﻿OF THE ACALEPHiE OF NORTH AMERICA. 233 



From this state of things as observed among Medusa?, we should be prepared to find them 

 endowed with the power of feeling, even if the sensitive cells were more diffuse, and did 

 not appear as continuous chains ; and this is probably the condition of the sensitive appa- 

 ratus in the Polyp ; in which, notwithstanding the care bestowed upon a thorough inves- 

 tigation of the tissues, I have not succeeded in discovering any thing similar to what is so 

 plainly seen in Medusae. There are, however, layers or heaps of peculiar cells in various 

 parts of the body ; but only the contractile cells assume a linear arrangement, a fibre- 

 like disposition in particular rows. 



We have here, therefore, a peculiar type of the nervous system, a type different from 

 all those types which have yet been recognized in the animal kingdom. And such a 

 nervous system I have already traced in all its details, as here described, in the genera 

 Hippocrene, Tiaropsis, and Staurophora. The illustrations accompanying this paper are 

 perhaps less complete for the Sarsia, though I first traced it in this genus ; but afterwards 

 some peculiarities of the genus Hippocrene, and especially the greater facility of keep- 

 ing it alive for a long time under the microscope, and of watching all the minute struc- 

 tural details upon the living animal, have induced me to represent them more extensively 

 from Hippocrene than from Sarsia, which, owing to its lively, active habits, is more dif- 

 ficult to keep in a steady position under the microscope, unless it be mutilated to prevent 

 it from running about. 



I may mention on this occasion, that all I have to say of the structure of Medusas 

 has been traced upon living individuals. I do not mean upon individuals taken out of 

 the water alive to be placed under the microscope, but upon individuals preserved alive in 

 narrow glass jars while under the microscope, surrounded by a sufficient quantity of water 

 to keep them alive during the whole period of investigation, and to preserve them after- 

 wards, to be placed again and again under observation ; so that comparatively few speci- 

 mens have been sacrificed for these studies, though a large supply was constantly at hand, 

 that their habits and development might be watched. 



In order to study the structure of marine animals in such a way, it is necessary to 

 plunge, without hesitation, the objective glass into the water, as it were otherwise im- 

 possible to follow with sufficient quickness the animal in its motions, or to adapt the mi- 

 croscope to the proper focus, if it were to be alternating through the media of air and 

 water, or through both together. By keeping it constantly under water, a great deal of 

 inconvenience is avoided ; and, with some practice, one succeeds soon in employing in 

 such investigations even higher magnifying powers, such as systems I. 6 of Ober- 

 hauser's microscope, and to reserve only the highest powers for more steady investiga- 

 tion, with a completely immovable apparatus. Without alluding to the advantage there 



