304: . CONTRIBUTIONS TO THE NATURAL HISTORY 



substance, contained in rather large cells, like those of the vitreous humors of the eye, 

 covered externally by a flat pavement epithelium of irregular nucleated cells, forming 

 a film which is deciduous soon after the animal has perished ; but even during life, 

 after injuries upon the surface, this coating is seen to peel off, and the animal grad- 

 ually decays by superficial decomposition. 



The disk is flat-hemispherical, its margin more or less expanded as the animal is in 

 a more relaxed or contracted state. Muscular fibres run in two different directions 

 through its whole mass ; part of the fibres being radiate, and others concentric. The 

 radiating fibres are in larger bundles, and appear particularly active upon three points in 

 each of the four intervals embraced by the four arms of the cross ; there is, however, 

 also a bundle in the prolongation of the ray of the cross itself, and similar smaller bun- 

 dles alternate again with those already mentioned. So that, upon contraction, its 

 radiating muscles may cause the disk, or at least its margin, to fold into more or 

 less numerous lobes, according to the intensity of contraction of the more powerful 

 of these bundles. 



The concentric muscles are disposed in such a manner as to form concentric squares 

 rather than concentric circles, the sides of which extend from one arm of the cross to 

 the other ; and upon strong contraction, assisted by the main radiating bundles which 

 alternate with the rays of the cross, they give the animal in its most powerful contrac- 

 tions the appearance of a square body rounded upwards. 



The diversity of forms and appearance which this animal may assume in its con- 

 tractions is so great, that I have been induced to multiply the figures representing it, in 

 order to give a full idea of their distinctive peculiarities. It was the more important 

 to do so, as in a dying condition they would at times assume permanently one or the 

 other of these forms, which, if observed singly, might easily be mistaken for a special 

 modification of structure, and give rise to an erroneous distinction of species. But 

 as I have observed these animals for days and weeks, I know that these are only tem- 

 porary modifications of the same species, as I have seen the same individual present, at 

 different periods, all the forms here illustrated. (Plate VII. Fig. 1 -8.) 



We must consider as the margin of the disk the edge from which the tentacles spring 

 and hang down, though this edge is more or less curved in when the animal is in gentle 

 activity, as this is the most prominent margin when the animal is in the greatest state of 

 relaxation, and spreads almost flat like a disk (Fig. 9). The tentacles are very numerous, 

 seemingly of equal length and size ; yet upon close examination it may be seen that 

 there are thicker and thinner, longer and shorter ones, alternating with each other ; but 

 the difference is so slight as, at first view, not to strike the observer. Their length is less 



