158 ZOOLOGY SECT. 



In the centre of the lower or sub-umbrellar surface is a four- 

 sided aperture, the mouth (mth), borne at the end of an extremely 

 short and inconspicuous manultrium : surrounding it are four long 

 delicate processes, the oral arms (or. a), lying one at each angle 

 of the mouth and uniting around it. Each arm consists of a 

 folded membrane, tapering to a point at its distal end, beset 

 along its edges with delicate processes, and abundantly provided 

 with stinging-capsules. The angles of the mouth and the arms 

 lie in the four per-radii, i.e. at the end of the two principal axes 

 of the radially symmetrical body : of the marginal notches with 

 their lappets, four are per-radial and four inter-radial. 



At a short distance from each of the straight sides of the 

 mouth, and therefore inter-radial in position, is a nearly circular 

 aperture leading into a shallow pouch, the sub-genital pit (s.g. ^;), 

 which lies immediately beneath one of the conspicuously coloured 

 gonads (gon). 



Digestive Cavity and Canal-System. The mouth leads by 

 a short tube or gullet (gul), contained in the manubrium, into a 

 spacious stomach (st), which occupies the whole middle region of 

 the umbrella, and is produced into four wide inter-radial gastric 

 pouches (g. p), which extend about half-way from the centre to 

 the circumference and are separated from one another by thick 

 pillar-like portions of the umbrella-jelly. In the outer or peri- 

 pheral wall of each gastric pouch are three small apertures, 

 leading into as many radial canals, which pass to the edge of 

 the umbrella and there unite in a very narrow circular canal 

 (circ. c). The canal, which opens by the middle of the three 

 holes, is of course inter-radial (i.r. c) : it divides immediately 

 into three, and' each division branches again: the canals from the 

 other two holes are adradial (a.r. c), and pass to the central canal 

 without branching. There is also an aperture in the re-entering 

 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. 114, B). The main mass of 

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

 is not structureless, but is traversed by branching fibres and 

 contains amoeboid cells derived from the endcderm. 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 is lined, not by endoderm, but by 

 an in-turned portion of the ectoderm, as we shall see to be the 

 case in Actinozoa and Ctenophora ; but this matter cannot be 

 considered as definitely settled. 



It was mentioned above that in the free medusa the gonads 



