>^2 THE DOGFISH. PHYLUM CHORDATA 



returned by two large ductus Cuvieri ^ which join the sinus 

 venosus, one on each side in the pericardium. Into these the 

 blood from the region of the body in front of the fore-fins is 

 conveyed by a pair of large dorsal anterior cardinal sinuses and 

 two smaller external or inferior jugular sinuses, below the throat. 

 Each anterior cardinal sinus communicates in front with an 

 orbital sinus around the eye, and this in turn with a nasal sinus 

 around the olfactory organ. A hyoidean sinus in the hyoid arch 

 joins the anterior cardinal and inferior jugular. At the outer 

 end of each ductus Cuvieri a subscapular sinus enters from the 

 fore-fin. On its hinder side a very large posterior cardinal sinus 

 brings back blood from the trunk. The two posterior cardinal 

 sinuses converge backwards, growing narrower, and lie side by 

 side between the kidneys, from which blood passes into them 

 by numerous renal veins. On each flank two lateral sinuses return 

 blood from the body-wall, and into one of these open vessels 

 from the fins. Blood from the tail is returned by the caudal 

 vein ; this divides opposite the hinder ends of the kidneys into 

 two renal portal veins, which run forwards along the outer sides 

 of the kidneys and supply them with blood. A portal vein is one 

 which takes blood from one set of capillaries to another, and is 

 named from the organ in which the second set is situated. Blood 

 from the alimentary canal and spleen is conveyed to the liver by 

 a hepatic portal vein, and thence, after passing through capillaries, 

 is discharged into the hepatic sinuses. It will be noticed that the 

 circulation of the dogfish makes a single circuit only, the blood 

 oxygenated in the respiratory organs being carried directly to 

 the rest of the body without first returning to the heart as in 

 animals with lungs : the pressure of this blood is therefore low. 

 The blood cells of the dogfish resemble those of the frog. 



WATER REGULATION 



The plasma is extremely rich in urea, and by that fact hangs 

 a surprising and interesting tale. The solids in the blood of most 

 fishes give it an osmotic pressure higher than that of fresh 

 water and lower than that of sea water. Freshwater fishes are 

 therefore liable to gain by osmosis from their surroundings more 

 water than is good for them. They put the matter right by 



* ' Ductus ' is a fourth, not second, declension Latin noun, and its plural is 

 ' ductus,' not ' diicti ' . 



