612 ZOOLOGY 



SECT. 



Ganoids tlicre arc some species witli a persistent notochord, others 

 with well ossified vertebra^ ; the conclusion being that the vertebral 

 column is a polyphyletic structure— that is, has been evolved 

 independently in various groups in accordance uith similar 

 conditions. 



Among extinct Elasmobranchs the Acanthodca and I'lcura- 

 canthea had bones investing the cranium, and Cladoselache 

 had no claspers. These facts seem to indicate as a probable an- 

 cestor of the Teleostomi and Dipnoi — the two sub-classes with 

 ossified skeleton — a generalised Elasmobranch in wdiich fusion of 

 dermal ossicles into investing bones had begun, but in which the 

 special reproductive phenomena of the existing members of the 

 group — internal impregnation and Few, large, well-protected eggs 

 — had not yet been acquired. The origin of the Dipnoi from such 

 a source is rendered more probable by the possession of the 

 characteristic biserial fin or " arch i pterygium " by Pleuracanthus. The 

 Holocephali and the existing Elasmobranchs may be considered 

 as having arisen from the same primitive stock along diverging 

 lines of descent. There is, however, at present no evidence to 

 trace or to explain the fusion of the palatoquadrate with the 

 cranium to form the characteristic autostylic skull of the Holo- 

 cephali and Dipnoi. 



The connection of the Ostracodermi with the better-known 

 groups of Fishes is very uncertain. It has been proposed to class 

 them wdth Cyclostomata on account of the absence — as far as our 

 present knowledge goes — -of jaws and limbs, and attempts have 

 been made to show affinities with the Xiphosura and with larval 

 Tunicates. They seem, how^ever, to be undoubted Fishes, but 

 with no clear relationship to any existing group. The Arthrodira 

 appear to be most closely allied to the Dipnoi. 



The question of the origin of Fishes from lower forms is involved 

 in the greatest obscurity. Practically the only assistance in the 

 solution of the problem is furnished by Amphioxus, wdiich seems to 

 indicate as the ancestral stock of Vertebrates, fish-like animals 

 having a skeleton in the form of a notochord, fin-rays, buccal 

 cartilages, and branchial rods ; a barely diiferentiated brain ; no 

 heart, but a contractile ventral vessel belovi^ the pharynx and a 

 dorsal vessel immediately beneath the notochord ; colourless blood ; 

 separate uephridia of the annulate type ; a coelome developed as 

 an enterocoele; metamerically arranged gonads devoid of ducts; and 

 alecithal eggs. The forward extension of the notochord, the 

 immense pharynx, the very numerous gill-slits, and the atrium, 

 are very probably characters special to the Acrania; but even 

 putting them aside as of no phylogenetic importance, it is obvious 

 that this group must have sprung from a point very low down the 

 chordate stem. The morphological ditierences between Amphioxus 



