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G. H. PARKER. 



quently a given individual may have more than one madreporite 

 as a preparatory step to division and may have arms of different 

 length as the result of regeneration after division. Crozier studied 

 the locomotion of Coscinasterias in relation to the length of the 

 arms and the position of the madreporites and found that both 

 factors had an influence in determining the axis of motion. Al- 

 though the animal could move in any direction, there was an 

 obvious tendency to move more generally in the direction of the 

 long arms and on the axis determined by the position of the madre- 

 porite, and of these two factors the position of the madreporite on 

 the whole predominated. Thus in all the starfishes that have been 

 investigated in this respect a physiological anterior has been dis- 

 covered, and this anterior seems to find a structural indication in 

 the position of the madreporite whereby an arm next this organ 

 commonly becomes a director. 



Among the crinoids Clark (1915) has shown that Comatula 

 fiurpiirca moves with its long arms anterior, but whether the- axis 

 thus established agrees with that in the starfishes can not be ascer- 

 tained because of the absence of the madreporite from crinoids. 

 Comatula must, however, be admitted to possess a physiological 

 anterior. 



Few observations have been made on the direction of locomotion 

 in the sea-urchins. Holmes (1912), in his study of the photo- 

 tropism of Arbacia, describes the locomotion of this sea-urchin as 

 though it were free to move in any direction, yet he makes no 

 specific statement on this point. Crozier (1920^), who studied 

 the bionomics of the sand-dollar, Mclllta, found that this animal 

 had a very definite axis of locomotion which corresponded with its 

 structural axis of bilaterality. In Mcllita, as in other sand-dollars, 

 the mouth is at the center of the underside of the disc, but the 

 anus is on the edge of the disc in an interradial position. A line 

 drawn through the anus and the center of the disc in Mcllita marks 

 the axis of locomotion and the forward end of this axis in creeping 

 and in digging is the end farthest from the anus. The madreporite 

 in the sand-dollar is central in position and hence can not be used 

 in homologizing rays as in other sea-urchins, but by comparing the 

 conditions in Mcllita with those in the spatangoids, in which the 

 symmetry is also bilateral but the madreporite is excentric, it can 



