Growth-changes in Brittle Stars. 113 



along both sides of the spine-bearing ridge of the side arm-plate. The 

 later behavior of this pigmented area varies more or less in different speci- 

 mens. In what may perhaps be considered typical cerstedii, the two 

 margins (adoral and aboral) of the area accumulate all the pigment and 

 the nearly or quite unpigmented area between them separates them sharply 

 from each other. In such specimens the upper arm-plate is crossed by two 

 heavily pigmented lines, one of which passes down the side arm-plate adoral 

 and the other aboral to the spine-bearing ridge. These lines extend across 

 the under arm-plates and thus form rings encircling the arms. Each arm- 

 segment, in an individual colored like this, has therefore three pigmented 

 rings, one near its aboral margin, distal to the spines, one near its middle 

 just proximal to the spines, and the third at its adoral margin. In many 

 specimens, however, the ring either just distal or just proximal to the arm- 

 spines is indistinct or lacking and the one which is present is very broad on 

 both the upper and under arm-plates. The same obscuring of purple 

 pigment by green and other coloring matters, referred to under angulata, 

 occurs in this species also. 



It is obvious from the above account that the color pattern of neither of 

 these species of Ophiothrix can be derived from that of the other very easily, 

 since one is due to a longitudinal and the other to a transverse distribution 

 of the pigment. While it is true that some specimens of angulata have the 

 dorsal white stripe so much broken up, and so much white present elsewhere 

 on each segment, that they are easily confused with cerstedii, such speci- 

 mens do not show definite transverse pigmented rings. Possibly further 

 rearrangement of their pigment might give rise to rings and we could thus 

 say the transverse pattern was evolved from the longitudinal. Evidence, 

 however, that such a development actually occurred is wanting, for cer- 

 stedii certainly goes through no stage of development wherein it shows any 

 longitudinal distribution of the pigment. 



PODIA. 



As is well known, the podia of adult Ophiothrix are remarkable for their 

 highly papillose surface. It is a matter of some interest, therefore, to note 

 that the podia of the 3 or 4 terminal segments of each arm are perfectly 

 smooth, those of the next 5 or 6 older ones are rough with small papillae, 

 while it is not until some 9 or 10 segments are developed that the character- 

 istic papillose podia are present. 



DISK-COVERING. 



One of the most interesting facts in the development of Ophiothrix was 

 discovered in the study of the disk-covering. Although no specimens seen 

 were small enough to show simply the primary plates, the specimen only 

 2 mm. across the disk shows certain primary plates, among which the 

 central one is conspicuous (pi. 3, fig. 18). In a specimen 3.5 mm. across 

 the disk (pi. 3, fig. 19), it will be noticed that the central plate is not only 



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