206 CGELENTERATA. 



solid tentacular processes, and the substance of the bell is traversed 

 by simple radiating canals which communicate above with the gastric 

 cavity of the polypite. The reproductive organs are developed in 

 the course of the radiating canals of the disc, and the embryo is 

 developed directly into the adult form, without the intervention of 

 an intermediate fixed and sexless stage. 



Though wholly devoid of hard structures, the Trachymedusce, as 

 will be more fully pointed out in dealing with the Acraspeda, are 

 capable of leaving traces of their past existence in the form of im- 

 pressions left by their stranded bodies in fine mud. Impressions 

 of this nature have been recognised in the fine-grained lithographic 

 limestone (Jurassic) of Solenhofen, in Bavaria, and have been referred 

 to the genera Palcegina and Trachynemites, representing respectively 

 the two existing groups of the sEginidce and Trachynemidce. 



Sub-class Lucernarida. 



The division of the Lucernarida comprises a large number of 

 recent Hydrozoa in which the base of the organism is developed 

 into a disc-shaped or cup-like structure (the "umbrella"), in the 

 walls of which the reproductive organs are developed, and which 

 carries on its concave side a single, more or less modified " poly- 

 pite." The "Telly-fish" thus constituted may develop itself directly, 

 or it may be only the generative form of a minute fixed organism 

 (" Hydra-tuba "), from which it is produced by fission. 



The peculiar group of Lucernarians (Calycozoa) of which Lucer- 

 naria itself is the type, is wholly unrepresented by fossil forms, and 

 needs no further consideration here. On the other hand, the group 

 of the Acraspeda — or, as it is often called, Discophora — has left 

 various traces of its past existence, and is in many ways of palaeon- 

 tological interest. The group of the Acraspeda comprises most of 

 the larger organisms which are usually spoken of as "Jelly-fishes," 

 though the structures so called are for the most part merely the gen- 

 erative zooids produced by a minute sexless Lucernarian. Such a 

 Jelly-fish (as, for example, the recent Aurelid) consists of a great 

 gelatinous swimming-bell or " umbrella," from the under side of which 

 hangs a single polypite, the structure of which is greatly modified 

 in the Rhizostomidce. The swimming-bell is traversed by a complex 

 system of canals, connected superiorly with the gastric cavity of 

 the polypite (fig. 89), and its margin usually carries more or fewer 

 "tentacles," which are often of great length. The mouth of the 

 polypite is extended into fringed or lobed processes, often of great 

 length (fig. 89, /). Lastly, the reproductive organs are in the form 

 of four folded bands, which project into four special cavities (the 

 " sub-genital pouches ") in the floor of the gastric cavity. 



