OF THE ACALEPHiE OF NORTH AMERICA. 317 



Unliappily, all these animals have been figured without reference to the natural position 

 in which they should be compared, and, no allusion to these prominent differences 

 being made, it is hardly possible to reconcile the descriptions of one author with 

 those of another. 



The genus Pleurobrachia is limited to those species of Beroid Medusa; in which 

 the body is nearly spherical or slightly elongated, the locomotive fringes extending 

 from the margin of the mouth all round the sphere, in eight vertical rows, towards 

 the opposite centre, where they approach very near to each other. This genus 

 differs from Cydippe chiefly in the extensive development of the rows of locomotive 

 fringes, which, in the latter, do not extend below two thirds of the whole height. 



I know at present only two species of this genus sufficiently well characterized to 

 be recognized as distinct species ; one, the Pleurobrachia common on the northern 

 shores of Europe, and the other that Pileus which I have observed on the eastern 

 shores of the Northern United States, and for which I propose the name of Pleuro- 

 brachia rhodoductyla, from its long, light rosy-colored tentacles. 



Though almost spherical, this species is slightly compressed in the vertical plane 

 of the two tentacles, so that one diameter, at right angles with the base of the ten- 

 tacles, is somewhat shorter than that which would pass through their points of at- 

 tachment. 



As it is of great importance to the full understanding of the internal structure 

 of this animal, and the correct appreciation of all its organs, to form a correct idea 

 of their respective location, I feel compelled to enter into some tedious details re- 

 specting this slight variation from the spherical form ; for though scarcely appreciable, 

 it has a direct connection with the bearing of all the organs, which, upon close ex- 

 amination, are found to preserve throughout a constant relation to this apparently in- 

 significant difference between the diameters ; so much so, that these globular animals 

 are truly bilateral in the arrangement of all their parts. 



In the first place, the mouth is split transversely, and there is upon the opposite 

 pole of the sphere an oblong, narrow, circumscribed area, placed also in the same 

 direction, transversely to the longer diameter. So that the two tentacles with their 

 bases are placed at right angles with the transverse split of the mouth and the op- 

 posite transverse area, the former being in the longer diameter, the mouth and the 

 area in the shorter. The rows of movable fringes alternate, two and two, with 

 these four radiating directions. So that there are four rows on one side of the 

 plane passing through the tentacles, and four on the other ; and also four on one side 

 of the plane passing through the mouth and the opposite area, and four on the other, 



