454 E. RAY LANKKSTER AND ARTHUR WTLLEY. 



The atrial groove becomes floored in first iu the region of the 

 atriopore. The growth of the sub -atrial folds extends gradually 

 forwards, and the closure proceeds along one side (the right) of 



Fig. 9. 



.^-^rny: 



Transverse section throusH an adult Ampliioxus. The atrium has grown 

 up so as to divide the primitive t-plauchuocoel into two portions — an inner 

 or splanchnic, and an outer or pleural, portion {sp' . and .7;".). sp. The 

 portion of tiie primitive splanchnocoel which is not so alTected by the 

 atrium, and which persists as the dorsal coclom. .7/'. Pleural ccelom. 

 y. Its perigonadial dilatation, f.r. Fin-ray. Other letters as in pre- 

 ceding figures. 



the pharynx. The whole atrium thus formed is a very small 

 tube-like space. The closure by means of the small horizontal 

 sub-atrial outgrowths in the region of the large gill-slits is 

 somewhat difficult to explain. The small left metapleur 



