186 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



Semaeostomeae in our cold northern waters. They have numerous 

 mouths and no marginal tentacles, and their gelatinous substance is 

 often as rigid as newly-formed cartilage. Often they occur in 

 swarms in the harbors, swimming strongly against the tide, and after 

 a storm great numbers are found stranded upon the shore. The rigid 

 gelatinous substance of these jelly fishes is very characteristic and was 

 seen in fossil forms found in the Jurassic lithographic slates of Stein- 

 heim in Bavaria, showing that in the age of the Reptiles these most 

 highly differentiated jellyfishes existed. Indeed, so old are the jelly- 

 fishes that their relationship to the echinoderms, sponges, and cteno- 

 phores remains unknown; all intergrading forms, if such ever ex- 

 isted, having died out long ago, as is often the case in very ancient 

 orders. 



In an early stage of development the higher animals commonly 

 pass through a condition in which they have only an outer cell layer 

 and a cavity lined by cells destined to form the intestine. Theo- 

 retically speaking, they are simply little 2-layered sacks, the outer 

 layer being the external skin with its nervous and sensory organs, 

 and the inner layer being the stomach ; and thus the name gastrula 

 is applied to this stage. Jellyfishes are essentially in the gastrula 

 stage, even when adult. Yet so extraordinary are the foldings, out- 

 growths, and adaptations that have arisen in their two body layers 

 during the vast time they have existed upon the earth that, ulti- 

 mately, simple as they are, no class of the animal kingdom exhibits 

 a more surprising variety of forms than do the jellyfishes and their 

 close allies the Siphonophorae. 



It is interesting to observe that the large jellyfishes, Scyphome- 

 dusae, which have gastric cirri and no marginal diaphragm or velum, 

 are probably only very remotely related to the small jellyfishes, the 

 hydromedusae, which have a velum and lack gastric cirri. Indeed, 

 we have good reason to believe that the jellyfish-shape and peculiar 

 locomotion through pulsation have been derived independently in the 

 two groups. The Scyphomedusae are probably allied to the actinians 

 or sea-anemones, while the hydromedusae have probably been derived 

 from hydroids. In fact a jellyfish shape and pulsating body have been 

 acquired independently in widely different kinds of animals, such as 

 Pelagothuria, a holothurian which bears a wonderfully close resem- 

 blance to a jellyfish and swims actively through the water in the 

 tropical Pacific; and in Craspedotella, a minute unicellular marine 

 animal, which would certainly have been mistaken for a jellyfish had 

 it not been of microsopic size. 



Indeed, there is reason to lead us to believe that the bell of the 

 Narcomedusae is a mere outgrowth from the sides of the pyriform 

 larva, and has thus been acquired in a manner quite different from 



