246 EVENINGS AT THE MICROSCOPE. 



motion, are not very large or conspicuous ; the cilia 

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

 visible ; 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 

 Polyarthra 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, how- 

 ever, a membrane may be seen for a short distance, which 

 doubtless protects the viscera from actual exposure. 



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

 ranged 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 — end-on, to speak familiarly — the fins being all ex- 

 panded, 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 

 outward : each is strengthened by a central rib. They 

 are jointed independently, on rounded shelly knobs, and 

 are doubtless moved by strong muscles. Under pressure, 

 the knobs and the fins are brought out with beautiful 

 distinctness. Here again we have true jointed limbs. 



On the front you may discern a pair of tiny antennae, 

 each bearing a pencil of very fine bristles. And just 



