CHAPTER XXV. 

 OF THE CLASS OF HYDROZOA. 



1164. OF the class of Hydrozoa of modern writers, which 

 includes the Hydroid Polypes and the Acalephae of Cuvier, it is 

 perhaps difficult to give a general character, as the animals be- 

 longing to it, however closely they may be related physiologi- 

 cally, present a wonderful variety of external form. They are 

 all, however, of a gelatinous texture, and usually of very simple 

 organisation ; the stomach is always hollowed out, as it were, in 

 the substance of the body ; the vascular system, when present, 

 is of a very simple nature ; and no trace of a nervous system can 

 be discovered in them. Like the Polypes, they capture their 

 prey by the agency of tentacles ; and their integuments are fur- 

 nished with a multitude of filiferous capsules, the stinging power 

 of which in many cases is very great. 



1165. The different forms presented by these animals having 

 been already referred to ( 1089), we may proceed at once to the 

 consideration of the Orders into which they may be divided, al- 

 though it must be confessed that the complex chain of relation- 

 ship, which in this class often binds the most dissimilar creatures 

 to each other, renders this a matter of some difficulty. Thus 

 many of the Polypoid forms give origin to free swimming oviparous 

 Medusae by a process of gemmation, so that these might almost 

 be regarded in the light of reproductive organs thrown off from 

 the parent polype in order to aid in the diffusion of the species ; 

 but they display an organisation so far superior to that of the 

 Polypes from which they are produced, that we can hardly help 

 regarding the latter as larval forms ; and besides, some of the 

 Medusae are capable of producing their like, not only by gem- 

 mation, but even by oviparous reproduction. To render this 

 part of our subject intelligible, therefore, it will perhaps be ad 



