492 EMBKYOLOGY OF THE LOWEE VEKTEBEATES oh. 



disturbance affects the development of the individual as a whole 

 much more than it does its component organs, the result being that 

 embryology frequently affords a much more perfect picture of the 

 evolution of single organs than it does of the organism as a whole. 



In reference to the ontogenetic record of phytogeny an interesting 

 question presents itself regarding the reliability or otherwise of the 

 information derived from the study of larval forms. To what extent 

 may a particular type of larva be taken as probably representing a 

 corresponding phase in the evolutionary history of the group : to 

 what extent are its features to be regarded as ancestral, to what 

 extent as mere modern adaptations to the environmental conditions 

 among which the particular creature now pursues its individual 

 development? In connexion with various groups among the 

 Invertebrata larval forms have played a conspicuous part in 

 phylogenetic speculation — in some cases without due discrimination 

 in interpreting their features as ancestral — the climax perhaps 

 being reached by the view which regards such pelagic larvae as 

 trochospheres or nauplii — precociously developed and free-swimming 

 heads without any trunk — as representing ancestral forms (of. 

 Graham Kerr, 1911). 



In considering whether a particular stage of development is to 

 be taken as probably repeating an ancestral stage of the adult 

 special attention should be directed towards its mode of life, with 

 the object of estimating the degree to which it diverges from the 

 probable mode of life of the ancestral stage. If its mode of life is 

 strikingly aberrant, e.g. parasitic where the normal habit of the 

 group is free-living, or pelagic where the normal habit is not pelagic, 

 then we must always keep in mind the possibility or probability 

 that its most conspicuous features are mere modern adaptations and 

 are therefore worthless as evidence of ancestral conditions. 



Again it should be considered whether in the main features of 

 its organization it agrees with animals which are admittedly 

 allied to it. 



Larvae occur in the following Vertebrates — Amphioxus, Petro- 

 myzon, Crossopterygians, Ganoids, many Teleosts, Lung-fishes and 

 the majority of Amphibians. Applying such criteria as are indicated 

 above we should rule out as probably devoid of phylogenetic 

 significance the larva of Amphioxus on account of its quite aberrant 

 " pleuronectid " asymmetry (see Vol. I. Chap. XVII.). We should 

 again rule out the Teleostean larvae on account of their extreme 

 diversity. In Urodele Amphibians and Dipneumonic Lung-fishes 

 on the other hand we see larvae which appear to be distinctly of a 

 common type. And in Crossopterygian and Actinopterygian Ganoids 

 we again find larvae which differ from these in detail rather than in 

 fundamental characteristics. Consequently we should incline towards 

 the view that the type of larva in question does not depart very 

 widely from the common ancestral type out of which existing 

 Vertebrates have evolved. 



