VI INTRODUCTION. 



a trailing fibre adherent to some body and rooting the 

 whole colony to its place, now as a simple or branching 

 trunk supporting the zooids, and multiplying and renewing 

 them. It is composed of two layers, an outer (ectoderm) 

 and an inner (endoderm) , which enter into every portion of 

 the structure. The endoderm lines the whole of the body- 

 cavity, and is chiefly concerned with nutrition : the ecto- 

 derm is much more susceptible of modification, and gives 

 rise to many important structures. Between these two 

 primitive layers a third is sometimes interposed, described 

 by Allman and Wright as a muscular coat, composed of 

 longitudinal fibres, and by Reichert as " a supporting 

 lamella a sort of inner skeleton." It has been observed 

 in the body of the polypites, and, if muscular, will explain 

 the rapidity with which they retract themselves*. In most 

 cases the ccenosarc is partially or wholly protected by a chiti- 

 nous covering (polypary) , which is a secretion from its outer 

 layer. In many families the polypary invests the whole of the 

 soft animal substance, and expands into elegant cups or caly- 

 cles around the body of the polypites ; and we have thus a 

 cast of the composite structure in chitine, which, in the 

 disposition of its parts and its general aspect, bears a close 

 resemblance to a plant. 



In other families the polypary is less developed, merely 

 investing the stolonic network and the base of the poly- 

 pites, or also clothing the trunk and branches, but never 

 forming a true calycle. The Hydra alone, if we except the 



* Vide a paper by Dr. T. Strethill Wright on Hydractinia echinata, Edinb. 

 N. P. Journ. N. S. for April 1857, paragraph 21. Also a paper by Eeichert 

 in the ' Monatsbericht der Akad. der Wissenchaft. zu Berlin ' for July 1866. 



For the histology of the Hydroicla, the student should consult Kolliker's 

 ' Icones HistologicaV part ii. 



