348 
POPULAR SCIENCE REVIEW. 
identical with that of the sexual buds, which continue attached 
to the colony, and mature their products in situ. He 
therefore views them as definite zooids, co-ordinate with the 
polypite and the medusa, and not as mere organs. In such 
cases the medusa is a mere carrier and nurse of the sexual 
buds, which form a separate term in the complex life-series of 
the species. But leaving these transcendental matters, whether 
in ovaries or zooidal sacs, the medusa originates the seed of 
new generations ; and with the escape of the embryos, and 
their dispersion through the waters of the sea, its existence 
probably terminates. 
The Hydroid embryo (or planula) (fig. 10) is an elongated, 
cylindrical body, thickly clothed with cilia, which after a short 
term of free life, fixes itself by one extremity, loses its loco- 
motive organs, and is moulded gradually into a polypite — 
the first term of a new series, from which by repeated buddings 
the complex, arborescent colonies, and the graceful medusan 
forms, will be evolved in due succession. 
Towards autumn chiefly the medusae seem to congregate 
for the purpose of spawning, and on warm, still days, certain 
species swarm in immense shoals near the surface. Their 
fragile forms are ill-suited to face the tumult of the waters, and 
large numbers of them are found floating dead after storms. 
The smaller kinds sometimes fall victims to rain-water ; and 
Agassiz has noticed that their total disappearance from the 
neighbourhood of the wharves, about which they congregate, 
44 uniformly coincides with heavy rainfalls, while the larger 
species survive.” Some are so frail as to be unable to resist 
the action of the light and heat near the surface, and con- 
sequently swim at a considerable depth below it. There seem 
to be diversities of temperament amongst them, but most of 
them are vivacious in their habits, moving about 44 with the 
greatest freedom and energy ; ” and it must be added that, 
with all their grace and tenderness, they are indubitably 
voracious ! 
The phosphorescence, for which they have longbeen celebrated, 
is confined to certain species ; and in them it is localized in 
certain portions of the structure. The painted bulbs of the 
tentacles sometimes glow with vivid lights, under the stimulus 
of irritation, without which the fires refuse to kindle ; and the 
margin of the crystal bell is girt with a wreath of brilliants. 
The phosphorescence also connects itself specially with the re- 
productive system, and sometimes the central pendant is all 
ablaze, and thousands of little lamps illuminate the dark 
waters around. Some species emit flashes of coloured light. 
The Hydroid medusae, like a host of other marine floaters, 
can only be obtained in their adult state by the use of the tow- 
