112 THE OCEAN WORLD. 



Medusae 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 Hydra 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 : Hydrides, where the polyps are locomotive and 

 consisting of such forms as Hydra viridis, Fig. 4. Corynida, where 

 the forms are attached and the ectodermic layer is generally firm ; 

 Tubularia indivisa is a well-known species. Sertulariadcz, 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. Calycophoridce, consisting of free 

 oceanic forms with nectocalyces, see Pray a diphyes, Fig. 42. Physo- 

 phoridce, also with free forms, but they are provided with a float or 

 pneumatophore, in addition to nectocalyces (see PLATE III., p. 134, 



