Chap. III. GENUS PLEUROBRACIIIA. 207 



have lieen identified Avitli each other by different writers in different genera, and 

 therefore require a complete revision. 



The type under consideration, for which I retain the name of Pleurobrachia 

 as the most ancient applied to species of this particular conformation, is one of 

 those which are deprived of peripheric lobes, that is to say, in which the gelatinous 

 body is undivided, and the mouth constantly turned upward or forward when in 

 motion; while the genus Bolina, to which I shall next call attention, is one of 

 those in which one extremity of the sphere is split into two lobes, between which 

 tlie mouth is situated, and in which this opening is almost constantly turned 

 downward when the animal is moving, though sometimes, when the animal is at 

 rest, it turns in the opposite direction, opening widely its two lol^es. It will be 

 oln'ioTis how great mistakes may arise from comparing two animals constructed 

 upon the same plan, but kept in a reversed position when contrasted. Unhappily, 

 all these animals have been figured without reference to the normal position in 

 wliicli 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 Pleuroln-achia is limited to those species of Cydippida^ in which the 

 body is nearly spherical or slightly elongated, and slightly compressed laterally ; 

 the locomotive flappers extending from near the margin of the moi;th all round 

 the sphere, in eight vertical rows, towards the opposite centre, where they approach 

 much closer to each other than on the side of the mouth. The tentacular sacs are 

 wide and arched sideways, their bottom rising toward the actinal pole, while the 

 aperture is turned towards the abactinal side of the spherosome, and opens in the 

 interambulacral space between the lateral pairs of spheromeres. Pleuroln-achia differs 

 from Eschscholtzia chiefly in the development of the rows of locomotive flappers, 

 which, in the latter genus, do not extend beyond half or two thirds of the whole 

 height. This is also the case with Dryodora (Gegenl)aur's Mertensia, and Mertens's 

 Beroe glandifonnis) ; but Dryodora has simple tentacles, while they bear lateral 

 threads of a imiform structure in Eschscholtzia and Pleurobrachia, and two kinds 

 of appendages in Hormiphora (Cydippe hormiphora, Gegenh.). Janira is chiefly 

 characterized by the j)i"ominence of the actinal diameter. 



I know at present only three species of this genus sufficiently well to char- 

 acterize them as distinct species : ^ one is the common Plcurohmchla of the shores 



^ Aftor defining the natural limits of the genus relating to them. All I am able to do is ti) point 



Pleurobrachia in accordance with its structural pe- out those I believe to belong here. Al)oui Pleuro- 



culiarities, it would be desirable to enter into a brachia Pileus Flem. there can be no doubt : it 



critical comparison (if its species ; but this is impossi- is the common species of the German (Jccan, with 



ble, owing to the imperfection of the illustrations which Cydippe Pileus Esch. is identical. Judging 



