CHAPTER VI. 
Hybrozoa. 
“In nova fert animus mutatis dicere formas corpora.’’—OvID, Ze. 
Tue class Hydrozoa includes the Acalephz (from éxaaf, a nettle, sa 
called from the stinging properties which many of them possess), as 
well as a great number of radiate animals, of which the Meduse 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 Medusidz 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 observed some structures 
which he thought could only belong to a nervous system, and 
Ehrenberg thought he observed eyes in Medusa aurita, as well as a 
nervous circle formed of four ganglion-like masses disposed round the 
