 
while the fourth or posterior efferent branchial vessel leaves 
the cerato-branchial a little below the dorsal-cephalic bend ; and 
the efferent branchial vessels of the second and third branchial 
arches come in midway between these extremes, forming a 
regular series of intergradations. Ventrally the first, second 
1 Each branchial filament is divided up into numerous parallel vascular 
plates or lamelle, which lie in dorso-ventral planes. 
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