CHAPTER THIRD. 



THE GENERA HYDRACTINIA AND IIALOCHARIS, 



SECTION I. 



THE HYDROID FORM OF HYDRACTINIA POLYCLINA AG. 



The first time tliat this species was met with, it was found upon the shells 

 of Gasteropods, whicli served for the retreat of Hermit-crabs ; but, subsequently, it 

 has been discovered and collected in great abundance from rocks in tide-pools. 

 In these latter habitats, it often covers several square feet with a rosy, velvet- 

 like carpet, presenting a delicacy and vividness of tint which can hardly be described. 

 The fact that it is often left by the tide, for five or six hours, in pools con- 

 taining not more than a pailful of water, is enough to negative the assertion that 

 the movable homes of Hermitrcrabs are necessary to the welfare of the colonies 

 of Hydractinia which settle upon them.^ In order to examine the specimens 

 without injuring them, a small pebble or shell on which a colony has developed 

 should be selected, instead of placing the animals, piecemeal, under the microscope. 

 Under such conditions it will be readily seen why the rosy tint of the colony 



' References to the genus Hydractinia of Hydractinia, Leidy, Marine Invert. Fauna, Journ. 



VanBeneden, or Synliydra Qitafr. Acad., Philad., 1855, PI. II. Fiff. 35. 



Hydractinia, VanBeneden, Bulletin Acad. Roy., ■" Wright, Edinb. New Phil. Jour., 1857, 



Brux., Feb., 1841, p. 80, PI. A, Vol. V. Pis. VIII. and IX. 



Fiffs. 1-4. " McCrady, Proc. Elliot Soc, Charles- 



" VanBeneden, Mem. Acad. Roy., Brux., ton, S. Carolina, 1858, p. 65. 



1844, Tom. XVII. p. 62, PI. VI. Echinocorium, Hassal, An. Mag. Nat. Hist., July, 



Johnston, Brit. Zooph., 2d ed., 1847, 1841, p. 371, PI. X. Fig. 5. 



p. 32, PI. I. Figs. 4, 5, and 6. Synhydra, Quatrefages, An. Sc. Naturelles, 1843, 



Alcyonidium echinatum, 1st ed. Tom. XX. p. 230, Pis. VIII. and IX. 



