MORPHOLOGY. 



21 



MORPHOLOGY OP THE HYDROIDA— TERMINOLOGY. 



I. The IIydrosoma in general. 



1 . Generalised conception of a Hydroid. 



We shall best bring this part of our subject before the mind, if we first endeavour to conceive 

 of hydroid organisation freed from all non-essential comphcation. 



Let us, then, imagine an open sac (woodcut, 

 fig. 1), whose walls consist of a double membrane ^'•'- ^• 



{x,y), and whose orifice {h) is surrounded by a 

 circle of tubular tentacles (c) formed by csecal off- 

 sets from its cavity. Let us further suppose that 

 there projects from some part of the walls of this 

 sac another sac (c/), also composed of a double 

 membrane. To the morphological conception 

 thus acquired, let us add the physiological one 

 which we obtain by regarding the walls as 

 endowed with irritability ; and as a further phy- 

 siological element in our ideal picture, let us ima- 

 gine that while the former sac subserves the func- 

 tion of a digestive cavity, the latter, which has 

 been emitted as an external bud from its side, is 

 destined to give origin to generative elements 

 which are formed between the two membranes 

 which constitute its walls ; and we shall then 

 have as general an idea, morphological and 

 physiological, as it is possible to form of a 

 hydroid. 



The hydroid organism, however, which we 

 have thus reduced to its simplest expression, 

 admits of much and varied complication, while 

 the actual forms which we meet with in nature 



Diagramatic section of a Hydroid. 



a. Body cavity ; 5, orifice serving for ingestion and egestion ; 



c, tentacle ; x, outer membrane of body walls (ectoderm) ; y, inner 



membrane of body-walls (endoderra) ; rf, generative sac, containing 



.... eggs. The endoderm is throughout indicated by a darker shading 



present a beautifully graduated series, in which than that of the ectoderm, 

 the law of specialisation is expressed with a 



distinctness and significance in the highest degree instructive, and whose study possesses a pecu- 

 liar interest in leading us to those wider generalisations in which other groups of the liydrozoa 

 may be included, and by which we are enabled to assert a morphological unity of the whole. 



2. General Structure — Ectoderm and Endoderm. 



Every hydroid is composed of two membranes, an outer or ectoderm (woodcut, fig. 1, ■?'), and an 

 inner or endoderm [y], the ectoderm having its free surface in direct relation with the medium in which 



