CRABS AND SHRIMPS. 175 



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 circumspec- 

 tion than insects have ; for besides the extensive con- 

 vexity 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 amusing 

 in examining microscopically the structure of the higher 

 Crustacea ; but we will now dismiss them in order to dis- 

 cuss 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 fila- 

 ments 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 withdraw- 

 ing 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 egg, 



