WHEEL-BEAUEES. 283 



The rotatory organs, tlie source of the common glid- 

 ing motion, are not very large or conspicuous ; the 

 cilia appear to be set all along the brow. The eye is 

 very noticeable : it is placed near the front and seems 

 to be of a deep bluish-black hue. 



I have not, however, as yet introduced the nimble 

 little stranger by name. We may call it familiarly the 

 Sword-bearer, but Professor Ehrenberg has named it 

 Polyarihra platyptera. 



This eminent authority on all that concerns these 

 minute forms has placed the species among those which 

 are destitute of a horny lorica or shell. But he is cer- 

 tainly in error here, for, as you may see, there is mani- 

 festly a stiff lorica, which covers the back and sides, 

 but which gapes widely in the middle of the under 

 side, throughout its length. From the lateral points, 

 however, a membrane may be seen for a short distance, 

 which doubtless protects the viscera from actual ex- 

 posure. 



The sword-like fins appear to be twelve in number, 

 arranged in groups, or bundles, of three each ; one 

 bundle being set on each side of the dorsal, and one 

 on each side of the ventral aspect, at about one fifth of 

 the entire length from the frontal points. These are 

 all that we can ordinarily count ; but I have seen 

 more : one day, while examining a specimen that 

 presented a vertical aspect to me — endron, to speak 

 familiarly — the fins being all expanded, I saw with 

 perfect distinctness a seventh pair, proceeding from the 

 middle of the breast. They are flat, thin, narrow 

 blades, of exceeding delicacy ; all distinctly serrated 

 on both edges, the teeth pointing from the base out- 

 ward ; each is strengthened by a central rib. They 



