586 INVEETEBEATA CHAP. 



nervous system, which differs so profoundly from the central 

 nervous systems of most invertebrates. The central nervous system 

 of Vertebrata must have been originally only a local intensification 

 of the general plexus of nerve fibres underlying the skin, which 

 exists both in Echinodermata and Enteropneusta. Its original 

 function was probably to act as a co-ordinating centre for the 

 activities of the tentacles of the two hydrocoeles. The importance 

 which it acquired in this way was retained when these tentacles 

 were lost, and it became the dominating nervous centre for all the 

 organs of the animal. 



CEPHALOCHORDA 



The Cephalochorda consist of a number of closely-allied species 

 of sand-inhabiting animals which are found in the tropical and 

 warmer temperate regions of the world. They have thus much the 

 same distribution as the Balanoglossida, but whereas these latter 

 live in soft black mud the Cephalochorda inhabit clean gravelly 

 sand. 



Most of the species are referred to the genus Amphioxus 

 (Branchiostoma). These animals, as is well known, possess the 

 general form of fish, and their muscles are arranged as in fish, in 

 series of blocks called myotomes. They possess a long tubular 

 spinal cord, underlaid by an indubitable notochord which 

 stretches from end to end of the body, whence the name Cephalo- 

 chorda. The pharynx is pierced by numerous long, narrow gill- 

 slits. 



No one has ever questioned the relationship of these animals 

 with the vertebrata. If, therefore, we can discover in their develop- 

 ment, features which ally them with the Enteropneusta, the question 

 of the relationship of this latter group with the Vertebrata will be 

 settled in the affirmative. 



The mode of development of the eggs of Amphioxus was first 

 discovered by Kowalevsky (1867-1877), and was more fully eluci- 

 dated by Hatschek (1881), whose account has been incorporated in 

 the text-books. The validity of this account was challenged "by 

 Lwoff (1894), who has been followed by other workers. We our- 

 selves criticized Lwoff (1898), but our account has in turn been 

 challenged by Cerfontaine (1907), who supports a modification of 

 Lwoff's view, and by Legros (1907). A final answer to Cerfon- 

 taine was given by us in 1909. The facts bearing on the controversy 

 will be given in the following pages. 



Cerfontaiue has given the best account of the segmentation of 

 the egg. He points out that the nucleus of the egg of Amphioxus is 

 nearer one pole of the egg than another, and that between this 

 nearer pole and the nucleus no yolk-granules are developed. On 

 this ground he asserts that the egg is not really alecithal, but 

 telolecithal, and regards this circumstance as a proof that the 

 ancestors of Amphioxus, like other Vertebrates, once had large 



