HYDROZOA 121 
Hydroids are very abundant, but are comprised in the few 
groups mentioned: namely, those which live only in the fixed 
colonial state; those which have alternation of generation, being 
first hydroids and then swimming-bells, or meduse; those 
which live always in the medusa state, the eggs of the jellyfish 
developing at once into other meduse; and the Siphonophora, or 
those which have a floating colonial state, the hydroid never being 
attached, but floating at large and capable of locomotion, some of 
the colony having the function of propulsion. 
Hydroids are particularly interesting as exemplifying the close 
resemblance that may exist in outward appearance between ani- 
mal and vegetable life and as illustrations of communal life and 
of the alternation of generation. A few examples of different 
types are given below. 
ORDER LEPTOLINIZ 
The members of this order agree in all essential particulars 
being branched colonies having two principal forms of zodids, 
the nutritive and the reproductive. Some genera attain the 
length of several inches, or even feet; others are very small tufts 
growing on shells and seaweeds. The cup may completely in- 
close the zodid and be close to the stem (sessile), as in sertula- 
rians; it may be on the end of a short stalk, as in campanula- 
rians; or it may not reach above the base of the zodid, as in 
tubularians. - The genera are based upon these differences in the 
perisare. 
The hydroids, like all other classes, exist in such great variety 
that it would be impracticable to describe here the many named 
species; but to recognize the genera is simple. A long tubular 
pedicel without a cup is characteristic of the tubularians ; the cam- 
panularians have an arborescent form and bell-shaped cups on 
stalks; the sertularians have sessile cups; and the plumularians 
have a feather-like form, with zodid-cups on one side only of the 
branches. 
The beautiful and varied structure of these “animal plants” is 
most interesting, and to be fully appreciated they should be seen 
