92 LOWER INVERTEBRATES. 



ially of the edges of its disk, can be plainly seen. Although fastened to the ground, 

 it still keejJS up a flapping motion of its hell probably for puri)oses of breathing, just 

 as is the case with free-swiiuming animals of closely allied genera. 



One of the functions of the marginal tentacles of the Discopliora is the capture of 

 tlie food. They wind themselves about their ])rey, sting it to death, and then, by con- 

 traction, draw it to the mouth. lu a medusa which is fastened to the ground, tentacles 

 would seem to be necessary if the food was large and cajjable of movement. The con- 

 struction of the mouth of Cassio2)ea shows that its food is of very small size. The 

 medusa feeds upon the animal and jjlant life which drifts past it, or which is caused to 

 move over it by the slow flapping of the bell margin. It is therefore evident that ten- 

 tacles would be of little service to an animal witli this mode of life, and accordingly we 

 find its bell margin is wholly destitute of those filaments called tentacles, which form 

 such a prominent feature in tlie adults of Cyanea, Aurelia, and several other genera. 



Throughout the animal world there are several examples which might be cited of 

 animals which uj)on becoming attached to the ground, after a free larval existence, 

 having no use for well-developed sense organs, lose the same or suffer a degeneration 

 in their complication. This can well be illustrated in the development of some well- 

 known genera of Aseidians, where the free larva has higher affinities throughout than 

 the adult, and where a highly-developed organ of sense is formed in a larva to be lost 

 in the fully-grown animal. The organs of sensation on the margin of the bell in Cas- 

 siopea are, however, as liighly developed as in any of its relatives which swim freely in 

 the water. Abnormal as its mode of life is, the otocysts, or organs of sensation, found 

 on the rim of the bell, have not disappeared, neither has their number diminished. 

 In Cassiopea there are sixteen of these bodies in normal specimens, and we also often 

 find monstrosities by which this number is increased to eighteen. Professor Agassiz 

 found twelve of these structures in Polyclonia, a closely related or identical genus. 



The structure of the mouth of Cassiopea is somewliat as follows : In the centre of 

 the oral surface of the bell there is a gelatinous cylinder in which there is a central 

 cavity, but no external opening, in a position wliich corresjjonds to the mouth of other 

 Disco])hora. On the side of this cylinder, however, there are openings, four in num- 

 ber, leading into as many cavities partitioned by a thin membrane from the main 

 cavity in which the sexual products are formed, and perhaps througii which they pass 

 wlien mature. From the oral cylinder there arise eight long arms which are commonly 

 extended at right angles to the cylinder parallel with the lower floor or aboral side of 

 the bell. Their tips extend a little beyond the bell margin, while the side adjoining 

 the bell is smooth. Eacli appendage is branched, and from its aboral surface there is 

 formed a great number of curious appendages of various functions. Two kinds of 

 appendages can be recognized. The former are simply little feeding mouths sur- 

 rounded by a circle of tentacles and resembling little ILjdTm. Of these there are a 

 large nun^ber on the or.il ajijiendages, and each and all ojien into a system of vessels 

 which ])ass through the ap])endages, and Tiltimately ])Our their contents into the cen- 

 tral cavity of the oral cylinder. All of these HydTai together make uj) the mouth of the 

 medusa, for they are the orifices through vi\w\\ food is taken into the stomach. Tlie 

 second prominent ap])endages to the oral arms are small, flask-shaped, and ovoid 

 bodies, with a central cavity which opens into the vessels passing tlirough tlie anus. 

 Tliey are, however, without an opening into the external water, and their true func- 

 tion is not yet definitely known. 



A most interesting family, tlie PelagidvE, is represented in our waters by two 

 genera called Pelayia and Bactylomctra. In Pelagia we liavc a siiherical-siia].ed 



