CoMPAHATIVE STIUV OF SCOMIiR')!!) FISHES. 451 



ID tbe case of the genus Katsuioomts, tlio hypaxial blood-vessels are much sinallei' 

 and shoi-ter than the epaxial, and the plexus of blood vessels suiTounding the dark 

 red portion of the lateral muscle are united to the epaxial blood vessaLs only. In- 

 deed the epaxial blood veasols of the Katsiiwonidae seem to correspond to the entire 

 cutaneoaq system of the Thunuidae, and the hypaxial vessels of the former seem 

 to l>e sui generis. The posterior cardinal vein joins the right Cuvierinu duct, and 

 joining this cardinal vein is a small renal vein. The iuterliaemal rod of the 

 vasciüar plexus atfciins the utmost development in Katsuiooims and Eulhi/nnns. 

 The rod is thicker than the diameter of the vei-tebral column, and is protected 

 by the bony trelhs formed by haemal processes of the colnmn, from the 

 enormous development of tlie inferior foramen. In the genus Avxis, however, 

 the interhaemal rod of the vasculai* plexus is very thin, and the inferior 

 foramen is formed in a few caudal vertebrae only, having no relation with the 

 vascualr plexus. 



Kithieys much elongated. Uretei"s are nearly separate, ninuiug almost 

 pirallel to each other in the posteiior slender portion of the kidneys. The 

 spleen is smaller than that of tunnies and is situated at the anterior portion 

 of the visceral cavity. 



In the hypaxal dai"k red portion of the lateral muscle, just Ixjlow the 

 series of intermuscular b.mes a laigo strong tendon fi"om the second vertebra 

 is sheathed with thin layers of some muscle segments, fi'om the myotomes of 

 the third and some succeeding vertebrae. Thus in each epaxial portion of the 

 lateral muscle two concentiic rings of muscle segments are foimd in the cross- 

 section. This is quite characteristic of the Katsuwonidae. The axial side of the 

 lateral muscle meets the axial skeleton in the epaxial as well as the hypaxial 

 jwrtion, and the dark red portion is more voluminous than in the Thimnidae. 



Tbe vertebral column is very firm, light, and compact, allowing no lateral 

 motion. In anterior precaudal vertebrae the neural canal is separated from 

 the canal of the spinal ligament by a thin bony septum as in the Scombridae. 

 Neural process of the first vertebra is more or less united to the centinmi and 

 the posterior dorsal zygapophyses are very well developed in the vei-tebra. 

 Parapophyses are quite abortive. 



Deutigeroas ossicles on the gill-arches are large and are arranged in one 

 row only. Internal gill-rakers are well developed. 



