404 



INVEETEBEATA 



CHAP. 



developmental facts which have been narrated in this chapter it is 

 obvious that we must commence with the life - history of the 



eft 



FIG. 321. Median sagittal sections through two fixed and metamorphosing larvae of 



Pedicellina echinata. (After Harmer. ) 



a.c, anal cone ; a;>, invaginated apical plate ; at, atrium ; afl, oral division of atrium ; aft, anal 

 division of atrium ; co, corona ; col, degenerating fragments of ciliated cells of corona ; do, dorsal organ, 

 i.e. cerebral pit ; ep, epistome ; hep, liver cells ; int, intestine ; Ir, lateral ridge which later, with 

 epistome, divides the atrium into two halves; m, position of the mouth; s.o.g, brain (supra- 

 oesophageal ganglion ; st, stomach ; ten, rudiments of tentacles of the adult. 



