{289}[Illustration: FIG. 137.—_Liriope rosacea_, one of the Geryoniidae, from the west side of North and Central America. Size, 15-20 mm. Colour, rose. _cp_, Centripetal canal; _gon_, gonad; _M_, mouth at the end of a long manubrium; _ot_, statocyst; _t_, tentacle; _to_, tongue. (After Maas.)] This statocyst is innervated by the outer nerve ring. There appears to be a very marked difference between these marginal sense-organs in some of the best-known examples of Trachomedusae and the corresponding organs of the Leptomedusae. The absence of a stalk supporting the statolith and the innervation of the otocyst by the inner instead of by the outer nerve ring in the Leptomedusae form characters that may be of supplementary value, but cannot be regarded as absolutely distinguishing the two orders. The statorhab of the Trachomedusae is probably the more primitive of the two types, and represents a marginal tentacle of the umbrella reduced in size, loaded with a statolith and enclosed by the mesogloea. Intermediate stages between this type and an ordinary tentacle have already been discovered and described. In the type that is usually found in the Leptomedusae the modified tentacle is still further reduced, and all that can be recognised of it is the statolith attached to the wall of the statocyst, but intermediate stages between the two types are seen in the family Olindiidae, in which the stalk supporting the statolith passes gradually into the tissue surrounding the statolith on the one hand and the vesicle wall on the other. The radial canals are four or eight in number or more numerous. They communicate at the margin of the umbrella with a ring canal from which a number of short blind tubes run in the umbrella-wall towards the centre of the Medusa (Fig. 137, _cp_). These "centripetal canals" are subject to {290}considerable variation, but are useful characters in distinguishing the Trachomedusae from the Leptomedusae. The tentacles are situated on the margin of the umbrella, and are four or eight in number or, in some cases, more numerous. The gonads are situated as in Leptomedusae on the sub-umbrella aspect of the radial canals. In _Gonionema murbachii_ the fertilised eggs give rise to a free-swimming ciliated larva of an oval shape with one pole longer and narrower than the other. The mouth appears subsequently at the narrower pole. The larva settles down upon the broader pole, the mouth appears at the free extremity, and in a few days two, and later two more, tentacles are formed (Fig. 138).