170 



ZOOLOGY 



SECT. 



^ angle between each two gastric pouches : this leads into a per- 

 radial canal (p.r.c), which, like the inter-radial, branches 

 extensively on its way to the edge of the umbrella. 



The general arrangement of the cell-layers in Aurelia is the 

 same as in a hydroid medusa (Fig. 128, B). The main mass of 

 the umbrella is formed of gelatinous mesogloea, which, however, 

 is not structureless, but is traversed by branching fibres and 



Fia. 128. Aurelia' .aurita. A, side view, one-fourth of the umbrella cut away ; B, dia- 

 grammatic vertical section, ectoderm dotted, endoderm striated, mesogloea black, circ. c. 

 circular canal ; g. f. gastric filaments ; gon. gonad ; g. p. gastric pouch ; gul. gullet ; 

 h. hood ; i.r. c. inter-radial canal ; mg. Ip. marginal lappet ; mth. mouth ; or. a. oral 

 arm ; s.g. p. sub-genital pit ; st. stomach. 



contains amoeboid cells derived from the endoderm. Both ex- and 

 sub-umbrellse are covered with ectoderm, and the stomach and 

 canal system are lined with endoderm, which is ciliated through- 

 out. Some observations seem to ,show that the short tube 

 described above as a gullet and a part of the gastric pouches 

 are lined, not by endoderm, but by an in-turned portion of the 

 ectoderm, but this matter cannot be considered as definitely 

 settled. 



