In these fishes the lateral muscles invariably. extend to the dorsal 

 surface of the head with their anterior ends coinciding with the anterior 

 edges of the frontal bones, In many cases the kidneys enter the haemal 

 canal where they run posteriorly, a characteristic which is not often 

 seen in other fishes. 



These species are said to be the most active and swiftest swimmers 

 among all of the fishes because their bodies offer the least resistance 

 to the water, their muscles are remarkably well developed, and their 

 skeletal structure permits only a lateral movement at the base of the 

 tail. It is also said that their blood is warm and that they maintain 

 their own body temperature. 



Although these fishes differ in their cutaneous vascular systems 

 and in the dark red lateral tissue developed in relation to these systems, 

 and in their kidneys and the vascular systems within the kidneys, they 

 are very close on all other points to the genera Gymno sarda and Sarda of 

 the family Cybiidae of the Acanthopterygii. 



Family Thunnidae 



The whole body is covered with scales,, The first vertebra is very 

 thin and is fused to the cranium A number of the thoracic vertebrae 

 have lateral processes. The second dorsal is ordinarily higher than the 

 first dorsal or is of at least equal height. The three lobes of the liver 

 are roughly equal in size and shape, and the intestine forms one loop. 

 There are pyloric caeca which are located only on the posterior side of 

 the duodenum. The cutaneous vessels pas_ through the myotomes of the fifth 

 or seventh vertebrae, and the dorsal and ventral branches are joined 

 anteriorly over the intermuscular bones of those myotomes, while posteriorly 

 they are joined over the 31st or 32nd vertebra. They are joined by trans- 

 verse branches to the great artery which runs along the central axis of the 

 body. The cutaneous arteries either join the great artery or join directly 

 to the Cuvierian ducts. A long branch of the kidneys enters the haemal 

 canal, but the portion of those organs which lies between the peritoneum 

 and the ribs is short. The ureters are separated into left and right 

 branches only anteriorly. These fishes are generally large, attaining 

 lengths of more than 3 feet. 



Family Katsuwonidae 



There are almost no scales except on the corselet. The vertebrae 

 have no lateral projections, and the first vertebra is thick. The second 

 dorsal is markedly lower than the first dorsal. The right lobe of the 

 liver is slender and, except in the genus Katsuwonus . long, reaching almost 

 to the anus. The intestine is straight and has no loops. There are 

 pyloric caeca on both sides of the duodenum. The cutaneous vessels of 

 the genus Katsuwonu s pass through the myotome of the third vertebra with 

 the inferior branch passing anterior to the first rib. In the other genera 

 they traverse the myotome of the fifth vertebra and there are no inferior 

 branches. In short, there is nothing which corresponds to the inferior 

 branches of the cutaneous vessels in the Thunnidae. In all of these fishes 



