4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



The theoretical planulalike gastrula postulated above as the com- 

 mon ancestor of the Coelenterata and the Annelida (fig. 2 A) pre- 

 sumably swam habitually in one direction by means of a covering of 

 cilia, and the mouth, or blastopore, was at the posterior pole where 

 food particles might be swept into the stomach with the eddy of 

 currents converging to the rear. 



In the ontogenetic development of the annelids, gastrulation 

 generally takes place by epiboly, which is the overgrowth of the endo- 

 derm by the ectoderm, and the primary open blastopore is at the 

 posterior pole of the embryo. There is no reason why this ontogenetic 

 stage should not represent an early phylogenetic stage, and one iden- 

 tical with the gastrula ancestor of the Coelenterata (fig. 2 A). With 

 the further development of the annelid embryo, however, the blasto- 

 pore elongates forward on the ventral surface of the gastrula (fig. 

 2 F) until its anterior end comes to be near the anterior pole (G) ; 

 but, at the same time, the lips of the blastopore grow together from 

 behind forward, leaving finally only the anterior end open into the 

 archenteron, and this opening is the primitive mouth (H, Mth). 

 Secondarily, an anal aperture {An) is formed later at the original 

 posterior end of the blastopore on the caudal extremity of the embryo. 

 The endodermal archenteron of the annelid thus becomes a simple 

 alimentary canal having the oral aperture located ventrally near the 

 anterior end of the body, and the anal aperture situated terminally 

 at the posterior end. 



If we visualize the change in the position of the blastopore as an 

 event in the phylogenetic history of the annelids, we must see a corre- 

 lated change in the habits of the animal. The actively swimming 

 gastrula (fig. 2 A) in its search for food, we may suppose, took to 

 brushing over the surfaces of stones or aquatic plants (B), where 

 food particles were more numerous and more easily obtained. For 

 this manner of feeding, a ventrally placed blastopore would be a 

 distinct advantage, or, even more efficient, a blastopore drawn out 

 lengthwise on the under surface (C). With a form thus modified in 

 habits and structure, there may easily have developed a creeping 

 habit, and an adaptation of the ventral cilia for progression on solid 

 surfaces (D). Finally, then, came a more complete adaptation to 

 feeding on a subsurface, resulting in an elongate flattened body, and 

 the establishment of an alimentary canal with a ventral mouth and 

 a terminal anus (E) produced by the closure of the intermediate 

 part of the blastoporic slit. 



A creeping mode of locomotion may be subserved entirely by a 

 ciliary coating of the body wall, as is shown in the Platyhelminthes, 



