OF THE ACALEPH^ OF NORTH AMERICA. 235 



upon itself in a revolving curve, and then dart again suddenly straight forwards in any 

 given direction. Of course, the changes of form which it assumes in these different 

 movements are almost endless ; and tiiough several are represented in Plate IV., they 

 hardly give a comj)lete idea of the beautiful diversity of aspect which this animal ex- 

 hibits in its movements. What increases the variety of its aspects beyond the change of 

 form of the main body, the shortening and elongating of the tentacles, and the shutting 

 and opening of the main cavity, is the disposition of the proboscis, which is either entirely 

 contracted within the main cavity near its upper centre, or hangs down to the margin of 

 the opening, or stretches out between the tentacles for nearly as great a length as the 

 whole diameter of the body, in either a straight line, or variously bent in graceful undula- 

 tions, or curved upon itself. Though the usual form of these animals is rounded, it may 

 be seen at times to contract in such a manner as to assume a flattened shape in its lower 

 part by the compression of its sides ; and this is especially the case when the animal turns 

 round upon itself, and changes its direction in its movements. (Fig. 8.) 



Again, when it pauses and remains in that state of rest for a longer time, the lower 

 margin is frequently seen to assume a square or quadrangular form ; especially when it is 

 perfectly immovable, and the tentacles are stretched out at right angles from the lower 

 margin for a considerable length. (Plate IV. Fig. 1.) 



On watching minutely its outline, it will be observed that the sides are not always 

 circular, but from the contraction of the layers or bundles of motory cells, it assumes a 

 quadrilobate appearance, especially when those radiating bundles which alternate with 

 the alimentary tubes are powerfully contracted. 



The main bulk of the body consists of a gelatinous mass, forming the bell-shaped, 

 central part of the animal. This is thickest above in the central part of the swollen 

 disk (Plate V. Fig. 7, 11) ; towards the sides it gradually tapers, and becomes very thin 

 near the lower margin about the origin of the tentacles, where it is suddenly turned 

 inward at right angles with its previous direction, and forms the transverse separation 

 between the main cavity of the body and the surrounding medium, a large hole, how- 

 ever, being left in the centre, through which the proboscis plays at ease. This lower 

 opening (Plate IV. Fig. 2, 3, 4) is comparatively much wider than in the genus 

 Hippocrene, and less so than in Tiaropsis and Staurophora. 



At first, when watching the animal in its movements, it would seem as if the gelati- 

 nous mass itself were the cause of locomotion ; but, upon close examination, it is easily 

 found that it is merely an elastic support for the active apparatus of motion, which con- 

 sists of layers and bundles of contractile cells diversely arranged. There is an external 

 system of these bundles immediately under the epidermis, through the agency of which 



