THE TWENTY-RAYED STARFISH. 



starfish does not only move with the same side all the time as 

 anterior end, but its speed is considerably greater than that of the 

 common star ; its highest speed, e.g., when moving to deeper water 

 during the receding tide, was 122.5 cms - P er minute. Opposite to 

 the largest number of rays, in early post-larval life, is one ray, 

 which is perhaps of the same age as the other primary rays, and 

 with this single ray are young ones, varying in strength and age. 

 As a matter of fact, it should be expected that if there are any 

 distance receptors at all, which act as influencing factors on be- 

 havior, they would be present in the earliest developed tentacles. 

 This seems to be probable in Pycnopodla: It always moves at right 

 angles to the zones of radial growth; the anterior end in adult life, 

 being always the side which possesses the fewest, but uniformly 

 sized, rays anterior to the radial interpolation zones ; in young 

 specimens the number of the large rays is greater for the anterior 

 end. In adults the number of the anterior rays is five; and the 

 youngest of all rays is a pair, one on each side of rays II. and I. ; 

 the number of rays of the posterior end varies with the total num- 

 ber of rays. In living animals it is quite easy to distinguish be- 

 tween the ages of the rays. There is a gradual increase in size 

 of the rays posteriorward from the interpolation zones, the young- 

 est being, of course, found at the latter points. This seems to 

 indicate that the so-called posterior end is the physiological ante- 

 rior end. 



Kjerschow-Agersborg ('18) demonstrated experimentally that 

 out of 50 righting reactions of Pycnopodia, 46 were toward the 

 anterior end with an average speed of about 57.1 sec. per normal 

 turn. In this species a physiological anterior point is actually 

 established (vide ut supra ct infra], the anterior ray is ray IV.; 

 the madreporite holds the relation as indicated by Ludwig, Agassiz, 

 Ritter, and Crocker for the five-rayed starfish (Figs, i, 4, 5, 6, 7). 



It may be noted that in Ritter and Crocker's diagram (Fig. 2) 

 the anus is in front of the base of the radial muscles, according to 

 the interpretation of these writers ; but by the establishment of the 

 physiological anterior end the anus comes nearer the posterior end 

 than before, while the madreporite is placed nearer the anterior 

 end. This, however, does not change their relation to the respec- 

 tive rays, but the relation remains the same as before. That is, the 



