Fate of Blastopore in Annelida 59 



respects. But the two phyla differ in respect to deuterogenesis. 

 In the Echinoderms it died out such elongation as there is is 

 probably interstitial, as possibly in Balanoglossus. In the caudate 

 Chordates it continued active dorsally, and as already described 

 produced the tail, see Fig. 31. 



Now it has probably been very different with regard to the 

 other phyla. Unfortunately at present there is no experimental 

 evidence to help us, but a good deal of accurate anatomical 

 evidence is available. 



What is the fate of the blastopore in Annelida ? The evidence 

 suggests that there is an elongation of the original gastrula mouth 

 in the plane of the blastopore, which elongated mouth becomes 

 reduced by a coalescence of the middle lips into two terminal 

 openings (as in the Urodele Amphibians) forming mouth and 

 anus respectively. 



And further it would appear that the deuterogenetic centre 

 remains active between the two openings, but more particularly 

 near the anal opening producing growth in length. The direction 

 of which elongation is therefore at right angles to the direction of 

 the Chordate elongation. 



Thus the fate of the blastopore in Hydroides uncinatus (Eupo- 

 matus) is described thus by Shearer: "The blastopore at first 

 lies exactly in the middle of the ventral plate... when fully formed 

 it is an elongated slit, somewhat enlarged at its anterior end. 

 This end never completely closes, but... becomes the mouth. 

 The posterior portion closes completely, the anus breaking through 

 almost immediately where the last portion of this part of the 

 blastopore disappears." So also in Polygordius. Conn says of 

 serpula : " the blastopore becomes an elongated slit, the lips of 

 which meet in the middle and close, forming the rudiment of the 

 future gut. For a short time the digestive tract remains attached 

 to the ectoderm throughout the length of the blastopore, but 

 after a little it only retains this connection at either end. The 

 digestive tract becomes hollow and acquires two openings to the 

 exterior at the two points of its previous connection with the 

 ectoderm." 



In Amphitrite ornata according to Mead the blastopore closes 

 by concrescence from behind forwards but the extreme anterior 



