Embryology i&c. o/Yoldia limatula, Say. 



271 



tip the ventral tube, v/, which opens to the exterior through 

 the blastopore, bl^ and is continuous with the raid-gut, rag. 



Fio-. 4. 



\ ■' ■'h':, 



Reconstruction of an embryo of one hundred and five hoiu-s ; cilia indi- 

 cated only at the margins, aa, anteiior adductor muscle ; «c, 

 apical cilia ; hi, blastopore ; cy, cerebral ganglia ; f, foot ; //, left 

 liver-lobe ; iiuj, mid-gut ; ot, otocyst ; pa, posterior adductor 

 muscle ; pg, pedal ganglion ; r, pouch extending from the 

 cerebral ganglia to the exterior ; s, shell ; t, test ; vg, visceral 

 ganglion ; vt, ventral tube. 



The embryo at this stage has acquired an anus, which has 

 broken through into the upper part of the blastopore. The 



