Embryology &c. o/Yoldia limatula, Say. 



271 



tip the ventral tube, vt, winch opens to the exterior through 

 the blastopore, bl^ and is continuous with the mid-gut, mg. 



Fig. 4. 



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

 cated only at the margins, aa, anterior adductor muscle ; ac, 

 apical cilia ; hi, blastopore ; eg, cerebral ganglia ; /, foot ; II, left 

 liver-lobe; vig, 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 



