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AN INTRODUCTION TO ZOOLOGY 



conus, and runs outwards on the second gill arch serving the second 

 holobranch. The fourth and fifth afferent branchials are given off 

 almost together just outside the pericardium, and running on the 

 corresponding gill arches supply the next two holobranchs. The 



afferent branchials break 

 up into a rich capillary 

 plexus in the gill filaments, 

 so enabling the blood to 

 give up its carbon dioxide 



and take up oxygen. 



The oxygenated 

 blood from the gills is 

 collected up by efferent 

 branchial arteries, vessels 

 running the complete 

 length of each hemibranch. 

 The first eight on each 

 side are joined at each 

 end so as to form complete 

 loops around the first four 

 gill pouches, and this 

 leaves a single vessel along 

 the anterior hemibranch 

 of the fifth pouch, none 

 being required along its 

 hinder edge as it bears no 



gill filaments. These loops are joined about half-way along their 

 length, i.e. laterally, by short longitudinal trunks passing across 

 the gill arches, and by means of the last of these the hindermost and 

 incomplete half loop drains into the preceding loop. From the inner, 

 dorsal ends of the loop come off four pairs of epibranchial arteries 

 which pass backwards and inwards in the sub-mucosa of the dorsal 

 pharyngeal wall to unite in the middle line and form the dorsal 

 aorta, which is the main big visceral artery and runs back to the end 

 of the tail. 



Three arteries form the main source of supply for the head 

 region. A slender mandibular artery is given off from the extreme 

 ventral corner of the first efferent branchial loop, and this passes 

 forward to the muscles of the lower jaw and also the hyoidean 

 region. From the middle of the first efferent branchial, i.e. in 

 line with the longitudinal trunks, arises a fair-sized vessel variously 

 termed the hyoid, the anterior or ventral carotid artery, which runs 

 forward dorsal to the hyomandibular cartilage and in the anterior 

 wall of the spiracle. It goes on across the floor of the orbit and 



FIG. 75. Ventral aorta and afferent bran- 

 chial arteries of Scy Ilium. 



A., auricle ; A.B., afferent branchial arteries ; G.C., 

 gill cleft ; H., hemibranch ; H.B., holobranch ; I., in- 

 nominate artery; M., muscles ; P., pericardium; S.V., 

 sinus venosus ; T., thyroid gland ; V., ventricle ; V.A., 

 ventral aorta. 



