BLOOD-VASCULAR SYSTEI^I 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. In a 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- 

 ies (PI. 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 arteries (Pis. I and II, figs, i, 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 theit 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 several 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 lamellse, which lie in dorso-ventral planes. 



