572 



INVEETEBRATA 



CHAP. 



wall of this sac lost their cylindrical form and became flattened, and 

 sent out pseudopodia by which the coelomic sac was anchored, so to 

 speak, to the ectodermal walls of the body. 



The anterior coelom now sent out a prolongation which reached 

 the anterior pole of the embryo ; at this point an apical plate was 



t—U)p 



Fig. 416. — Later stages in the development of Balanoglossus cfaOTsrenss.]^^ (After Heider.) 



A, formation of tlie apical plate. B, formation of mouth and of pore-canal. C, mouth anus and 

 water pore formed, a, anus ; ap, apical plate ; coe^^ anterior coelom ; i%t, intestine ; o, mouth ; oes, 

 endodermal section of oesophagus ; 'p.c, pore-canal ; st, stomach ; stwn, stomodaeum ; w.p, water-pore. 



formed by a thickening of the ectoderm, and the cells forming this 

 thickening developed long stiff cilia, so that in this way a typical 

 apical plate was formed. Erom the coelomic sac also a dorsal pro- 

 jection was developed which grew backwards and fused with the 

 ectoderm, and here, somewhat later, an opening was effected which 

 became the proboscis-pore or water-pore of the adult (w.p, Fig. 

 416, C). 



