Chap. m. GENUS PLEUROBRACHIA. 209 



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 position, 

 I feel compelled to enter into some tedious details respecting this slight variation 

 from the spherical form ; for, though scarcely appreciable, it has a direct liearing 

 upon the connection of all the organs, which, upon close examination, are found to 

 l^reserve, throughout the order of Ctenophora?, a constant relation to this apparently 

 insignificant difference between the three diameters of the body, — so much so that 

 these gloljular animals are as truly bilateral in the arrangement of all their parts, 

 as any other species of the whole group. 



The mouth opens transversely ; and there is upon the ojjposite pole of the 

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

 transversely to the greater diameter. The two tentacles with their fusiform sockets 

 are placed at right angles with the transverse split of the mouth and the opposite 

 oblong area, the tentacles being in the diacoeliac diameter, the mouth and the area 

 in the coeliac diameter, and the main axis of the digestive cavity trending vertically 

 between them. The rows of locomotive flappers 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, — no one being placed either in the prolongation of the mouth or in that 

 of the bases of the tentacles (PI. II^ Flys. 20, 21, 22, and 23). 



Owing to the compression of the Ijod}-, and the difierence in the curvature of 

 its actinal and abactinal sides, the eight rows of locomotive flappers have their upper 

 and lower halves bending in a somewhat different manner. Again, two pairs, per- 

 fectly equal, inclosing the base of the tentacles, stand in antitropic relation to one 

 another along the prominent side; while two other pairs, inclosing the prolongation 

 of the angles of the mouth and the circumscribed area at the opposite pole, extend 

 in a similar manner along the flattened side. The consequence of this arrangement 

 is, that each side of the body has two equal rows of locomotive fringes placed 

 in a symmetrical manner opposite each other from side to side and crosswise, 

 those of opposite sides being identical, but differing from those which stand at right 

 angles with them. 



Having thus ascertained that the body of this animal is neither vertically nor 

 transversely cn-cular, and that there is a medial axis with reference to which the 

 arrangement of all the parts is regulated, the question at once arises how we 

 should consider these diameters : whether the mouth shoidd be placed upward 

 or downward, or whether it should be considei'ed as the anterior extremity, and 

 what are the relations of the sides. As with the other Medusa?, whatever view 

 we take of the subject, when we compare these animals with either Polypi or 



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