OF THE ACALEPHiE OF NORTH AMERICA. 271 



The outer coating consists of several layers of minute nucleated cells, some of which 

 among the superficial ones are more prominent, giving the tentacle a ragged appearance 

 under a high magnifying power. Among these cells, there are some minute ones, prob- 

 ably stinging cells, the nucleus of which is like an arrow-head, though I have not been 

 able to see any of them throw out their arrows, as they do in Sarsia. 



The tip of the tentacles (Plate II. Fig. 2, 4) is rounded, and there is no trace of an 

 aperture or puncture, the whole extremity being surrounded by these small superficial 

 cells. The last cell of the interior row, by its hemispherical form, precludes all sup- 

 position of a central canal in these tentacles. The terminations of two of these ten- 

 tacles are represented in Plate II. Fig. 2, 4; and the elongation of these cells, es- 

 pecially as seen in Fig. 2, can leave no doubt respecting the correctness of the view 

 I have expressed above, that this middle row is the active portion in the contractions 

 of the tentacles. This description leaves no doubt as to the difference existing in the 

 structure of the tentacles in different genera of the family ; as we have seen in Sarsia 

 the circular chymiferous tube radiating into the four marginal tentacles, whilst in Hip- 

 pocrene there is no such communication between the two organs. 



The nervous cord (Plate I. Fig. 2, 3, 4 ; Plate III. Fig. 1, 2, 3, 4, 5, 7, 8, 9), which 

 extends from bud to bud, with a ganglion at the base of each bud, is not the only part of 

 the nervous system which I have been able to trace in this animal ; and though the other 

 portions of this system differ widely from this main cord, I entertain scarcely a 

 doubt as to their real nature. On examining minutely the walls of the vertical tubes, 

 which are so uniform in their homogeneous structure, it is easy to discover on their inner 

 surface one or two threads (Plate III. Fig. 5), following along the whole length of these 

 tubes. Below the curve where the tubes bend to form the loop which is connected with 

 the central digestive cavity, these threads combine into a plexus (Plate III. Fig. 6) of 

 intricate fibres, from each of which arises another thread extending horizontally from 

 one plexus to another (Plate III. Fig. 5), so that there is a circle of such threads all 

 round the upper part of the chymiferous system, or below the centre of the gelatinous 

 disk. This circular thread, considered in itself, may therefore be viewed as a ring 

 encircling the upper part of the chymiferous system, forming a plexus of similar 

 threads under the curve of each radiating tube, from which identical threads may be 

 traced downwards to the sensitive bulbs, in which they also merge. From the 

 middle of each upper horizontal thread, there arises a vertical thread extending down- 

 wards, and following the main course of the large inner vertical bundles of muscles. 



I am not fully satisfied of the real nature of this apparatus, on account of the great 

 difference there is between these threads and the sensitive cord of the lower margin. 



