Chap. HI. GENUS IDYIA. 275 



mouth to the utmost, and woi'king its prey down into the digestive cavity by 

 repeated contractions, slowly overlapping with the edge of the mouth more and 

 more of the large morsel it is attempting to swallow, until it is finally engulfed. 

 Fig. 10 represents an Idyia immediately after it had swallowed a Bolina nearly as large 

 as* itself, the outlines of which may be seen in its distended cavity, in a position 

 transverse to the vertical diameter of its own body. If the animal seized upon 

 is too large to be swallowed entire, after forcing into its digestive cavity what is suf- 

 ficient to fill it, our Idyia will, by powerful contractions of the margins of the mouth, 

 cut off the parts which cannot be swallowed. I have once seen an Idyia, of about 

 the size of that of Fig. 7, seize upon a Bolina nearly double its own size, and, 

 after working the abactinal part of the Bolina into its digestive cavity, cut off in 

 that way about two thirds of the actinal side of the Bolina and let it drop. This 

 operation lasted for about an hour; and while portions of the swallowed body showed 

 signs of life in the contraction of the locomotive flappers during three quarters of 

 an hour, the process of digestion was nevertheless going on so fast, that, after an 

 hour and a half, fragments of the indigestible parts, such as the locomotive flappers, 

 began to be discharged through the mouth. In four hours, the whole portion 

 introduced into the digestive cavity had disappeared from it, the more fibrous cell 

 walls and the locomotive flappers being thrown out through the mouth and the 

 more fluid portions passing into the chymiferous system, so that the main chy- 

 miferous cavity and all the ch3Tniferous tubes were distended to the utmost, and 

 the fluid contained in them was moving rapidly up and down through the ambu- 

 lacral tubes into the oral tube and back through the coeliac tubes. Shortly 

 afterward the two coeliac apertures opened successively and discharged some more 

 of the indigestible matter, and the animal seemed as empty as before, with this 

 difference only, that the interambulacral zones, which, when the animal has been 

 fasting, are depressed, and the digestive cavity itself very much flattened, were now 

 distended and presented a rounded outline, as in Fig. .3. On another occasion I 

 noticed a large Idyia swallowing a whole Bolina of sufficient size to fill its cavity; 

 and yet, after five hours, no trace of the prey coidd be observed within it. 



From the preceding remarks it may be inferred how difficult it is accurately 

 to describe these animals without prolonged study, under the different circumstances 

 which may modify their appearance. But after collecting many hundreds and 

 keeping them together for weeks at different periods of their growth, in a large 

 tank well supplied with food, I may well say, that the different illustrations pub- 

 lished of allied animals observed in other parts of the world, though showing the 

 existence of the genus Idyia in all seas, do not j^et furnish us with the means 

 of distinguishing the species inhabiting different zoological provinces with sufiicient 

 precision. For not only do the young differ from the adult in the manner already 



