xm PHYLUM CHORDATA 59 



blindly in the roof of the oral hood : it disappears completely in the 

 adult except in Amphioxides, in which it is said to contain soleno- 

 cytes. 



On the floor of the archenteron in the neighbourhood of the mouth 

 a depression appears giving rise to a structure known as the club- 

 shaped gland (k), which may be a modified gill-cleft. Posteriorly 

 the neurenteric canal closes and the anus appears. 



We left the mesoderm in the form of separate paired somites, 

 arranged metamerically in the dorsal region of the embryo. These 

 increase in size, and extend both upwards and downwards, each pre- 

 senting a somatic layer (Fig. 760, D, mk 1 ) in contact with the external 

 ectoderm, and a splanchnic layer (mk 2 ) in contact with^the nervous 



"*, 



; / .' / 



^ 



. N 7771* 



Ch. 



\t 

 _-___^ . _ St> 



sv 



Fio. 762. Amphiosois lanceolatus. A, young larva ; B, anterior end more highly 

 magnified, c. provisional tail-fin ; ch. notochord ; en. neurenteric canal ; d. enteric canal ; 

 h. coelome of head ; k. club-shaped gland ; k'. its external aperture ; ks. first gill-slit ; m. 

 mouth ; mr. neuron ; np. neuropore ; sv. sub-intestinal vein ; w. pre-oral pit. (From 

 Korschelt and Heider, after Hatschek.) 



system and notochord dorsally, and with the enteric canal 

 ventrally. At about the level of the ventral surface of the notochord 

 a horizontal partition is formed in each coelomic sac (Fig. 760, D), 

 separating it into a dorsal and ventral portion. The dorsal section 

 is distinguished as the protovertebra or myotome (ns), and its cavity 

 as the myoccele or muscle-cavity : the ventral section is called the 

 lateral plate or splanchnotome, and its cavity forms a segment of the 

 ccelome. 



The lateral plates now unite with one another in pairs below the 

 enteric canal, their cavities becoming continuous : at the same 

 time the cavities of successive lateral plates are placed in communica- 

 tion with one another by the absorption of their adjacent (anterior 

 and posterior) walls. In this way the cavities of the entire series 



