154 CXELENTERATA. 



the Luccrnarida, the Oceanic Hydrozoa, and the Ctenophora] 

 are almost or quite without hard parts, and therefore only 

 capable of preservation in the fossil condition under very 

 exceptional circumstances, the geological history of the sub- 

 kingdom is very imperfect. 



FOSSIL HYDROZOA. 



Of the living orders of Hydrozoa, the Fresh-water Polypes 

 (Hydrida) and the Oceanic Hydrozoa (Calycophoridce and 

 Physophoridw) have left no traces of their former presence, 

 as might have been anticipated from their want of hard 

 structures. The order of the Medusidce and the sub-class 

 Lucernarida (Jelly - fishes and Sea - blubbers) are equally 

 destitute of hard parts, and their absence from the palse- 

 ontological record might have been confidently predicted. 

 Curiously enough, however, traces of both groups have been 

 detected in the fine-grained lithographic slates of Solenhofen, 

 Pappenheim, and Eichstadt. Of the Medusidce, the two 

 living families of the ^Eyuoridce and Trachynemidce have 

 been recognised by their impressions ; and an ancient 

 member of the order Ehizostomidce^ (Hexarhizites) represents 

 the Lucernarida in the same formation. With these excep- 

 tions, however, the only living orders of Hydrozoa which 

 have fossil representatives are the Corynida and Thecaphora, 

 both of which possess a chitinous or horny integumentary 

 skeleton. In neither case, however, can the evidence be 

 said to be w T holly free from suspicion, except in the instance 

 of Hydractinia and its immediate allies. 



I. CORYNIDA or TUBULARIDA (Pipe -corallines). Animal 

 simple, consisting of a single polypite ; or compound, consisting 

 of several polypites united to one another by a common flesli or 

 ccenosarc. The ccenosarc generally secretes a hard chitinous 

 outer covering or " polypary ;" but the separate polypites are 

 never protected by cup-like expansions of the polypary. As a 

 rule, the polypary extends only to the bases of the polypites 

 (fig. 46, A) ; but in one case it is continued far enough up- 

 wards to include the bases of the tentacles (fig. 46, B). 

 Generally the polypary is more or less plant-like, and is 



