74 MARINE BIOLOGICAL LABORATORY. 



taken to mean that bilateral animals have arisen from 

 radial forms by elongation in one of the transverse 

 axes of the latter, the oral face becoming the ventral 

 aspect, and the aboral face the dorsal. The mouth, 

 meanwhile, shifted its position so as to lie near the 

 anterior extremity of the new long axis, and the lateral 

 portions, growing together more or less completely 

 along the region formerly occupied by the mouth, gave 

 rise to the process of concrescence in the ontogeny. 

 What, then, was the origin of the anus of bilateral 

 forms ? Here again the answer of embryology appears 

 to be nearly or quite conclusive. The blastopore gives 

 rise sometimes to the mouth, sometimes to the anus, 

 sometimes to both. The only possible interpretation 

 of these facts would seem to be that the blastopore 

 originally gave rise to both mouth and anus, the case 

 of Peripatiis being an interesting and apparently iso- 

 lated remnant of the ancestral mode of development, 

 or perhaps a reversion to it. Under any other view, as 

 has often been pointed out, we should be reduced to the 

 absurdity of regarding the mouth of one animal as 

 homologous with the anus of another, perhaps closely 

 related, form ; or we should be involved in other difficul- 

 ties, equally great. The original mode of closure of the 

 blastopore has been secondarily modified in the great 

 majority of cases, but the mesoblastic bands and the 

 neural cords still follow the original mode of develop- 

 ment, being laid down separately on either side of the 

 region of the blastopore, and growing together along its 

 line of closure. 



Let us now turn, in the last place, to the significance 

 of the trochophore. A very little consideration will 



