72 MARINE BIOLOGICAL LABORATORY. 



to include the genealogy of the Bilateralia — 2>. of all 

 forms above the Coelenterata. It is generally agreed 

 that the bilateral type of structure arose by the modifi- 

 cation of a radial type, but as regards the mode of transi- 

 tion two totally different views are maintained. Under 

 the older view, still maintained by some morphologists, 

 the long (or principal) axis of the radiate body — e.g. of 

 a Hydra or a sea-anemone — corresponds to the long or 

 antero-posterior axis of the bilateral body; and hence the 

 oral face corresponds to the anterior end, and the aboral 

 face to the posterior. The mouth must therefore corre- 

 spond in the two cases, and the anus of Bilateralia is 

 a new formation. Under the more recent view — which 

 is held by the greater number of morphologists — the 

 long axis of the bilateral body corresponds to one of the 

 transverse axes of the radiate body, and the oral face of 

 the latter is represented by the ventral aspect of the 

 former.^ The justification of this view — which to me 

 appears the only possible one — lies in the facts of em- 

 bryological development. The gastrula stage of devel- 

 opment is all but universally regarded as being, in a 

 broad sense, the embryonic representative of the radial, 

 two-layered, ancestral type. The blastopore (or gas- 

 trula mouth) represents the ancestral mouth (or protos- 

 tome). Hence the mode of transition from the radial 

 gastrula to the bilateral adult should give us decisive 

 evidence in regard to the ancestral transition. Now, it 

 has been shown in the clearest manner that, in the great 

 majority of cases, at any rate, the blastopore occupies 



1 This applies, of course, to the bilateral invertebrates only. The verte- 

 brates are left out of consideration, as having, in all probability, arisen 

 from forms to which the statement would equally apply. 



