XIII PHYLUM CHOJtDATA (51 



with the nervous system and notochord dorsully, and with the 

 enteric canal ventrally. At about the level uf the ventral .surface 

 of the notochord, a horizontal partition is formed in each coelomic 

 sac (Fig. 751, D), separating it into a dorsal and ventral portion. 

 The dorsal section is distinguished as the ■protovertclra (ns), and 

 its cavity as the myoccclc or muscle-cavity : the ventral section 

 is called the lateral plate, and its cavity forms a segment of the 

 coelome. 



The ventral plates now unite with one another in pairs below 

 the enteric canal, their cavities becoming continuous : at the 

 same time the cavities of successive ventral plates are placed 

 in communication with one another by the absorption of their 



in -fc vw ch. 



! / 



> 



.0 



: ^-__, /- / /- ■^--r^-'r' :,':-;l.. 



en 



ks 



SP 



B 





jnr 

 ck 



d 



SI? 



ji i \ ^s 



sv 



jn 



Fig. 753.— Amphioxus lanceolatus. A, young larva ; B, anteiior end more highly 

 magnified, c, im>y\Auua\ tail-thi ; ch. notochord ; f», neurentoric canal ; <l, enteric canal ; 

 h. cielome of head ; L, club-shaped gland ; k', its external aperture ; Ics, lirst gill-slit ; m. mouth ; 

 mr, neuron; np, nouropore ; sv, sub-intestinal vein; w, prc-oral pit. (From Korsehelt and 

 Heider, after Hatschek.) 



adjacent (anterior and posterior) Avails. In this way the cavities 

 of the entire series of ventral plates, right and left, unite to 

 form the single unsegraented coelome of the adult, their walls 

 giving rise to the coelomic epithelium. 



At the same time the cells of the splanchnic layer of the 

 pro to vertebrae become converted into muscular fibres, which 

 nearly fill the myocoele, and give rise to the myomeres : the 

 myocommas arise from the adjacent anterior and posterior walls 

 of the protovertebrse. An outpushing of the splanchnic layer, at 

 about the level of the ventral surface of the notochord, grows 

 upwards between the myomere externally and the notochord and 

 nerve-tube internally : from the cells lining this pouch the 



