52 CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. [April 4, 



considered as fixed in position. It is highly probable however that 

 both movements occur and that both the apical pole and the blas- 

 topore pole are shifted forwards, while the middle of the gastrula 

 axis is shifted backwards, the gastrula axis thus being doubled on 

 itself exactly as is the case in Turbellaria, Annelida and Mollusca. 

 Since the polar bodies have long since disappeared it is impossible 

 to locate exactly on the embryo the point which corresponds to the 

 animal pole of the egg. It is probable, however, that this point 

 lies on the ectoderm directly opposite the apex of the gastrula 

 invagination, and therefore anterior to the ridge which develops 

 into the dorsal mantle fold. In the forward shifting of the apex of 

 the gastrula invagination it is highly probable that this point is also 

 shifted forward and continues to lie opposite the apex of the enteron. 

 If this be true the animal pole of the egg coincides very nearly 

 with the point where the line from Ce in Figs. 20 and 24 touches 

 the ectoderm. 



The bending of the gastrula axis which has just been described 

 shows that Terebratulina, like most bilateral animals, belongs to 

 the group designated by Hatschek (^^^^ Heteraxonia, and by 

 Goette ('82) Hypogastric forms. Goette has divided bilateral 

 animals into two groups: (i) the pleurogastric, in which the chief 

 axis of the egg becomes the chief axis of the larva or adult, as in 

 Sagittadccidi the echinoderms, and (2) the hypogastric, in which one 

 of the ''cross axes" of the egg becomes the chief axis of the 

 larva or adult, as in worms, mollusks and arthropods. There can be 

 no doubt that Terebratulina should be classed among the hypogas- 

 tric forms, and if it be true, which however seems questionable, 

 that Sagitta and the echinoderms belong to the pleurogastric type, 

 it shows a very important difference between the embryo of the 

 brachiopod and of the chaetognath. 



V. Development and Organization of the Larva. 



There is of course no natural Ime of demarcation between the 

 embryo and the larva, but for the sake of convenience we shall 

 designate those stages which precede the closure of the blastopore 

 as embryonic, while those which extend from the closure of the 

 blastopore to the end of the free-swimming life we shall call lar- 

 val stages. 



The flattening of the embryo in the dorso-ventral axis and its 



