318 CONTRIBUTIONS TO THE NATURAL HISTORY 



no one being placed either in the prolongation of the mouth, or in that of tlie bases 

 of the tentacles (Plates III. and IV. Fig. 3 and 4). 



Again, owing to the slight flatness of the bodj, the four rows of fringes have 

 their upper and lower arms bending in a somewhat different manner, so that there 

 are two pairs perfectly parallel with each other along the prominent side, inclosing 

 the base of the tentacles, which are perfectly equal, and two other pairs along the 

 flattened side, inclosing the prolongation of the angles of the mouth and the trans- 

 verse projections of the opposite area. The consequence of this arrangement is, that 

 each segment of the body has two unequal rows of locomotive fringes placed in a cor- 

 responding symmetrical manner opposite each other, side by side, or crosswise. 



Having thus ascertained that the body of this animal is not truly circular or spheri- 

 cal, and that there is a medial axis, with reference to which the arrangement of parts 

 is regulated, their four planes meeting at right angles, one of which passes through the 

 longitudinal diameter of the mouth and the corresponding diameter of the ojjposite 

 area, and another through the bases of the tentacles, which are prolonged into the 

 interior of the body, the question at once arises how we should consider these rays ; 

 whether the mouth should be placed upwards or downwards, or whether it should 

 be considered as the anterior extremity. As with the other Medusae, whatever view 

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

 noderms to ascertain their homologies, we must, as a matter of necessity, bring them 

 all into the same respective position, and contrast the arrangement of their parts in 

 their mutual correspondence. There is, therefore, no difficulty about this point, in- 

 asmuch as the mouth is made in every case the central point of comparison. But 

 it has been ascertained that Polypi, though absolutely radiated animals, have in most 

 of their types, if not iu all, a rudimentary indication of a longitudinal axis in the 

 oblono- form of their mouth, which is the first indication in the animal kingdom of a 

 bilateral symmetry, occurring even among the lowest Radiata, while in Echinoderms 

 it rises hif^her and higher, and becomes so prominent in Spatangoids as to influence, 

 not only the general form, but even the number and arrangement of the internal 

 parts, and the length and special development of the external appendages and the 

 ambulacral rows. 



The class of Acalephse, which is intermediate between those of Polypi and Echi- 

 noderms, holds in these respects also an intermediate position. Here we have a slightly 

 compressed body and an oblong mouth. But the mouth opens in a direction trans- 

 verse to the elongation of the body. The question therefore is. Does the mouth, 

 with the plane which passes through the mouth and the opposite area, indicate the 



