486 



MANUAL OF ELEMENTARY ZOOLOGY 



a ventral half. The cavity of the 

 ventral portion is known as the 

 splanchnocxle. The septa between 

 the splanchnocceles break down 

 so that they form a continuous 

 perivisceral cavity, which after- 

 wards becomes broken up in the 

 pharyngeal region into a series of 

 tubes by the appearance of the 

 gill-clefts (p. 334). The cavities 

 of the dorsal parts of the somites 

 remain separate and are known 

 as myocxles. Their inner walls, 

 against the notochord, become 

 greatly thickened to form each a 

 structure, known as a muscle plate, 

 which gives rise to a myomere, the 

 walls between successive myocceles 

 giving rise to connective-tissue 

 septa between the myomeres, 

 and the outer walls of the 

 myocceles to the dermis. From 

 the inner wall of each dorsal 

 division of a somite, below the 

 muscle plate, an outgrowth 



Fig. 365. — The development of the atrial 

 chamber in Amphioxus. — After Lan- 

 kester and Willey. 



In I. the metapleural folds are seen sending a 

 slight projection inwards. In II. the two 

 projections have united and enclose a small 

 space {A T.), which is the rudiment of the 

 atrial chamber. In III. this space is 

 enlarging at the expense of the coslom. 

 A comparison of this figure with the 

 cross-section of the adult (Fig. 237) will 

 show the relation of coslom and atrial 

 chamber. 



FR., Ccelomic space within dorsal fin ; AL., 

 gut ; 5., ccelomic space of metapleural fold ; 

 AfP., metapleural fold; SA 7\, projection 

 which forms floor of atrial chamber; Ao. t 

 aorta ; B.C., coelom ; S.I. V. y sub-intestinal 

 vein ; N. t nerve cord; .ST/., sheath of noto- 

 chord ; M 1 '. , myotome ; C. , cavity of scler- 

 otome ; A T. y atrial chamber. The dotted 

 line indicates the mesodermic wall of the 

 ccelom. 



