CHAP. XI 



TRACHOMEDUSAE 



289 



statolith. 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 innerva- 

 tion 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 stat- 

 orhab of the Trachomedusae 

 is probably the more primi- 

 tive of the two types, and 

 represents a marginal ten- 

 tacle of the umbrella re- 

 duced in size, loaded with 

 a statolith and enclosed by 



c/on 



the mesogloea. Interme- 

 diate stages between this 

 type and an ordinary ten- 

 tacle 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 tlie 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 



VOL. I ^ 



Fig. 137. — Liriope rosacea, one of the Geryo- 

 niidae, from tlie 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.) 



