LARVA 



627 



two limbs fusing to form a single strip. As the clefts move down- 

 wards the endostyle between them moves also. 



rr tr 



Fig. 485. — Three larva stages of Branchiostoma. — 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. 



ap., Atriopore ; co., ciliated pit derived from the left anterior coelomic division of the coelom ; g.e., gill 

 slits ; /./., left metapleural fold ; m., mouth ; r.f., right metapleural fold. 



MESOBLASTIC SOMITES 



We must now consider the fate of the mesoderm rudiments. 

 The median outgrowth divides into right and left halves, of which 

 the right becomes a coelomic cavity in the snout of the adult, 

 while the left opens to the exterior and becomes Hatschek's 

 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 and by backward outgrowths 

 to spaces in the metapleural folds. The walls of the spaces give 

 rise to mesodermal tissues around the mouth and in the meta- 

 pleural folds, and to the first myomere. The remaining meso- 

 blastic somites all behave alike. They extend downwards on each 

 side (Fig. 483) till they meet below the gut. The outer or somatic 

 wall of each lies against the ectoderm, and the two together are 

 known as the somatopleure ; the inner or splanchnic wall lies 

 against the endoderm and is known with it as the splanchnopleure. 

 The longitudinal septum or ventral mesentery between the cavities 

 of the two sides now breaks down, so that they become continuous. 

 The coelom is thus formed by the coalescence of the cavities of 

 pouches which have grown out from the archenteron, and is called 

 an enterocoele. Meanwhile there has formed in each of them a 

 horizontal septum which divides it into a dorsal half or epimere 

 and a ventral half, the hypomere or splanchnotome. The cavity of 

 the ventral portion is known as the splanchnocoele. The septa 

 between the splanchnocoeles break down so that they form a 



