170 LECTURE IX. 



and even variegated, in different species. Yellow with a red dot is 

 a common condition and is well defined in Oceania.* 



In some Acalephaj the cysticles are not complicated with pigment- 

 cells. In the JEquorea violacea the marginal tentacular interspaces 

 are divided by a mammillary process, on each side of which are two 

 cysticles, of an oval or spherical form, containing each two or three 

 spherical corpuscles.f In Geryonia, Thaumantias, Oceania and 

 Polyxenia the cysticles are sessile upon the circular vessel, and 

 placed between its inner and outer membranes : in Phacellophora as 

 in CyancEa, each cysticle is placed at the extremity of a short double- 

 walled tubular pedicle projecting downwards, the under margin of 

 the fissure in which it is lodged being prolonged into two overlapping 

 fringes, whilst the pedicle is a prolongation of the marginal system 

 of canals. In Cephea and Rhizostoma the cysticle is placed in a notch 

 between two lobe-like processes of the margin of the disc and looks 

 upwards. On the upper surface a semilunar fold extends from one 

 lobe to the other and covers the cysticle. | The cysticles are below 

 the marginal tentacles in Thaumantias, but alternate with tliem in. 

 the nearly allied Geryo7iia.^ The cysticles are yellow in Pelagia 

 noctiluca, but colourless in Cassiopcea and Aurelia, and the colourless 

 pedunculate mai'ginal cysticles of Polyxenia leucostyla contain each a 

 single round corpuscle, whilst the cysticles of Cytaeis polystyla con- 

 tain a cluster of yellowish calcareous corpuscles. 1| Will and Siebold 

 regard these contained corpuscles as homologous with the otolites of 

 higher animals and the cysticle as an organ of hearing^ ; its cavity is 

 lined by vibratile cilia in Oceania **, which impress a vibratile move- 

 ment upon the contents, like that which characterises the otolitic 

 corpuscles in the Mollusca. As the pigment-cell, when present, is 

 distinct from the cysticle it may do the office of a light-appreciating 

 organ, and the cysticle that of a simple organ of hearing. We may 

 with much reason regard as organs of touch the labial and marginal 

 tentacles. 



The ciliograde Acalepha^ are beautifully represented in seas that 

 wash our coasts by the little semi-transparent, delicately-tinted, sphe- 

 roidal animals {Jig- 77.) called Beroii by Miiller, and now the types 

 of many genera, which have this in common, that tlieir chief or- 

 gans of motion consist of unusually lai'ge vibratile cilia, aggregated 

 in lamelliform groups ( ib. c, c), which seeming plates are arranged 



• CXLVII. p. 9. t CXLVI. p. 196. pi. 1. fig. Ic, e, e. 



% CXLV. p. 41G. § CXLVII. p. 8. 



II CXIV. pp. 64. 68. «[[ XXIV. p. 61. 



** Observed by Kollikcr, quoted in CXLVII. p. 9. 



