OF THE ACALEPHiE OF NORTH AMERICA. 223 



be made out only from living or perfectly frrsli specimens, there are others which 

 may he best observed in those which have I)cen for some time dead. We should, there- 

 fore, be in no hurry to place living Medusae under the microscope ; or to take them out 

 of the water to observe; them more closely. We may first watch their movements, and 

 trace, as far as possible, in the complete specimen, the connection of parts, and the in- 

 fluence wliich the movements have upon the circulation of lluids, and upon the changes 

 of form of the whole body. >Ve may next proceed to the investigation oi" those ex- 

 ternal details which are best traced upon fresh specimens, such as the microscopic 

 structure of the tentacles, the fringes of the mouth, the eye-specks, by cutting such parts 

 from living specimens, which will survive for some time after this operation. But we 

 should carefully preserve the dying and dead bodies, and closely watch the process of 

 decomposition ; for, during the cadaveric stiffening of the animal, tissues may be dis- 

 covered which remain unnoticed in the living or fresh animal. Such is particularly the 

 case with the muscular, or rather contractile, system of cells by which the voluntary 

 movements are effected, and which are best seen when strongly contracted. Of course, 

 the forms exhibited in the contracted state are neither natural nor frequently observed 

 during life, as they only exemplify the action of the contractile tissue in its utmost state 

 of contraction. But if we do not obtain in that way a satisfactory view of the object, 

 we, at least, learn where to look in order to study these parts in a more natural con- 

 dition, and gradually to discern them where their existence was unsuspected before. 

 Even fragments in a decayed state should not be neglected, as the manner in which the 

 parts separate during decomposition affords the means of ascertaining the limits of the 

 different tissues ; some decomposing more readily than others, under the same cir- 

 cumstances. For instance, the easiest way to study the epidermis, or simple layer of 

 epithelial cells, which cover the whole surface of the body of MedusiP, is to allow decay 

 to commence without disturbing them ; when it will be easy, after a certain time, to 

 take off a thin, continuous layer of the most beautiful polygonal, nucleated cells, forming 

 a single layer over the whole surface. 



The most active species, when it is desirable to study them alive, can easily be 

 brougiit to a state of innnobility by ether or chloroform. I have frequently seen even 

 the smallest species, such as Hippocrene, recover from a state of perfect immobility and 

 insensibility to active life again. Even had I not succeeded in finding distinct sensitive 

 apparatus, I should, from this very fact, have been led to claim for them the power of 

 feeling, so plainly shown by such experiments. 



Another method of studying these animals, used frequently to great advantage, has 

 been, to drop them into liquids of various densities, — into fresh water, for instance, or into 



