CHORDATA. 



animal kingdom ; and in the arrangement of their coelom they 

 come close to the Cephalochorda and Vertebrata. 



The notochord itself is a rod-like structure in all cases 

 developed from the dorsomedian endoderm of the embryonic 

 enteron. This streak of tissue undergoes a modification of 

 structure almost identical with that presented by the axial endo- 

 derm of the tentacles of many Coelenterata. The modification, 

 which may be described as being of a skeletal nature, consists 

 in both cases of a a vacuolisation of the protoplasm of the 

 endodermal tissue (Fig. 1) and of a 

 considerable development within this 

 tissue of cuticular structures (vide 

 Vol. i, p. 101). Indeed the function 

 of the notochord, like that of the ten- 

 tacular endoderm referred to, is a sup- 

 porting one : it supports the axis of 

 the body and particularly the central 

 nervous system beneath which it lies. 

 In the Cephalochorda this function is 

 discharged by the notochord during 

 the whole of life ; in the Vertebrata 

 and Tunicata however it is purely 

 embryonic or larval in its duration. 

 In the Vertebrata the notochord, 

 though it may in some forms, e.g. 

 Pisces, persist throughout the whole of 

 life, becomes surrounded by a stiff 

 sheath, which takes over its function of 

 axial support and becomes, especially m those forms in which 

 the endoskeleton acquires rigid texture, divided up into segments 

 corresponding with those of the embryonic muscular system. 

 The central nervous system develops from the ectoderm of 

 what is usually called the dorsal surface, and at first nearly 

 always has the form of a groove, which, excepting in the Entero- 

 pneusta, extends along the whole of the dorsal surface and closes 

 completely to form a canal— the central canal of the nervous 

 system. It is characteristic of Cephalochorda, Vertebrata and 

 Tunicata that this canal opens in the embryo for a shorter or 

 longer period unto the enteron (neurenteric canal). This 

 neurenteric communication is however never maintained in 



FlQ.'^'f 1. — Transverse section 

 Oirough notochord and spinal 

 cord of the larva of Bombina- 

 tor igneus (after Gotte, jroni 

 Claus). C/i,S notochordal 

 sheath ; Ch notochord ; Sk 

 skeletogenous layer ; JV^ sjiinal 

 cord. 



