COELENTERATA. 



113 



centripetal vessels arising at the umbrella edge and ending blindly (Carmarina, 

 Fig. 101). 



Excretion pores may be present at the umbrella-edge in both craspedote and 

 acraspedote medusae. In the Craspedota (most frequently in the Lcptomcdusae) 

 they are placed on warts on the circular canal and open on the sub-umbrella 

 surface. The endoderm cells near the openings are of a glandular nature, and 

 contain concretions which are emptied through the pore. In the Acraspeda 

 there are eight excretion pores at the distal end of the eight adradial canals. 



The ectoderm which is composed of a flat epithelium on the dorsal surface, 

 and of a muscular epithelium on the sub-umbrella surface and velum, is 

 thickened and stiffened along certain lines and contains a large number of 



r 



RK 



FIG. 100. Aurelia aurita from the oral surface. MA the four oral tentacles with the mouth 

 in the centre; GK gonads ; GH aperture of sub-genital pit; RK marginal body; RG radial 

 vessel ; T marginal tentacles. 



nematocysts. An annular thickening of this kind is found round the umbrella 

 margin in some forms (Fig. 101), and similar thickenings ascend for a short 

 distance on the ex-umbrella surface from the roots of the tentacles the 

 peroniums (Fig. 101, pe) and from the roots of the auditory tentacles 

 the otoporpas (Fig. 96, x). The peroniums and otoporpas are continuations of 

 the first-mentioned circular thickening. 



The gastrovascular apparatus is lined by endoderm, which in places carries cilia 

 for the movement of the contained matter. The digestion seems to be, partly 

 at least, intracellular. The endoderm is without muscular structures and nema- 

 tocysts, except on the gastral filaments, which are actively moveable and contain 

 both. Possibly the four longitudinal muscular bands, found beneath the four 

 taenioles and septa in some acraspedote medusae, are of endodermal origin. 



I 



