I12 THE OCEAN WORLD. 
Medusez are charged with numbers of very minute eggs, of the most 
lively colours, which are suspended in large festoons from their 
floating bodies. In some cases these eggs develop themselves while 
still attached to their bodies, and are only detached at maturity. In 
some cases the larval forms produced bear no resemblance to the 
parent ; they are elongated and vermiform, broad at their extremity, 
with vibrating cilia, scarcely perceptible, by which they execute the 
most lively motions. At the end of a certain time they are 
transformed into polyps, and furnished with eight tentacula. This 
preparatory sort of animal seems to possess the faculty of reproduction 
by means of certain buds or tubercles which develop themselves on 
the surface of the body, so that a single zooid form originates a 
numerous colony. The Aydra tuba is subjected to a transformation 
still more remarkable ; its structure becomes complex, its body 
articulate, and it seems to be composed of a dozen discs piled one 
upon the other, like the jars of a voltaic pile; the upper disc is 
convex, and is separated from the colony after a convulsive effort ; 
it becomes free, and an excessively small, star-like Medusa is the 
result ; every disc, that is, every zooid form, is isolated one after the 
other in the same manner. 
Thus some of these forms propagate their kind according to the 
usual laws, like producing like; but others bring forth young which 
_have no resemblance to the parent at all; others are produced by 
budding, or fission, from individuals like themselves. These can also 
acquire sexual distinctions ; but before this change takes place, the 
creature, which was simple, is transformed into a composite animal, 
and it is from its disaggregation that individuals having sexual organs 
are produced, the process being that which has been called alternate 
generation. It goes on in a perfectly regular manner, although it 
is a fact that the young never resemble their mothers, but their 
grandmothers. 
This great class of the Hydrozoa is divided, by Professor Greene, 
into seven orders :—Hydride, where the polyps are locomotive and 
consisting of such forms as Hydra viridis, Fig. 4. Corynide, where 
the forms are attached and the ectodermic layer is generally firm ; 
Tubularia indivisa is a well-known species. Sertulartade, where the 
animal is plant-like and much branched, and where the ectodermic 
layer forms cups in which the polyps dwell; as an example, Sertu- 
laria cupressina may be mentioned. Calycophoride, consisting of free 
oceanic, forms with nectocalyces, see Praya diphyes, Fig. 42. Physo- 
phoride, also with free forms, but they are provided with a float or 
pneumatophore, in addition to nectocalyces (see PLare III., p. 134, 
