144 BOYS AND GIRLS IN BIOLOGY. 



as well as mechanics can learn from the lobster. So 

 much for the heart and its circulation. The lobster is 

 well supplied with nerves ; in front of the gullet you 

 will find his brains, in the form of a bunch, or ganglion, 

 on each side (Fig. 116) ; they correspond to the ce- 

 phalic or head ganglia of the mussel, though they are 

 much larger. Passing back from the nerve-bodies 

 (ganglia) you will see white nerve-cords, reaching over 

 the gullet on each side to the floor of the head breast- 

 plate (cephalo-thorax). Here there is a chain of six 

 nerve-bunches (Fig. 116) ? or ganglia, joined by double 

 cords. The first of these bunches (ganglia) answers 

 to the foot-nerves, or pedal ganglia, of the mussel. 

 From the last of the breast, or thoracic ganglia, a 

 single nerve-cord goes back and joins another chain of 

 six ganglia^ lying in the floor of the abdomen, one in 

 each somite, and the last one is very large. These are 

 united by single cords, so altogether there are thirteen 

 pairs of nerve-bunches or ganglia (Fig. 116) ; seven in 

 the head and breast (cephalo-thorax) and six in the 

 abdomen. From these ganglia, which answer to your 

 brain and spinal cord, nerves pass off to all parts of the 

 body ; but your spinal cord lies at the dorsal or back 

 part of your body and your heart in front, while in the 

 lobster these positions are just the opposite (Figs. 110- 

 121). .You see what a perfect system of nerves the 



