87 
thick, and its internal surface is produced into longi- 
tudinal folds. 
The Afferent Branchial Vessels.—The ventral aorta 
(Ao. V.) continues forward the bulbus arteriosus. It 
runs forward in the middle line of the body beneath the 
esophagus and the ventral extremities of the gill arches. 
Its wall is composed of fibrous connective tissue appa- 
rently without muscle fibres. Like all the larger blood 
vessels in the plaice, it contains black pigment. Three 
afferent branchial vessels are given off at nearly equal 
intervals on each side. The first of these almost immedi- 
ately divides into two vessels of equal calibre (Af. Br. 4; 
Af. Br. 3) which supply the 4th and 3rd_holobranchs. 
Separate vessels (Af. Br. 2; Af. Br. 1) are given off to 
the 2nd and lst holobranchs. ‘The ventral aorta ter- 
minates by dividing to form the Ist afferent branchial 
vessels. Each afferent branchial vessel enters the gill at 
about one-third of the length of the latter from the ventral 
extremity, and immediately divides into two branches 
which traverse the whole length of the gill, running on 
the concave surface of the gill arch. 
The Structure of the Gills. —It will be convenient to 
describe here the minute anatomy of the gills before con- 
sidering their vascular arrangements. In the Plaice, as 
in most Teleostean fishes, there are four functional gills. 
Each gill is a holobranch, and consists of two separate 
series of gill filaments borne on the same branchial arch, 
each of which represents the demibranch or single series 
of filaments found on the one side of a gill pouch of an 
Elasmobranch fish. In the Teleostomi the septum which 
in the Elasmobranch separates the two adjacent demi- 
branchs has disappeared, with the result that the two series 
of filaments borne by the same arch have become closely 
opposed. 
