XVII 



PKOTOCHORDATA 



621 



8 o> X 



11 |& 



,3 I f 



a 





5 if 



bp$i g ^ 



endst 



mn 



FIG. 452. Anterior portion of 

 free-swimming tadpole of 

 the simple Ascidian Phal- 

 lusia mammillata, seen from 

 above. (After Kowalevsky. ) 



a, position where anus will be 

 formed ; at, atrial involutions ; 

 6.v, brain-vesicle ; endst, endostyle ; 

 fix, fixing processes ; 3*1, gs*, the 

 first two gill-slits ; int, intestine ; 

 m, mouth ; mn, chin (mental pro- 

 cess) ; ms, longitudinal muscles of 

 the tail ; n.t, nerve tube ; oc, eye- 

 cup; ot, otolith ; /)/i, pharynx. 



