254 Description of the Plates. 



h. Mouth, 'trompe bueeale'' of Van Beneden ; a clear line mark- 

 ing the line of junction of endoderm and ectoderm. 



c. Body-cavity, directly continuous with the digestive, and so, 

 indirectly, with the tentacular cavities. 



(l. Conical testicular glands, which set free their spermatozoa by 

 dehiscence outwards, and can be approximated by flexion of 

 the animaFs body to the inferiorly placed ovaria. 



e. Large protxiberance containing a single ovum, as seen in the 

 autumn, proliferation taking place in the way of gemmation 

 and separation of the buds during the summer months, just 

 as agamogenesis prevails during tlie same period in the 

 Daphnidae and Botifera, but is replaced by the sexual im- 

 pregnation of the ' winter ova'' at the latter end of the year. 

 In the Spongilla, and the fresh-water Polyzoa, on the other 

 hand (see pp. 163 and 238, supra), this history is reversed, 

 agamogenesis by means of gemmules inclosed in a resistent 

 capsule, taking place towards the close of the warmer months 

 of the year, whilst a true sexual process takes place during 

 the summer months. The ovum here is seen to be sur- 

 rounded by a polygon ally-shagreened capsule, which, how- 

 ever, is not spinous here as in Hydra vulgaris. The ovum is 

 still enclosed within another capsule, furnished to it by the 

 substance of the body-walls, in which, between the endo- 

 derm and ectoderm, the generative products are developed. 



The Hydra is not always monoecious, as figured here ; most ordinarily 

 the Hydroid Zoophytes are dioecious ; and as the Hydra possesses a cer- 

 tain power of movement from place to place, while the other Hydroids 

 do not, we should, h 'priori, have expected to find the allocation of her- 

 maphroditism to be the reverse of what it actually is. The history, how- 

 ever, of Cerianthus, one of the Anthozoa which possesses some slight power 

 of locomotion, which is rare in the class it belongs to, and is at the 

 same time hermaphrodite, which no other Anthozoon is known to be, is 

 similarly surprising. The analogy of the language universally employed 

 by us when S})eaking of animals in any one of the higher sub-kingdoms, 

 would cause us to speak of the structures here lettered d and e, as 

 * organs ;' wdiilst the history of the development of the various Hydroid 

 Zoophytes, or an observation of the unbroken chain of gradations which 

 connects these ' organs,' through such fixed gonozooid forms as those of 



