CHAPTER XIV. 



GEPHYREA 1 . 



IT is convenient for the purposes of embryology to divide the 

 Gephyrea into two groups, viz. (1) Gephyrea nuda or true Gephyrea; 

 and (2; Gephyrea tubicola formed by the genus Phorunis. 



GEPHYREA NUDA. 



Segmentation and formation of the layers. 



An embolic or epibolic gastrula is characteristic of the Gephyrea, 

 and the blastopore appears, in some cases at any rate (Phascolosoma, 

 Thalassema), to become the mouth. 



Bonellia. In Bonellia (Spengel, No. 370) the segmentation is un- 

 equal but complete, and, as in many Molluscs etc., the ovum exhibits 

 before its commencement a distinction into a protoplasmic and a yolk 

 pole. The ovum first divides into four equal segments, each of them 

 formed of the same constituents as the original ovum. At the animal 

 pole four small cells, entirely formed of protoplasm, are next formed 

 by an equatorial furrow. They soon place themselves in the intervals 

 between the large spheres. Four small cells are again budded off from 

 the large spheres and the eight small cells then divide. By a further 

 continuation of the division of the existing small cells, and the forma- 

 tion of fresh ones from the large spheres, a layer of small cells is 

 eventually formed, which completely envelops the four large spheres 

 except for a small blastopore at the vegetative pole of the ovum (fig. 

 160 A). The large spheres continue to give rise to smaller cells which 

 however no longer take a superficial position but lie within the layer 

 of small cells, and give rise to the hypoblast (fig. 160 B). The 

 small cells become the epiblast, and at the blastopore they curl 



1 The following scheme shews the classification of the Gephyrea adopted in the 

 present chapter: 



T n i j ( (1) Iiiermiti. 



i. Gephyrea nuda. {[ 2 J AniMta . 

 II. Gephyrea tubicola (Phoumis). 



