87 



thick, and its internal surface is produced into longi- 

 tudinal folds. 



The Afferent Branchial Vessels. — The ventral aorta 

 (Ao. V.) continues forward the hulhus arteriosus. It 

 runs forward in the middle line of the body beneath the 

 oesophagus 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- 

 atelj divides into two vessels of equal calibre {Af. Br. 4 ; 

 Af. Bi'. 3) which supply the 4th and ord holobranchs. 

 Separate vessels [Af. Br. 2 ; Af. Br. 1) are given off to 

 the 2nd and 1st holobranchs. The ventral aorta ter- 

 minates by dividing to form the 1st 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. 



