iv PHYLUM CCELENTERATA 177 



Sub-Order a — Semostomce. 



Discomedusaj in which the square mouth is produced into four long oral 

 arms. 



Sub- Order b — Rhizostomcv. 



DiscomedusoB having the mouth obliterated by the growth across it of the 

 oral arms : the stomach is continued into canals which open by funnel-shaped 

 apertures on the edges of the arms. 



Systematic Position of the Example. 



Aurelia aurita is one of several species of the genus Aurelia, 

 and is placed in the family Ulmaridce, the sub-order Semostomce, 

 and the order Discomedusai. 



Its saucer-shaped umbrella and eight tentaculocysts place it at 

 once among the Disco-medusae : the presence of a distinct mouth 

 surrounded by four oral arms places it in the first sub-order 

 or Semostomae. This group contains six families, characterised 

 mainly by differences in the canal system : the Ulmaridae are 

 distinguished by narrow branched radial canals opening into a 

 circular canal. Of the eight genera in this family, Aurelia stands 

 alone in having its tentacles attached on the dorsal or ex-umbrellar 

 side of the margin, and in the oral arms showing no trace of bi- 

 furcation. Eight species of Aurelia are recognised, A. aurita 

 being distinguished by having the oral arms slightly shorter 

 than the radius of the umbrella, and by possessing a trichotomous 

 inter-radial canal and two unbranched adradial canals springing 

 from each gastric pouch. 



Order 1. — Stauromedus^e (Lucernarida). 



Tessera (Fig. 130), formerly regarded as the simplest member of this group, 

 is now looked upon as probably not a mature form. It is described as a small 

 medusa about 4 mm. in diameter having the same general characters as the 

 scyphula-stage of Aurelia, except that the bell-shaped body is free-swimming. 

 The edge of the umbrella is surrounded by eight tentacles, four per-ratlial 

 {p.r.t.) and four inter-radial (i.r.t.), and movement is effected by a well-developed 

 system of circular and radial muscles. 



Lucernaria (Fig. 131), a genus not uncommon on the British coasts, is in one 

 respect even more like a scyphula, since it is attached by a peduncle developed 

 from the centre of the ex-umbrella. The margin of the umbrella is prolonged 

 into eight short hollow adradial arms, bearing at their ends groups of short 

 adhesive tentacles (/. ). As in the scyphula, each gastric ridge contains an 

 infundibulum, lined with ectoderm and opening on the sub-umbrella. The 

 gastric filaments (g.f.) are very numerous — a distinct advance on Tessera — and 

 the gonads (gon. ) are band-like. There are no sense-organs in Lucernaria, but 

 in an allied genus, Halicystus, there are eight per-radial and inter-radial 

 marginal bodies (anchors) of the nature of reduced and modified tentacles, each 

 surrounded at its base by a cushion-like thickening containing many adhesive 

 cells. Internal to each anchor on the sub-umbrellar side is a pigment spot 



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