66 ZOOTOMY. 



153. The curved hard bars which separate the gill clefts 

 from one another : these 'are the branchial arches. 



154. A similar hard bar of cartilage separating the spiracle 

 from the first gill cleft: this is the hyoid arch. 



155. The relations of the branchial filaments to the bran- 

 chial arches : there is a set of filaments on both the anterior 

 and posterior walls of all the gill cavities except the last, the 

 posterior wall of which is devoid of filaments. Thus there 

 are nine sets of filaments or half-gills in all, one of which 

 occurs on the posterior face of the hyoid arch, and one on 

 each face of each of the first four branchial arches, the fifth 

 branchial arch being without filaments. 



XXXVIII. Carefully dissect away the mucous mem- 

 brane from the roof of the mouth ; remove the 

 dorsal ends of the gill arches or pharyngo-bran- 

 chials ; and follow out 



156. The efferent branchial arteries (Figs. 20 and 

 21, e.br. i 9), of which there are nine, one for each demi- 

 branch : after leaving the dorsal ends of the gills they unite 

 with one another in pairs, that from the hyoidean demi- 

 branch uniting with that from the anterior demibranch of 

 the first branchial arch, that from the posterior demibranch 

 of the first with that from the anterior demibranch of the 

 second branchial arch, and so on. Four trunks are thus 

 formed, the last of which receives the vessel from the 

 posterior demibranch of the fourth branchial arch, and the 

 first two of which soon unite with one another, so that there 

 are now three main efferent branchial trunks on each 

 side. These take a direction inwards and backwards. The 

 most anterior of the three pairs unite with one another in 

 the middle line and form a short trunk : with this the next 

 pair unite and produce a somewhat larger trunk, with which, 

 finally, the third pair join. The longitudinal vessel thus 



