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, 4) 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, 4g. Nerves pass to each eye, and others to 
the four antennz, 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 antenne (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 #y 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. 
Braun. I. First stage: a, the two old cuticular layers; 4, the layer of 
casting hairs ;'c, the epidermis cells. II. Second stage: a, 4, c, asin 1; 
between 4 and ¢ the new cuticle @ 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 
antenne. Crustaceans moult or cast their shell at differ- 
ent periods. The old shell is pushed up by what are called 
5 
