II 



THE GENERAL PLAN OF CHORDATE 

 ORGANIZATION: AMPHIOXUS 



1 . The variety of chordate life 



The Chordata occupy a greater variety of habitats and show more 

 complicated mechanisms of self-maintenance than any other group in 

 the whole animal kingdom. They and the arthropods and the pulmon- 

 ate molluscs have fully solved the problem of life on the land — which 

 they now dominate. This domination is achieved by most delicate 

 mechanisms for resisting desiccation, for providing support, and for 

 conducting many operations that are harder in the air than in water. 

 By even more wonderful devices the body temperature is raised and 

 kept uniform and thus all reactions accelerated. Finally, use is made of 

 this high rate of living for the development of the nervous system into 

 a most delicate instrument, allowing the animal not only to change its 

 response to a given stimulus from moment to moment, but also to 

 store up and act upon the fruits of past experience. 



Besides these more developed types of chordate that dominate the 

 land and air there are also great numbers of extremely successful 

 aquatic and amphibious types. The frog is often referred to as a some- 

 what lowly and unsuccessful animal, but frogs and toads are found all 

 over the world. The sharks and bony fishes share with the squids and 

 whales the culminating ecological position in the food chains of the 

 sea, while the bony fishes are the only animals that have achieved con- 

 siderable size and variety in fresh water. Among the still more lowly 

 chordates the sea-squirts take a very important, though not dominant, 

 position among the animal and plant communities that occupy the 

 sea bottom, but they have never entered fresh water. 



One could continue indefinitely with particulars of the amazing 

 types produced by this most adaptable phylum. Yet through all their 

 variety of structure the chordates show a considerable uniformity of 

 general plan, and there can be no doubt that they have all evolved 

 from a common ancestor of what might be called a 'fish-like' habit. 

 In the very earliest stages only the larva was fish-like, and the life- 

 history probably also included a sessile adult stage, such as the tuni- 

 cates still show today (p. 66). This bottom-living phase was then 

 eliminated by paedomorphosis, the larvae becoming the adults. There- 

 fore the essential organization of a chordate is that of a long-bodied, 



