HYDRACTINIA. 



Hydr actinia. (Hydractinia polyclina AG.) 



This is another Tubularian, covering the surface of rocks ill 

 tide-pools, or attaching itself upon shells inhabited by hermit 

 crabs. Indeed it was iipon these shells that the Hydractinia was 

 first noticed, and it was long supposed that the wanderings to 

 which the little colony was thus subjected were necessary for its 

 healthy development. But subsequent observations have shown 

 that it attaches itself quite as frequently to the solid rock as to 

 these nomadic shells. It has a rosy color, and, being very small, 

 it looks, until one examines it closely, more like a thick red car- 

 pet of soft moss, than like a colony of animals. These communi- 

 ties are distinct in sex, the fertile individuals in each being either 

 all male or all female. In Fig. 100 we have a portion of a fe- 

 male colony, representing one fertile head, in which the buds are 

 crowded with Medusae ; one sterile head, surrounded by its 



Fig. 100. Fig. 101. 



wreath of tentacles ; and still another member of the society 

 whose office is not fully understood, unless it be that of a kind 

 of purveyor, catching food for the rest. Fig. 101 represents the 

 corresponding individuals taken from a male colony. The sex 

 makes little difference in the appearance of the reproductive 

 heads. All the individuals of a Hydractinia colony are con- 

 nected at the base by a horny network, rising occasionally 



Fig. 100. Female colony of Hydractinia ; a sterile individual, b fertile individual producing female 

 Medusae, c fertile individual with globular tentacles without Medusas, d efg h i Medusae in different 

 stages of growth, o mouth tentacles. (Ayassiz.) 



Fig. 101. Male colony ; a a sterile individuals, b fertile individuals producing male Medusas, d ; 

 globular tentacles, t slender tentacles of sterile individual. (Agassis.) 

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