220 



THE SCIENCE OF LIFE. 



small bristles, in the form of recurved hooks on each 

 ring of the body, which assist in locomotion. It pos- 

 sesses no external gills, but respires 

 by internal ciliated processes. The 

 nervous system is often but little de- 

 veloped. The mouth is on the second 

 segment, and the digestive canal is a 

 straight tube, which is wide, and al- 

 ways full of earth, which these animals 

 devour for the sake of the organic par- 

 ticles contained in it ; the remaining 

 part being cast out and heaped at the 

 outlet of their burrows, as " worm- 

 casts." For better division of the 

 FIG. .-serpuia. ma terial swallowed the digestive canal 

 lias a muscular gizzard about fifteen rings from the 

 mouth. They are propagated by eggs. 



3. The class of CRUSTACEA, (crusta, a crust or shell,) 

 includes all Articulates with jointed legs and gills. They 

 have a double, or complete circulation of blood ; a dor- 

 sal tube, or heart, sending off a system of arteries, not 

 found in insects; but the blood, as it leaves these tubes, 

 escapes into the general cavity, as in other Articulates. 

 (Fig. 123.) The shell, or carapace, has for its base a 

 horny substance called Chitine. It is also found in the 

 covering of Insects. In the Crab and Lobster there is 

 a large proportion of carbonate of lime combined with 

 this, rendering the carapace extremely hard. In others, 

 a mixture of chitine and albumen gives rise to a softer 

 integument. The rings of the body have considerable 

 freedom of motion, by means of striated or voluntary 



