C 195 ) 



CHAPTEE VIII. 



ACALEPH^E, OR SEA NETTLES. 

 In nova fert animus mutatis dicere formas corpora." OVID, MET. 



THE class Acalephse, from aKaXrifyy, a nettle, so called from the 

 stinging properties which many of them possess, include a great 

 number of radiate animals of which the Medusae are the type. They 

 form the third class of Cuvier's zoophytes. The Acalephse, forming 

 the first order, are characterised as floating and swimming in the sea 

 by means of the contraction and dilation of their bodies, their substance 

 being gelatinous, without apparent fibres. 



The great genus Medusa is characterised by having a disk, more or 

 less convex above, resembling a mushroom or expanded umbrella 

 the edges of the umbrella, as well as the mouth and suckers, being 

 more or less prolonged into pedicles, which take their place in the 

 middle of the lower surface; they are furnished with tentacula, varying 

 in form and size, which have given rise to many subdivisions, with 

 which we need not concern ourselves. 



The substance of the disk presents an uniform cellular appearance 

 internally, but the cellular substance being very soft, no trace of 

 fibre is observable. Taken from the sea and laid upon a stone, a 

 Medusa weighing fifty ounces will rapidly diminish to five or six 

 grains, sinking into a sort of deliquescence, from which Spalanzani 

 concluded that the sea- water penetrated the organic texture of its 

 substance, and constituted the principal volume of the animal. Those 

 which have cilia round their margins have also cellular bands running 

 along their bases, and most of the projectile and extensile tentacula and 



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