PHYLUM CHORD A.TA 



61 



with the nervous system and notochord dorsally, and with the 

 enteric canal centrally. At about the level of the ventral surface 

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

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

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

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

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

 ccelome. 



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 



mr ch 



B 



m 



Fig 753 -Ampliioxus lanceolatus. A, young larva; B, .anterior end more high y 

 mamlfled. c, pvovlsional tail-fin; ch. notochord ; «., neuronteric canal ;d, enteric canal 

 " c^lome of head ; I, club-shaped gland ; k', its external aperture ; /« ^rstffill-slt;m mouth 

 mr, neuron; «p, neuropore ; m, sub-intestinal vein; w, pro-oral pit. (From Korsohelt and 

 Heider, after Hatschek.) 



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

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

 form the single unsegmented ccelome of the adult, their walls 

 giving rise to the ccelomic epithelium. 



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

 protovertebrse become converted into muscular fibres, which 

 nearly fill the myoccBle, and give .rise to the myomeres: the 

 myocommas arise from the adjacent anterior and posterior walls 

 of the protovertebriB. 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 



