OF THE ACALEPH^ OF NORTH AMERICA. 247 



that the main cord of the sensitive system extends round the lower margin between the 

 eyes, and is connected with otiier cells forming a row behind each vertical tube, and 

 another row round the central digestive cavity ; and that distinct muscular contractile 

 cells occur in four different systems, following different courses, which are easily dis- 

 tinguished in the main from the sensitive cells. 



After arriving at such results, and upon comparing the elements of these tissues with 

 each other, and finding them so near alike, it may even be questioned whether there is 

 so strict a distinction between their functions in these low animals as we notice in higher 

 types. And I should not be at all surprised, if it should be ascertained that even the 

 cells of the circular, sensitive cord below contract in the motions of the animal, as well 

 as the truly muscular cells ; and if these cells, in their turn, should be found to enjoy the 

 power of feeling or perceiving impressions similar to those which act upon the truly sen- 

 sitive cells. So much, at least, is certain, that the nettling cells, which are truly epithe- 

 lial cells, are capable of contracting or throwing out the coiled thread within them, and of 

 being excited to perform their functions ; — which would indicate in them, also, a certain 

 degree of sensitiveness. Indeed, there is no philosophical ground for considering the 

 structural elements in any way more distinct from each other in their nature, than their 

 real structure warrants, which is throughout cellular ; and where we have animals of a 

 large size, living as these do, and performing such complicated functions as these are 

 known to perform, without any other constituent elements besides cells, there is no reason 

 to consider each set of these cells as absolutely distinct in its functions. I would men- 

 tion in particular, that I have ascertained, by direct observation, that many of the threads 

 which Dr. Will has described as connected with the upper ganglion in Beroe, are con- 

 tractile threads, notwithstanding their connection with a distinct ganglion. Should it 

 be objected, that this would only go to show that Dr. Will has mistaken muscular fibres 

 for nerves, so much would remain clear, — that here muscular tissues combine intimately 

 with nervous tissues, a mode of combination unknown in other animals. This circum- 

 stance, at all events, goes to support the view which I entertain of the subject in general, 

 that the elements of structure in Medusae have not yet reached that degree of distinctness 

 and independence which they present in other groups of the animal kingdom. 



There is another point in the structure of Sarsia which deserves particular notice. 

 Above the central cavity may be observed a little knob rising into the gelatinous sub- 

 stance of the upper portion of the disk, brightly colored, and somewhat resembling 

 the four bulbs of the lower margin, consisting, as the bulbs themselves, of a number of 

 pigment-cells. The form of this knob varies in different individuals of this species, ac- 

 cording. to their size and age. In the youngest, when about a line in diameter, just when 



