THE EVOLUTION OF PARTICULAR SKULL TYPES 



Lower Chordates 



The main object of this paper is the study of the origin and evolution of the teleost 

 skull, but for the sake of completeness it seems desirable to insert brief notes on the skull 

 structure of the lower chordates, chiefly as a key to the literature but also in order to put 

 the teleost skull in its proper historical perspective. 



Amphioxus was long supposed to be an ideally simple chordate and to furnish the key 

 to the origin of the vertebrates (see Willey, " Amphioxus and the Origin of the Vertebrates " ; 

 Delage and Herouard, "Les Procordes"). Even now Professor Howard Ayers (1921), a 

 great authority on the minute anatomy of Amphioxus, tries to derive the fish skull and 

 and jaws from this source. But in the light of the illuminating investigations of Kiaer and 

 of Stensio on the anatomy of the Silurian ostracoderms we may well suspect that the ex- 

 cessive simplification of the skull structures of Amphioxus is due, at least in part, to ex- 

 treme degeneration and specialization acquired perhaps in connection with the habit of 

 darting into the sand and drawing in microscopic food by ciliary ingestion. In order to 

 establish his theme that Amphioxus is really primitive, with respect to the gnathostome 

 chordates. Professor Ayers rejects the view that the jaws of fishes have been derived from 

 gill-arches and sets up his hypothesis that they have been derived from parts of the velar 

 skeleton of Amphioxus. This view, however, can hardly be taken seriously by those who 

 appreciate the force of the evidence for the derivation of the primary vertebrate jaws from 

 the branchial series. In brief, present evidence seems rather to favor the suggestion that 

 Amphioxus is an extremely simplified descendant of some such early chordate as Lasanius, 

 which has lost all of its exoskeleton and a good part of its endoskeleton as well. And the 

 suggestive points of agreement of Amphioxus with the larvae of the partly degraded cyclos- 

 tomes (Delage and Herouard, 1898, pp. 340-342) seem not averse to the possibility that 

 Amphioxus represents a greatly degraded cyclostome, just as the latter is assuredly a 

 degraded ostracoderm. 



The skull of the cyclostomes has been intensively studied by many authors, including 

 W. K. Parker (1883), Gaskell (1900, Pis. LVI, LVII), Goodrich (1909, 1930) and Stensi6 

 (1927). Both Gaskell and Stensio have shown the striking similarities of the larval lamprey 

 head to that of the cephalaspid ostracoderms and in the light of much evidence it seems 

 highly probable that the lamprey skull type has been derived from a cephalaspid-like type 

 in the following way: (1) the bony exoskeleton has lost its bone cells and become mem- 

 branous; (2) thorny epidermal teeth have developed around the sucker-like mouth; (3) a 

 rasping apparatus has developed out of the so-called tongue, which is a specialized part 

 of the branchial apparatus; (4) the reSt of the branchial arches have been displaced baick- 

 ward; (5) the cartilages that support the sucker and its teeth have also been enlarged; (6) 

 the originally continuous cartilaginous septa between the gill-pouches have become fenes- 

 trated, giving rise to the branchial basket; (7) a special hydraulic organ, described by T. E. 

 Reynolds (1931) has been developed in the oral chamber to assist in the sucking action of 

 the mouth. 



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