REPORT ON THE DEEP-SEA MEDUSAE. liii 



simplest organs of this sensation are the pigment spots ("ocelli"), chiefly those placed on 

 the umbrella margin. They consist partly of pigment cells, partly of optical sense 

 cells or root cells, which belong to the sense epithelium of the dorsal nerve ring. Whilst 

 these ocelK are originally simple centres for the perception of heat, they are developed 

 later on into true light eyes. As experiments showed, it is principally the swollen bases 

 of the tentacles which bear such pigment eyes, and that chiefly in the order of the Antho- 

 medusae and in those Leptomedusae which have no marginal vesicles (" ocellatae "). Such 

 ocelli are more rarely found in the Trachomedusae, Narcomedusas, and Stauromedusse. 

 On the other hand they are widely spread among the three higher orders of the 

 Acraspedae and usually found at the base of the sense clubs or rhopalia described 

 below. In many Acraspedae and a few Craspedotaa (Anthomedusae) a lens is also found 

 in the pigment body of the eye and in the Cubomedusae a crystalline lens or retina is 

 developed between the lens and the pigment cup. We also find " composite eyes " in 

 the Cubomedusae, as, e.g., in Charybdea, where each sense club bears two large unpaired 

 and two small paired eyes. Moreover, Medusae perfectly devoid of colour, which have 

 neither marginal ocelb nor other pigment spots, are sensible to light ; in this case it is 

 probably the sense epithellium of the umbrella margin which discharges this function. 

 We therefore find in the class of the Medusae a long series of different phylogenetic 

 stages of development of optical apparatus, from the simplest beginning up to very 

 composite eyes. 



§ 82. Organs of hearing (" organa acustica "). In the majority of Medusae we find 

 organs of sense on the umbrella margin, which must be indubitably regarded as organs 

 of sense as they possess both otobtes (" otobthi ") and auditor) 7 cells (" otocellae ") 

 bearing bristles. In the minority of the Medusae, in which the otolites are absent, 

 it is possible (or rather probable) that a lower degree of acoustic functions are 

 exercised by part of the cells bearing tactile bristles (" palpocellae ") already described. 

 As, on the one hand, we know of no definite morphological distinction between such 

 tactile cells, bearing bristles and auditory cells which also bear bristles, and as on the 

 other hand, the latter must be regarded as merely special modifications of the former, it 

 is possible that many apparently indifferent tactile cells are sensible not only to fluctua- 

 tions of pressure, but also to vibrations of sound. Considering, however, the immense use 

 which the capacity of hearing must be to the free swimming Medusae (e.g., the perception 

 of the noise of the tempestuous breakers on nearing the coast), it is most probable that 

 a lower or higher degree of sensibility to sound is generally spread in this class. In this 

 case we must consider the cells with tactile bristles, which are found in the Medusas 

 devoid of otolites, in the whole of the Anthomedusae, Stauromedusas, and also in the Ocellatae 

 (Thaumantidae and Cannotidae) must among the Leptomedusas be regarded as " lower 

 acoustic organs." All other Medusae, on the contrary, possess " higher acoustic organs " 

 or true " organs of hearing," consisting of auditory cells and otolites ; these are found in 



