280 EVENINGS AT THE MICROSCOPE 



jumps are effected are the long breast-spines, which are 

 suddenly thrown out in various directions, and they may 

 frequently be seen extended the instant after a leap. 

 When we consider that the creature is jerked often four 

 or five times its own length, through so dense a fluid, we 

 shall perceive how strong the muscular action must be 

 which moves the lever-like spines. The creature is thrown 

 irregularly, often with the side foremost, or the back, or 

 made to perform a somersault in the act. It is probably 

 a sensitiveness to danger or annoyance that prompts these 

 violent leaps; at least, it frequently performs them, after 

 a momentary examination of any floating matter with 

 which its course brings it into contact. 



The rotatory organs, the 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. 



1 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 mi- 

 nute forms has placed the species among those which are 

 destitute of a horny lorica or shell. But he is certainly 

 in error here, for, as you may see, there is manifestly 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, 



