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CHAPTER VI. 



Hydrozoa. 



" In nova fert animus mutatis dicere formas corpora." — Ovid, Met. 



The class Hydrozoa includes the Acalepha^ (from aKaXri^rj, a nettle, so 

 called from the stinging properties which many of them possess), as 

 well as a great number of radiate animals, of which the Medusae are 

 the type. Many of them are remarkable as floating and swimming in 

 the sea by means of the contraction and dilatation of their bodies, 

 their substance being gelatinous, without apparent fibres ; many of 

 them, again, are attached and are only locomotive in their young state. 



The great order of the Medusidse is characterised by having a disc, 

 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 disc presents a 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 filaments have sacs and canals containing fluids at their roots. 

 The indications of nerves or nervous system are too slight to be 

 received as evidence, although Dr. Grant obsei'ved some structures 

 which he thought could only belong to a nervous system, and 

 Ehrenberg thought he observed eyes in Medusa auriYa, as well as a 

 nervous circle formed of four ganglion-like masses disposed round the 



