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 



B 



i. no 



FIG. 128. Aurelia aurita. A, side view, one-fourth of the umbrella cut away ; Ji, 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-umbrellae 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. 



