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MANUAL OF ELEMENTARY ZOOLOGY 



shift downwards with the clefts, from the right side of 

 the body to their permanent position. The atrium is at 

 first small, but enlarges so as to enclose the sides of 

 the body (Fig. 365). The endostyle appears at the begin- 

 ning of the larval period as a band of columnar ciliated 

 cells on the right side of the anterior end of the pharynx 

 above the first gill-cleft. It becomes folded as a V with 

 the apex directed backward. When the two rows of clefts 

 are established, the apex of this v grows back between 

 them, the two limbs fusing to form a single strip. As the 

 clefts move downwards the endostyle between them moves 

 also. A structure known as the club-shaped gland is formed 



Fig. 363. — Three larva stages of Amphioxus. — After 

 Lankester and Willey. 



In A the metapleural folds are still separate ; in B they are united 

 behind ; in C they are united along their whole length. 



aj>., Atriopore ; Co., ciliated pit derived from the left anterior ccelomic 

 division of the coslom ; g.e., gilkslits; l.f., left metapleural fold; 

 wz., mouth ; r.f., right metapleural fold. 



from the wall of the pharynx on the right side above the 

 gill-clefts. It disappears while the second series of clefts 

 is forming. 



We must now consider the fate of the mesoblastic 

 somites. The anterior median outgrowth 

 divides into right and left halves, of which the 

 right becomes a cavity in the snout of the adult, 

 while the left opens to the exterior and becomes a small pit 

 in the wheel organ. Each of the somites of the first pair 

 sends forward into the snout an outgrowth, which gives rise to 

 a cavity in the head of the adult, while its walls form part of 

 the mesoderm of the same region. The rest of the somite 

 gives rise to other spaces in the neighbourhood of the mouth 



Mesoblastic 

 Somites. 



