(560 PROF. HUXLEY ON THE [DeC. 14, 



next lower stage of vertebrate evolution ; and it is extremely in- 

 teresting to observe that even the existing Amphibia present us 

 with almost every degree of modification of the type, from such forms 

 as the oviparous, branchiate, small-lunged Siredon and Menobran- 

 chus, which stand in the same relation to it as Gymnura to the 

 Eutheria, to the exclusively air-breathing Salamanders and Frogs, 

 in which the period of intraovular development, either within the 

 uterus itself or in special receptacles, may lae as much prolonged as 

 it is in the Mammalia. 



A careful study, on full materials, of the development of the young 

 of such forms as Hylocles will probably throw great light on the 

 nature of the changes which ended in the suppression of the branchiae, 

 and the development of the amnion and of the extra-abdominal part 

 of the allantois in the foetus of the higher Vertebrata. 



The recent researches of Boas^ on the structure of the heart and the 

 origin of the pulmonary arteries of Ceratoclus fell into my hands when 

 I happened to be working afresh at the subject, and had arrived, so 

 far as the heart is concerned, at results which are entirely confirma- 

 tory of his. This wonderful creature seems contrived for the illus- 

 tration of the doctrine of Evolution. Equally good arguments might 

 be adduced for the assertion that it is an amphibian or a fish, or both, 

 or neither — the reason of this being that, as it appears to me, Cera- 

 todus is an extraordinarily little-modified representative of that 

 particular stage of vertebrate evolution of which both the typical 

 Fishes and the tjpical Amphibia are special modifications. I think 

 it will be convenient to have a name for the representatives of this 

 stage ; and I propose that of Ilerpetichthycs . 



If we were to take away from Ceratodus the membrane-bones of 

 the head and the pneumatoccele, and slightly simplify the structure 

 of the heart, the result would be an animal which would undoubtedly 

 be classed among the Chimeroidei ; and if, in such a Chimseroid, the 

 lamellar septa of the branchioe were not reduced, as thzy are in the 

 Chimajroids, while the opercular fold remained undeveloped, the 

 product would be a little-modified representative of the Selachian 

 group, to which, among actually known forms, Heptanchus and 

 Cestraciun present the nearest approximations. Vertebrated animals 

 in this stage of evolution may be termed Chondrichthyes. 



Suppose the limbs and the genital ducts of the Chondrichthyan 

 stage to be undeveloped, and let the two nasal sacs be repre- 

 sented by a partially divided sac with a single external aperture, the 

 result will be a still lower grade of vertebrate life, which may be 

 termed Myzichthyes, represented only by the greatly modified Lam- 

 preys and Hags of the existing fauna. 



Finally, let the head retain its primitive segmentation, and the 

 heart its primitive character of a contractile tube, and we reach, in 

 the Ilypichthycs, a stage of simplification of the vertebrate type, 

 from which it would be difiicult to remove any essential feature 

 without reaching a point at which it is questionable whether an 



1 Ueber Horz unci Avterienbogeu bei Ceratodt's xukI Protoptems;' Morph, 

 Jahrbuch, 1880. 



