GENERAL CONSIDERATIONS. 



as in that of the adult Phoronis, a cephalic section and a trunk 

 section. Further, the body is unsegmented, and there are no indica- 

 tions of the descent of Phoronis from a segmented ancestor. 



The metamorphosis of Phoronis affords important data for the 

 orientation and interpreta- 

 tion of the adult animal. 

 We have seen that the 

 principal parts of the body 

 owe their origin to an 

 excessive growth of the 

 ventral side. The longi- 

 tudinal axis of the adult 

 therefore lies at right angles 

 to that of the Actinotrocha. 

 The dorsal surface is 

 shortened, being restricted 

 to the short tract lying 

 between the mouth and the 

 anus (Fig. 5, m-a). Al- 

 though the larval organs 

 are cast off, we may regard 

 the organs that appear in 

 their place as their full 

 equivalents. We shall 

 therefore have to derive 

 the tentacle-crown of the 

 adult from the post-oral 

 ciliated ring of a Trocho- 

 phor-e-like ancestor, and to 

 regard the epistome as the 

 transformed pre-oral lobe. 



The remarkable meta- 

 morphosis of Phoronis 

 certainly does not corre- 

 spond to any phylogenetic 

 condition. We shall have 

 to assume that a very 

 gradual shifting of the 

 anal aperture along the 

 dorsal middle line took 



Fig. 5.— Diagram representing a median longitudinal 

 section of Phoronis (constructed after Cori). a, 

 anus ; eh, epistomal cavity ; ep, epistome ; g, gan- 

 glion ; i, intestine ; I, fenestrae in the mesentery ; 

 Ih, lophophoral cavity ; m, mouth ; mg, second 

 stomach [intestine of Benham] ; ms, dorso-ventral 

 mesentery ; n, nephridium ; oe, oesophagus ; r, 

 circular nerve ; t, tentacles ; vm, first stomach. 



place in the worm-like ancestor of Phoronis, which perhaps lived in 



