92 LOWER INVERTEBRATES. 



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

 it still keeps up a flapping motion of its bell probably for purposes of breathing, just 

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



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

 the food. They wind themselves about their prey, sting it to death, and then, by con- 

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

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

 struction of the mouth of Cassiopea shows that its food is of very small size. The 

 medusa feeds upon the animal and plant 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 with 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 the adults of Gyanea, Aiirelia, and several other genera. 



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

 animals which upon 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 Ascidlans, where the free larva has higher afiinities 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 highly developed as in any of its relatives Mhich 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 somewhat 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 which corresponds to the mouth of other 

 Discophora. 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 through which they pass 

 when 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. Each appendage is branched, and from its aboral surface there is 

 formed a great number of curious ajipendages 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 Sydrce. Of these there are a 

 large number on the oral appendages, and each and all ojjen into a system of vessels 

 which pass through the appendages, and ultimately pour their contents into the cen- 

 tral cavity of the oral cylinder. All of these Hydrce together make up the mouth of the 

 medusa, for thej^ are the orifices through which food is taken into the stomach. The 

 second prominent appendages to the oral arms are small, flask-shaped, and ovoid 

 bodies, with a central cavity which opens into the vessels jiaseing through the arms. 

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

 tion is not yet definitely known. 



A most interesting family, the Pelagid^, is represented in our waters by two 

 genera called Pelagia and Dactylometra. In Pelagia we have a spherical-shaped 



