20 ARKIV FÖR ZOOLOGI. BAND 7. N:0 25. 



Diodon, Spheroides, Tetrodon, and Orihagoriscus . The single 

 hepatic vein in Spheroides must also be referred to a second- 

 ary stage, while the condition in Balistes is more primitive. 

 Judging from the numher of primitive characters that we have 

 found in some Plectognaths we have all reason to believe, that 

 this fishgroup has branched off rather early from the Teleostean 

 main stock. The evolution within the group seems to have 

 taken place in the following manner, when we base our view 

 on the conditions shown by the bloodvascular system. The 

 ancestors of the Plectognaths have probably been provided 

 with a heart in which sucli primitive characters as a number 

 of four valves at the auriculo-ventricular opening, and the 

 conus arteriosus, furnished with more than two valves, but 

 arranged in a single row were still preserved. The circulas 

 cephalicus was very small, involving only the first pair of 

 efferent branchial arteries. The coronary artery was supplied 

 from several branchial vessels as in the Elasmobranchs. Lateral 

 veins were present. More than one hepatic vein existed. 

 All these characters are not present in any recent Plecto- 

 gnath. Very soon a branch must have been given off from 

 the Plectognath-tree that has led to the Orthagoriscus-gronp, 

 in which the primitive characters of the heart, the origin of 

 the coronary artery were preserved. A specialization has on 

 the other hand taken place within this group as concerns 

 the circulus cephalicus and with all probability even in other 

 instances which are not yet known. In the main trunk of 

 Plectognaths the heart soon obtained the characters common 

 to the Teleosts but the primitive circulus cephalicus was still 

 preserved for some ti me. Lactophrys and Balistes represent 

 these stages. After this follows a number of important spe- 

 cializations. The circulus cephalicus enlarges and involves 

 all the efferent branchial arteries. These become reduced in 

 number, owing to the loss of the fourth gill. In Spheroides, 

 the second and third of them coalesce at their entrance in to 

 the circulus. Parallelly to the liver decreasing in size only 

 one of the hepatic veins persists. These stages are represent- 

 ed by Diodon, Tetrodon and Spheroides. Whether the lateral 

 veins are preserved throughout the whole group of Plecto- 

 gnaths has not been shown. Another condition that may prove 

 to be of phylogenetical interest is the run of the aorta for 

 a long distance within a »haemal» canal, but this is best 



