Common Hydra. 253 



Figure 7. 



Common Hydra {Hydra Viridis) showing the perfectly free communication or con- 

 tinuity with the body cavity which the digestive tract possesses ; the continuity of 

 the cavities of the tentacles with the digestive sac, and their contractility, as indi- 

 cated by the annulation of their external surface as at a ; and the reproductive 

 organs in situ. After Greene ; Manual of Coelenterata, iS6i, p. 24. 



The animal is drawn as attached by its 'hydrorhiza' to a piece 

 of weed with the oral end downwards, as in the position very ordi- 

 narily assumed by it during* life. It is much enlarged, its natural 

 size being", at the greatest, little over three quarters of an inch in 

 length. The Hydra differs from most other Hydrozoa, except the 

 free forms of the class, in the three following particulars : firstly, 

 in consisting of but a single polype, in which point only a few 

 Corynidae resemble it; secondly, in being totally destitute of any 

 external polypary; and, thirdly, in possessing the power of loco- 

 motion as well as that of fixation to one spot. It differs again from 

 all Coelenterata, with the exception of Cordylopliora lacustris^ by its 

 fresh-water habitat. 



a. One of the tentacles, externally annulated, owing to the con- 

 traction of the external layer or ectoderm, which, in the 

 terminal part of the tentacles, can be very well seen to be 

 bounded internally by a structureless 'basement membrane'- 

 like layer, which again forms here the innermost layer, the 

 cellular endoderm not being developed. The ectoderm is very 

 richly bestudded with the thread cells, so characteristic of 

 all Coelenterata (except Ctenophora?), but, besides a few 

 pigment cells, it is said by Reichert to contain no other 

 morphological element, its contractility depending upon a 

 perfectly homogeneous and hyaline substance, as in Rhizo- 

 poda. The basement-like membrane is an excretion from 

 the ectoderm, just as the chitinous external poljq^ary is also 

 in other Hydroid Zoophytes, but has been supposed to be 

 muscular in character. As this figure shows that con- 

 tractility resides in parts of the organism of the Hydra, 

 where the purely cellular endoderm is absent, that layer of 

 the body at all events may be supposed with some prob- 

 ability to be devoid of this faculty. 



