46 CHORDATE ORGANIZATION n. 11-12 



coelomic cavities, but it is probable that the many segments of verte- 

 brates arose in order to provide a set of muscles able to contract in a 

 serial manner for the purpose of swimming. Their segmentation would 

 thus be a relatively late development, not related to the segmentation 

 of annelids, which divides the whole body into rings. Accordingly the 

 ventral part of the vertebrate coelom usually remains unsegmented. 

 But in amphioxus (and in the lamprey) it is subdivided from its first 

 appearance and only becomes continuous later. The best interpreta- 

 tion of this condition is to suppose that in order to provide a series of 

 myotomes a rhythmic process subdividing the mesoderm was adopted. 

 In its earliest stages this affected the whole mesoderm, ventral as well 

 as dorsal, but later became restricted to the dorsal region. New 

 morphogenetic processes may often pass through stages of refinement 

 and simplification in such ways. 



12. Amphioxus as a generalized chordate 



Amphioxus provides us, then, with a valuable example of a chordate 

 that retains the habit of ciliary feeding, which was probably that of the 

 earliest ancestors of our phylum. No doubt in connexion with this, 

 and the bottom-living habit, there are many specializations; the 

 enormously developed pharynx with its atrium, the asymmetry, and 

 so on; but the general arrangement of the body is almost diagram- 

 matically simple, and it may well be that amphioxus shows us a stage 

 very like that through which the ancestors of the craniates evolved. 

 Perhaps next the larva remained longer in the plankton and became 

 mature there. The Amphioxides larvae show signs of such a change. 



This might give rise to a suspicion that amphioxus is not an 

 ancestral type but a simplified derivative of the vertebrates, perhaps 

 a paedomorphic form. It possesses, however, sufficient peculiar 

 features to make this view unlikely. Neoteny might explain the 

 regular segmentation, separate dorsal and ventral roots, and other 

 features, but can hardly account for the method of obtaining food, 

 for the condition of the skin, or for the presence of nephridia. It may 

 be, therefore, that amphioxus shows us approximately the condition 

 of the early fish-like chordates, living in the Silurian some 400 million 

 years ago, and that it has undergone relatively little change in all the 

 time since. 



