CRABS, ETC. 



79 



bailer," that moves back and forth, creating a current over 

 the gills that finds its way out through an opening near the 

 mouth. The colorless blood is pumped by the heart (Fig. 

 83, ti) to the gills, where it takes up oxygen, returning to 

 the heart by numerous venous channels. 



Nervous System. The brain or head ganglia is seen 

 in Fig. 83, hg. Nerves pass to each eye, and others to 

 che four antennae, while a chain of nerve-masses extend 

 through the body (Fig. 83, g), having branches to the 

 principal parts. 



Organs of Touch, Hearing, etc. The ears are at the 

 base of the smaller or first antennae (Fig. 81), and are 

 little sacs in the upper side, containing a thick fluid, in 

 which float grains of sand. On a ridge projecting into the 

 interior of the sac are numerous hairs, not over ^ of an 

 inch in length, that are connected by nerves with the brain. 

 The sound-wave sets the sand-grains in motion, the vibra- 

 tion in turn affects the hairs, and the sound is carried to 

 the brain. 



II. 



FIG. 84. Stages of casting in the carapace of the freshwater cray-fish, from 

 Braun. I. First stage : <z, the two old cuticular layers ; b, the layer of 

 casting hairs ; c, the epidermis cells. II. Second stage : <z, d, c, as in I ; 

 between b and c the new cuticle d has intervened. 



The organs of touch are the delicate hairs about the 

 mouth-parts and legs. The organs of smell are supposed 

 to be on the under side of the outer branch of the small 

 antennae. Crustaceans moult or cast their shell at differ- 

 ent periods. The old shell is pushed up by what are called 

 7 



