iin PHYLUM CHORDATA 577 



ossified skeleton a generalized Elasmobranch in which fusion of 

 dermal ossicles into membrane 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 palato-quadrate 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 ; 

 distinct nephridia ; a ccelome developed as an enteroccele ; meta- 

 merically 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 phylo- 

 genetic importance, it is obvious that this class must have sprung 

 from a point very low down the chordate stem. The morphological 

 differences between Amphioxus and a Hag are, in fact, of a more 

 fundamental character than those between a Hag and a Mammal. 



Still lower must have been the point of origin of the Urochorda, 

 with the notochord confined to the tail, the dorsal mouth, and the 

 absence of myomeres and of nephridia. The huge pharynx with 

 its innumerable stigmata is undoubtedly a secondary character; 

 but the atrium, endostyle, dorsal lamina, and peripharyngeal bands 

 seem undoubtedly to indicate an affinity with the Acrania. So 

 also do the earlier stages of development, but the later stages, and 

 especially the mode of origin of the atrium, are quite different in 

 the two cases. 



VOL. II P P 



