386 THE DOGFISH LESS. 



capillaries, unlike the arteries, have no muscle or connective 

 tissue in their walls, which are formed of a single layer of 

 epithelial cells. The blood in these respiratory capillaries 

 is therefore brought into close relation with the surrounding 

 water, and as the blood flows through them it exchanges its 

 carbon dioxide for oxygen. 



From the respiratory capillaries the blood is collected into 

 minute arteries, which join into larger and larger trunks, and 

 finally unite into efferent branchial arteries (ebr. a two to 

 each gill by which the purified blood is carried from the 

 gills. The efferent arteries of the right and left sides unite 

 in a median longitudinal artery, the dorsal aorta (d. ao), 

 which passes backwards, immediately beneath the vertebral 

 column, to the end of the tail. 



From the efferent branchial arteries and the dorsal aorta 

 are given off numerous arteries supplying the whole of the 

 body with blood. The most important of these are two pairs 

 of 'carotid arteries (c. a) to the head, a pair of subclavians 

 (scl. a) to the pectoral fins, unpaired cceliac (cl. a) and mes- 

 enteric arteries (ms. a) to the enteric canal, liver, pancreas, 

 and spleen, numerous paired renals (r. a) to the kidneys, 

 spermatics (sp. a) to the gonads, and a pair of iliacs (it. a) 

 to the pelvic fins. The posterior part of the dorsal aorta, 

 supplying the tail, is contained in the haemal canal of the 

 caudal vertebrae, and is known as the caudal artery (cd. a). 



All these arteries branch and branch again in the various 

 parts to which they are distributed, their ultimate ramifica- 

 tions opening, as in the case of the gills, into a capillary 

 network with which every tissue, except the cartilages and 

 the epithelia, is permeated. In traversing these systemic 

 capillaries the blood parts with its oxygen and various 

 nutrient matters to the tissues, and receives from them 

 carbon dioxide and other waste matters. 



