392 



CEPHALOCHORDA 



[CH. 



The notochord is a smooth cylindrical rod lying above the 

 gut and running from end to end of the animal. It consists of 

 cells, the greater part of the bodies of which are changed into a 

 gelatinous substance, and which are surrounded by an exceedingly 

 firm membrane termed the chordal sheath. In the embryo 

 the notochord first appears as a groove in the dorsal wall of the 

 gut, so that we may say that the notochord of the Hemichorda 

 retains a form which is passed through in development by that of 

 Amphioxus. 



In the very young embryo also an indication is seen of the 

 division of the body into the same three regions as we found in the 

 Hemichorda. Just as in the embryo of Balanoglossida so here, the 

 embryonic gut gives rise to five outgrowths from which the coelom of 



FIG. 187. Diagrammatic longitudinal section of an embryo of Amphioxus. 



1. Neuropore anterior opening of the neural canal. 2. Neural canal. 



3. Neurenteric canal. 4. Coelomic groove. 5. Somite divided off 

 from coelomic groove. 6. Collar cavity. 7. Head cavity. 8. Ali- 

 mentary canal. 



the adult is derived. These outgrowths are (1) a median anterior 

 bilobed pouch, corresponding to the proboscis cavity of Balanoglos- 

 sida; this divides at once into two, giving rise to the so-called head 

 cavities; (2) an anterior pair of pouches, the collar cavities, 

 corresponding to the similarly named sections of the coelom of 

 Balanoglossida ; and finally (3) a pair of groove-like extensions of 

 the dorso-lateral angles of the gut cavity, called the coelomic 

 grooves, developed only at the hinder end of the gut. From 

 the last-named the coelom of most of the body arises, and they 

 correspond to the trunk coelom of Balanoglossida (Fig. 187). The 

 proboscis or prae-oral region is however very small and bent down 

 ventrally ; its cavity becomes more or less obliterated in the adult. 

 Dorsally the collar region is narrow from before backwards, but 

 it extends obliquely downwards and backwards, and here becomes 

 fused with the lower divisions of the trunk cavities, (v. infra.) 



