166 



ACTINOZOA. 



by the title of rudimentary eyes. The radiating 

 system of ganglia and nerve-fibres which Spix 

 described as existing within the base of Actinia 

 has not come under the notice of other observers. 

 But in the Ctenophora occurs a well-marked 

 sense-organ, the 'ctenocyst,' upon whose precise 

 function, whether oculiform or auditory, naturalists 

 are far from being agreed. Such differences of 

 opinion are in truth based on the prejudices which 

 most anatomists acquire from a too exclusive 

 attention to the structural peculiarities which the 



higher 



im 



present. The ctenocyst, in all 



probability, neither sees nor hears, but would seem 

 to be the localised recipient of those obscure 

 general impressions to which its lowly-organised 

 possessor is capable of responding. 



The ctenocyst occupies a central position amid 

 the soft substance of the ectoderm, immediately 

 within the apical pole of the body. In form it is 

 ovate or spherical, smooth externally, but, in some 

 cases, invested with an adventitious layer of pig- 

 ment granules. Its wall appears to be very firm 

 and elastic, so as quickly to recover its proper 

 figure, should this be changed in accordance with 

 the ordinary contractions of the body. Within, the 

 ctenocyst is hollow, and apparently distended with 

 a fluid. In the midst of this fluid lie a number of 

 rounded or polyhedral concretions, semitransparent, 

 colourless or somewhat tinted, occasionally coa- 

 lesced into a single mass, and composed, probably, 

 of carbonate of lime. Each granule is little more 

 than -0003 of an inch in diameter. The concre- 

 tions appear subject to a peculiar vibratory move- 

 ment, but some observers have disputed the fact of 

 its occurrence. 





