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, however, 

 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 dis- 

 tinctness. 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 



