156 



CHAS. B. WILSON. 



made up of ectoderm cells much smaller than the average, and 

 elongated at right angles to the surface. In its formation the 

 cubical epithelium becomes columnar and invaginates into 

 the body-cavity as a thimble-shaped papilla. As invagina- 

 tion proceeds the columnar cells become wedge-shaped, with 

 their bases toward the body-cavity, and, withdrawing a little, 

 leave a depression at the centre of the papilla, which furnishes 

 the requisite support for the flagella. The latter arise singly 

 from the inner ends of the wedge-shaped cells, and are 

 wholly unlike the rowing flagella possessed by so many larvas, 

 never being used for locomotion. They are rather enlarged 

 cilia, soft and very flexible, and they originate, so far as can 

 be determined, like the cilia, by a spinning out of the proto- 

 plasm from within the ectoderm cells. Consequently they 

 are sensitive, and serve as tactile organs of considerable 

 delicacy and power. 



The Change from Radial to Bilateral Symmetry. 

 — It has just been stated that the apical plate is eccentric in 

 position. 



This first appearance of bilateral symmetry is quickly 

 emphasised by an elongation of the embryo parallel with the 

 eccentricity, and the long diameter thus determined becomes 

 the longitudinal axis of both larva and adult. In later 

 development the entodermal invagination is elongated along 

 this same axis, but in one direction only, i. e. away from the 

 apical plate. 



This completes the differentiation, and we are able to dis- 

 tinguish anterior and posterior, dorsal and ventral, right and 

 left, and the second stage in the formation of a pilidium is 

 complete. 



Such a transition from radial to bilateral symmetry is of 

 interest in its bearing on phylogeny. Embryologists gener- 

 ally agree that several of the largest groups of Metazoa show 

 enough resemblance in their larval development to warrant 

 the conclusion that they are all descended from a common 

 ancestral form. These groups include Annelids, Molluscs, 

 Rotifers, Gephyreans, Nemerteans^ and possibly others. 



