184 CASWELL GRAVE. 



the perfect radial symmetry of other structures, which are char- 

 acteristic of the adult condition and which arise, in every known 

 case, by the remodelling of the structures of a larva which is 

 bilateral in its entire organization, than by assuming that the 

 group has been derived from a bilateral pelagic organism, similar 

 to the one described above, which at a very remote period in its 

 existence exchanged its free swimming life for one on the bottom 

 during which it became fixed. 



Briefly stated, the steps by which the present organization of 

 an echinoderm are generally accounted for, and for which there 

 is more or less evidence, are as follows : Pelagic life was given 

 up for one on the bottom because of less competition and a 

 greater food supply in the latter place. The preoral lobe be- 

 came gradually modified into an organ for fixation. The mouth, 

 at first directed downward as was its position in pelagic life, 

 gradually moved to the left until it took up a position in which 

 it was directed upward. This, in a fixed animal, feeding upon 

 microscopic organisms, is its most favorable position as is shown 

 by its position in animals which exist at present under these con- 

 ditions. During the migration of the mouth and oesophagus, 

 those organs of the left side which would obstruct such a move- 

 ment (left middle and posterior body cavities) were carried along 

 and each became drawn out into the shape of a horseshoe and 

 greatly hypertrophied. In the final position of each, the opening 

 of the horseshoe was directed anteriorly. The middle and pos- 

 terior body cavities of the right side also became changed in 

 position and correspondingly reduced in size. The left middle 

 body cavity retained its connection with the exterior through the 

 greatly reduced anterior left body cavity and its duct, but the 

 duct of the right side disappeared. During this period when 

 food was plentiful and easily accessible and when no energy was 

 used in locomotion, a rapid increase in the size of the creature 

 took place and radial symmetry was developed. There is such 

 a diversity of opinion, however, as to the details of this process 

 that I will attempt to give but one ; that which has been sug- 

 gested by my own observations. 



The ciliated rings were useful to the free-swimming animal not 

 only as organs of locomotion but were used in feeding as well, 



