CHAPTER XIV. 



GEPHYREA 1 . 



IT is convenient for the purposes of embryology to divide 

 the Gephyrea into two groups, viz. (i) Gephyrea nuda or true 

 Gephyrea ; and (2) Gephyrea tubicola formed by the genus 

 Phoronis. 



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 unequal but complete, and, as in many Molluscs etc., the 

 ovum exhibits before its commencement a distinction into a 

 protoplasmic and a yoke 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 

 formation of fresh ones from the large spheres, a layer of small 



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

 present chapter : 



I (i) Inermia. 



I. Gephyrea nuda. j (a) Arma ^ 

 II. Gephyrea tubicola (Phoronis). 



