1869.] 107 [Agassiz. 



lines ; they meet them floating about a short distance above the bot- 

 tom of the sea. The Astrophyton is not as sensitive to disturbance 

 as the starfishes are ; hence in the same experiment made with a star- 

 fish, the animal would be more likely to sink if disturbed when 

 expanded. The solid limestone shell of these Echinoderms is not so 

 solid at it looks; a section made across the plates of an Echinus, of an 

 Ophiuran or starfish, still reveals to us the original mode of building 

 up of this limestone structure, so apparent in the younger stage of 

 the Echinoderms, which is nothing but a system of net work of lime- 

 stone meshes, becoming closer and closer as the animal grows older; 

 and still leave, even in the adult, an immense area for the absorp- 

 tion of the water which all these animals are able to draw into their 

 cavities, and must permeate all their interstices, thus equalizing thb 

 pressure to which they must be subject at great depth, by thus pene- 

 trating everywhere, and permeating every part of their frame. The 

 large percentage of water which starfishes and sea-urchins are capable 

 of taking into their cavity, is shown by the following figures, made up 

 by weighing fully expanded Echinoderms which have been in water 

 a considerable time, and then weighing them again after they had 

 been exposed to the air, but without allowing them to dry. 



The average loss of thirty-three specimens of various sizes of 

 Asteracanthion berylinus Ag., weighed as mentioned above, was .795 

 due to the water which the specimens lost when exposed to the air. 



The average loss of seventeen specimens of A. pallidus Ag., was 

 .815. The greater loss in this species is due to the greater flexibility 

 and extensibility of the abactinal area than in the preceding species. 



The average loss of eight specimens of Toxopneustes drobachiensis 

 Ag., was .554. 



When we come to other classes, such as the Acalephce, the loss of 

 water is still greater, being, on the average for our large Aurelia 

 Jlavldula Ag., no less than .997, so that the jelly fishes seem nearly to 

 be in the condition most favorable to resist great pressure, that of a 

 spheroidal shell with an infinitely thin envelope suspended in the 

 water at any depth it may be placed. 



