LUCERNARIDA. 



207 



The Acraspedote Jelly-fishes are wholly marine, and are often of 

 very large size. They usually swim near the surface, but they have 

 been observed to crawl about on the sea-bottom in shallow water, 

 and they have also been seen to lie at the bottom with the base of 

 the umbrella turned downwards and the tentacles directed upwards. 

 Though they possess no hard structures, and though their tissues are 

 saturated with water, and therefore singularly perishable, they are 

 nevertheless capable of producing impressions in sand or mud by 

 which their past existence, and even their form, may be determined. 



Fig. 89. — Under side ot the umbrella of Aurelia aurita, one of the Acraspedote Medusas, 

 reduced in size. /, One of the four oral lobes, in the centre of which is placed the mouth ; /, 

 Tentacles attached to the margin of the umbrella ; c, One of the radiating canals of the umbrella ; 

 in, Sense-organ; r, Reproductive organ, projecting into a " sub-genital pouch" {go). (After 

 Claus and Sedgwick.) 



The markings by which the former existence of Jelly-fishes may be 

 recognised are of different kinds in different cases. In one group of 

 cases the body of the Jelly-fish has fallen after death upon the fine 

 mud of the sea-bottom, or has been thrown up on the shore, and an 

 impression representing the swimming-bell, and in some cases the 

 tentacles, of the animal, has thus been formed in the soft sediment. 

 Upon impressions of this nature in the lithographic slates (Jurassic) 

 of Solenhofen, Haeckel has founded various genera. Some of these, 

 as previously noted, are believed to belong to the group of the 

 Trachymedusce ; but others, such as Rhizostomites (fig. 90) and 



