76 INTRODUCTION TO ZOOLOGY. . 



division, termed, from the circumstances, "gill-footed,"* tlie 

 surface of the legs is extended, and made subservient to re- 

 spiration. From this cause, in the minute tribes in which this 

 structure prevails, the feet are sometimes seen in motion when 

 the body is at rest. The more actively the body moves, the 

 more brisk wiU be the circulation; " and since," as Mr. Owen 

 remarks, "the muscular energy directly depends upon the 

 amount of respiration, the two functions are brought into 

 direct relation with each other by the simple connexion of 

 their respective insti-uments."f 



In those tribes that live partially or altogether on the land, 

 the respiratory apparatus is modified, but is still in its most 

 essential features, aquatic. In the Wood-louse {Oniscus,X 

 Fig. 50), which lives in dark and damp 

 situations, respiration is effected by a 

 series of plates, at the lower side of the" 

 abdomen. § In the Land-crabs, contri- 

 vances of different kinds exist, to retain 

 so much water as wiU supply the gills 

 with the amount of moisture needful for 

 the due performance of their functions. 

 But the quantity of oxygen which water 

 only can furnishis insufficient for animals 

 whose respiration is so active. They 



Fig. 50.— Oniscus. i 1, X • xi • • 



must nave access to air, or they inevi- 

 tably perish. Hence we are able to understand why it is that 

 they are drowned, if immersed for any long time in water. 



Vision. — In the eyes of the Crustacea a great diversity of 

 structure is exhibited. Some species are furnished with two 

 placed upon distinct peduncles or stalks ; others have eyes of 

 the same formation, but the peduncle is wanting; such eyes 

 are therefore described as being ' ' sessile ' ' or sitting. In one 



* Phyllopoda. 



f I-ectures, page 182. 



t The Oniscus is well-known, in the North of Ireland, by the provincial 

 name of Slater. 



§ Some of these animals have been found in a fossil state in Wiltshire, 

 in those secondary rocks termed the Wealden formation. The eyes which, 

 like those of the Trilobite, hereafter mentioned, are composed of a num- 

 ber of separate lenses, form beautiful objects when magnified. They are 

 sometimes found not attached to the head, but loose in the limestone- 

 Fossil Insects in the Secondary Kocks of England, by the Kev. P. B. 

 Brodie. London, 1845. 



