5o6 Evolution and Distribution of Fishes 



as traced by Kerr, leads up to the more complex brain of 

 elasmobranch and ganoid fishes. The dorsal, the lateral, 

 and the ventral pairs of nerves in metanemerteans are re- 

 garded as originating Reissner's fibre, the two lobed mass 

 of the spinal cord, and the sympathetic system of fishes and 

 other chordates. So in this as in various other details of 

 structure, the Urochordata and Cephalochordata are evi- 

 dently degraded and derivative groups. In study of such 

 sense-centres as the nasal sac, the paired and unpaired eyes, 

 the auditory organs and related structures, these are re- 

 viewed in light of the Author's published statements in 

 "Causes and Course of Organic Evolution." 



The blood-vascular system not only shows numerous 

 points of homological resemblances, Dendy's microscopic 

 study of the walls of the vessels of metanemerteans reveals 

 like structural details with those of chordate animals. 



In considering the excretory system the writer seconds 

 the earlier views of Dendy and Spencer that It and the 

 vascular system probably had a common origin, and that 

 gradually the functions of oxygen and food conveyance — 

 or the anagenetic jfunction — was assumed by the majin 

 vessels, while that involving removal of the effete products 

 — or the katagenetic function — was assumed by the finer 

 and terminal tubes. So the derivation, in succession to the 

 archinephros, of a pronephros, a mesonephros, and a me- 

 tanephros is sketched. 



Graded transitions are traced for the reproductive 

 organs from metanemerteans through cyclostomes and 

 amphibians as one line of evolution, and through ganoids 

 to teleosteans as another line. Holoblastic segmentation 

 of the egg in metanemerteans, Amphioxus, Petromyzon 

 and some higher fishes, and evolved meroblastic in Myxine 

 and In other groups of higher fishes are compared. 



In embryonic development five mesoblastic cell-plates 

 arise during "direct" development In metanemerteans and 

 Amphioxus, or two rather Irregular plates — regarded by 

 the writer as due to fusion amongst the five — arise In 

 Petromyzon and other fishes. These originate the meso- 

 blastic tissue of the adult. 



