112 THE AMERICAN ARBACIA 



The most absorbing regions are the digestive tract and the two trans- 

 verse ciliated bands, the oral band around the mouth and the postoral 

 (or ventral) band between the two long(anal) arms above the anal 

 opening. These regions are the most active physiologically, being con- 

 cerned with procuring and digestion of food, and there may be some 

 correlation between physiological activity and ultraviolet absorption. 

 In fixed preparations stained with haematoxylin, these same regions, 

 the alimentary canal and the two transverse ciliated bands, are deeply 

 stained. 



When photographed by infrared light (8,000-10,000 A), the struc- 

 tures of the pluteus appear as they do with visible light except that 

 the red pigment spots are not distinguishable (E. B, Harvey and Lavin, 

 1951a). 



An interesting though somewhat involved mathematical or geo- 

 metrical explanation of the form of a pluteus larva may be found in 

 D'Arcy Thompson's Growth and Form (1948, p. 625). 



The normal pluteus is quite uniform in shape, both in different 

 batches and in individuals of the same batch. There do, however, occur 

 in some cultures, and usually in the entire culture, plutei of a different 

 shape. Sometimes the anal arms are widely divergent, and sometimes 

 they are close together. The cause for these abnormalities has not been 

 determined. Such abnormalities can, however, be produced by experi- 

 mental conditions, such as KCN, acids, alkalis, salts, alcohols, etc. 

 These have been studied by Child (1916b, 1941, p. 197-21 1) with 

 relation to axial gradients, and also by Medes (191 7). Abnormal plutei 

 have also been produced by acetylsalicylic acid (aspirin), probably an 

 acid effect (Budington, 1935); dinitrophenol, iodoacetic acid, pyo- 

 cyanine, methylene blue (Waterman, 1938); malonic acid (Rulon, 

 1948). Tennent (1910a) has made a statistical study of variations in 

 Lytechinus plutei. 



Most substances have a harmful effect on the plutei, but M. M. 

 Brooks (1943) reports that methylene blue increased the length of the 

 anal arms; those treated with methylene blue averaged 420 (x while 

 the controls averaged 280 [x. 



An interesting effect of KCN on the plutei has been reported by 

 Lyon (1902). Ciliary motion is stopped, and when the larvae are re- 

 turned to sea water, it starts up again, but the cells, either singly or 

 in small masses, break loose and swim for a moment; then the pluteus 

 disintegrates. The same result follows anaerobiosis caused by prolonged 

 exposure to hydrogen gas. 



