OF THE ACALEPH^ OF NORTH AMERICA. 353 



animal. With the power whicii these animals enjoy of opening widely or shutting elosely 

 their anterior and posterior lobes by contraetion and dilatation, bringing them alternately 

 close together or stretching them forwards and backwards, the general appearance of 

 the animal is constantly so completely changing, that it requires long acquaintance with 

 them fully to appreciate the connection of all the parts in their diflerent attitudes, and 

 the influence of the movements of certain parts upon the position of others, and upon 

 their functions. The activity of the circulation through the chymiferous tubes, and the 

 position the main branches of the central cavity assume in these different changes of 

 the general form, are constantly modified, as are also the width of the body and the 

 power of its contractions. But in the same proportion that the extent of the longitudinal 

 diameter is modified by the expansion and contraction of the anterior and posterior lobes, 

 the height of the animal, compared to its width and length, is also constantly chang- 

 ing. If we add to tliis the diversity of images which are brought before us, when we 

 watch these animals in their various movements, from different sides, facing alternately 

 the longer or the sliorter diameter, the sides, or the upper or lower surface, I venture 

 to say that it is impossible to make correct descriptions, and to give true representations 

 of such animals, unless they have been watched for a long time in a living state ; for it is 

 utterly impossible to examine their forms out of the water, as all parts then collapse, fall 

 together, break in pieces, or dissolve into a shapeless mass. And, although 1 acknowl- 

 edge the great interest of the descriptions published by travelling naturalists, making us 

 acquainted with the great diversity of types of these remarkable animals all over the 

 world, satisfactory illustrations cannot be expected from any quarters save those where 

 able observers have resided for a long time, and the accounts of the generic and specific 

 characters of most Medusae must be considered as provisional, so long as they are not 

 revised under favorable circumstances. 



Viewed from above, that is to say, from the anal extremity, with the lobes con- 

 tracted, Bolina appears very much like Pleurobrachia, assuming then the form of a 

 slightly compressed sphere (Plate VII. Fig. 5) ; and were it not for the opposite direc- 

 tion of the circumscribed area, which runs in the longer diameter, while it is transverse 

 to it in Pleurobrachia, the identity would be almost perfect. Seen from below, how- 

 ever, (Plate VII. Fig. 6,) even when the lobes are contracted, the difference is already 

 marked, owing to the circumstance, that the vertical rows of locomotive fringes do not 

 extend uniformly from one extremity of the animal to the other, the two ambulacra of 

 the anterior and posterior lobes being much longer than those of the sides, which 

 terminate at about half the height of the body. 



Viewed in the same position with slightly opened lobes (Plate VI. Fig. 5), the 



48 



