154 CCELENTERATA. 



ORDER DISCOPHORA. 



These soft, gelatinous Hydrozoa, known as " Jelly-fishes," 

 " Sea-nettles," " Sea-blubbers," " Medusse," etc., vary in size 

 from an almost invisible dot to a yard in diameter. Large ones 

 often weigh 50 lbs. ; yet they are little more, as it were, than 

 "coagulated water," for when dried, nothing is left but a film of 

 membrane, thin as a gossamer, and weighing but a few grains. 

 The animal is covered by a very delicate epidermis, undei' which 

 are situated the nettling capsules. There is a distinct muscular 

 system ; but the only motion consists of an alternate contraction 

 and dilatation of the disc, performed with great regularity about 

 fifteen times a minute. 



Acalephs have left few traces in sti-atified deposits, owing to 

 the extraordinai-y softness of their bodies. 



No. 489. [962, CastJ. Acalepha deperdita, Bey. 



Two SPECIMENS. The remains of this frail creature are circular imprints, 

 composed of very regular concentric circles, on the borders of which are eight 

 rays. These are the only sure remains of a true Acaleph which have been 

 found fossil. From the Lithographic limestone (Middle Oolite), Eichstadt, 

 Bavaria. 



SUB-CLASS GRAPTOLITID.C. 



The Graptolites were first considered of vegetable origin; sub- 

 sequently they were regarded as extremely slender Orthoceratites; 

 but Portlock has pointed out their analogy with Sertulariay 

 or the lowest forms of compound Hydroids. They consist of 

 sessile polypite cells (hydrothecfe) arranged in one or two rows 

 on flexible, tubular stems, which radiate from a central disc. 

 They seem to have been free-floating colonies. The specimens 

 usually observed are fragments of stems in a flattened condition, 

 presenting only a serrated edge ; they seldom preserve more of 

 their substance than a carbonaceous or corneous film of extreme 

 tenuity. Graptolites are exclusively and characteristically Cam- 

 brian and Silurian fossils, and prevail in argillaceous deposits. 



