degenerate and disappear and the dorsal branch to develop longitudinally,, 

 they would present exactly the same situation as that seen in the posterior- 

 part of the subcutaneous arteries in the genera Euthynnus and Auxis of the 

 family Katsuwonidae „ (cap in fig»2). The same explanation can be given 

 of the relationship between the subcutaneous veins and tne segmental veins 



In the sailfish ^Istiophorus orientaiis/ 7 all of the segmental vessels 

 make their appearance in the subcutaneous tissues along the lateral median 

 line of the body whence they proceed posteriorly along the boundaries bet- 

 ween the myotomes and soon disappear , The arteries originate from the 

 dorsal aorta and ordinarily run anterior to the veins, and the capillaries 

 /"?_/ branch off from the great vein, the renal portal veins, and the caudal 

 vein» 



Now when we compare the segmental vessels of the third myotome of 

 this species (figure 4 sa, sv) with the posterior part of the subcutaneous 

 vessels of the genus Auxis as described by Kishinouye figure 2 cap,, eov), 

 there is no marked difference except that the vessels of the former are 

 shorter and smaller than those of the latter , for this reason, if the 

 vessels of the third myotome in the former were prolonged until they reached 

 the caudal portion of the body, and at the same time the rest of the seg- 

 mental vessels were hidden within the lateral muscles, they would present 

 an identical condition with the posterior part of the subcutaneous vascular 

 system in the latter » furthermore, the anterior part of the subcutaneous 

 arteries in the genus Auxis is almost identical with the arteries of the 

 second myotome in the sailfisho 



In view of these facts, the subcutaneous vascular system is homologous 

 with the segmental system, of which it is merely a marked development in 

 the subcutaneous tissues „ Accordingly the subcutaneous vessels and the 

 segmental vessels do not differ fundamentally but only in the degree of 

 their development, and they are bascially identical 



III. Do Differences in the Size of rilood Vessels have Any 



Value as the Principal Characteristics of Orders in the 

 Taxonomy of fishes? 



No other part of the internal organs of animals is as rich in 

 variations as the vascular system, a fact which is recognized by all 

 comparative anatomists and which needs no further discussion,, 



Kishinouye regarded the presence of the subcutaneous vascular system 

 as the only major characteristic of the Plecostei, but, as shown above, 

 the subcutaneous vessels and the segmental vessels are identical in origin, 

 differ only in size, and do not differ at all basically. If this is so s 

 is it possible after all for a difference in the degree of development, or 

 in other words in the size and length, of anything as prone to marked 

 variations as the vascular system to be the main cnaracteristic of an 

 Order? In particular is it possible to consider such a characteristic as 

 having a taxonomic value equal to or greater than the differences which 

 exist between the other Orders of the Teleostomi such as the Crossopterygii, 

 the Chondrostei, and the Holostei? These are propositions on which there 

 is room for a good deal of doubt „ 



