592 ZOOLOGY SECT. 



primitive Cyclostome stock, in which a vertebral column had been 

 independently acquired. The latter conclusion seems the more 

 probable, and is supported by the fact that in all three orders of 

 Ganoids there are some species with a persistent notochord, others 

 with well ossified vertebrae ; the conclusion being that the vertebral 

 column is a polyphyletic structure that is, has been evolved 

 independently in various groups in accordance with similar 

 conditions. 



Among extinct Elasmobranchs the Acanthodea and Pleura- 

 canthea had bones investing the cranium, and Cladoselache 

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

 ancestor of the Teleostomi and Dipnoi the two sub-classes with 

 ossified skeleton a generalised Blasmobranch in which 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 " archipterygium " 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 with 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, however, 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, which 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 differentiated brain ; no 

 heart, but a contractile ventral vessel below the pharynx and a 

 dorsal vessel immediately beneath the notochord ; colourless blood ; 

 separate nephridia of the annulate type ; a co3lome developed as 

 an enteroccele ; 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 



