CRABS AND SHRIMPS. 175 



former you see that the facets into which the cornea is 

 divided are hexagonal, like those of most Insects, but 

 in the latter they are square. 



But Crustacea have a far greater faculty of circum- 

 spection than insects have; for besides the extensive 

 convexity and numerous facets of their eyes, these organs 

 are placed at the extremity of shelly foot-stalks, which 

 are themselves movable on hinges, capable of being- 

 projected at pleasure, and of being moved in different 

 directions, and of being packed snugly away, when not 

 in active use, in certain grooves hollowed out expressly 

 for them in the front margin of the shell. 



We might find much more both instructive and amus- 

 ing in examining microscopically the structure of the 

 higher Crustacea ; but we will now dismiss them in order 

 to discuss some of the lower forms, many of which are 

 so minute that their whole bodies may be watched with 

 ease performing all the functions of life, while confined 

 under our eye, on the stage of the microscope. I refer 

 to the tiny active little creatures known as Water-fleas, 

 which are abundant in both fresh and salt water. 



In this jar of fresh water, which has been standing in 

 the window for weeks, you may see among the green 

 filaments of Chara many little atoms, which scuttle 

 hither and thither with a rapid succession of short leaps. 

 These belong to the genus Cyclops, and are Crustacea, 

 belonging to the order Entomostraca. 



By the aid of a glass tube which I stop at one end 

 with my finger, I will endeavour to catch one. It is no 

 easy matter, as you see, for the instant the end of the 

 tube is brought near to one, he takes the alarm and 

 leaps nimbly away before I can make the water rush in 

 by withdrawing my finger from the other end. But I 

 have one at length. 



Here it is ; a living atom, not more than a sixteenth 

 of an inch in length, looking something like a pellucid 



