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 medusae; those 

 which live always in the medusa state, the eggs of the jellyfish 

 developing at once into other medusas ; and the 8iplionopTiora, 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 LEPTOLINLZE 



The members of this order agree in all essential particulars 

 being branched colonies having two principal forms of zob'ids, 

 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 zooid 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 zooid, as in 

 tubularians. The genera are based upon these differences in the 

 perisarc. 



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 zooid-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 



