NATURAL HISTORY OF AMERICAN LOBSTER. 
265 
dactvl and afford a surface for the attachment of the huge flexor and smaller extensor 
muscles. Each tendon is a keeled plate which is developed in a flattened pocket of the 
skin, but the closing muscle of the great claw being the largest and the strongest in the 
body requires the largest tendons. The tendon of the flexor (t. fl. e ) is a broad leaf- 
shaped plate, keeled above and below, while that of the weaker opening muscle is narrow 
and strap-shaped. 
At the time of molt these huge tendons, like all others in the body, are drawn out, 
attached to the cast-off shell, and leave deep open pockets into which in a large animal 
the little finger can be easily inserted. As soon, however, as the soft claw becomes 
tense with blood, the water is driven out and, the opposed surfaces of the pocket uniting, 
a new tendon is gradually formed. (Compare fig. 1 , t p, pi. xliii.) 
The coarser flesh of the claws represents, as we have indicated, the characteristic 
flexor and extensor muscles, while the “ fine meat ” of the dactyl (fig. 3, pi. xlvi) and distal 
half of the propodus is composed of a sponge work of involuntary muscle fibers in addition 
to fine-blood vessels of the arterial system, nerves, glands, and connective tissue, the 
whole being enveloped by the soft pigmented skin (pi. xl). No special sense organs, 
aside from the setae, have been detected in it. The meshes of the sponge work form 
a system of communicating sinuses into which the arteries appear to open through very 
small branches or capillaries. 
During the molting process, when the fleshy mass of the claw is drawn through a 
series of narrow rings as if it were a piece of candy, the blood is of necessity withdrawn 
from these parts. The sponge work is an adjustment which meets this prime need of the 
molting period. At the time of molt the muscles are extremely tense and the flesh hard, 
and the contraction of the fibrous sponge work apparently keeps back the flow of blood 
until the animal escapes from its old shell, when it again becomes completely relaxed 
(see p«2o6). 
The abundant blood always found in the large claws, except when molting, is supplied 
by a large artery, which at the point of entry from the fifth segment divides into an inner 
and a smaller outer branch. The inner division passes between the two muscles, and 
gives off small twigs in its course; then as it curves outward over the distal end of the 
flexor muscle, it sends off somewhat irregularly a branch to the upper and lower division 
of each muscle, and to upper and lower parts of dactyl and propodus. 
The nerves of the great cheliped (pi. xl) consist of two main bundles ( n 1 and n 2 ), 
made up of a number of closely related strands. In the basal segments of the limb the 
larger and more complex bundle (■ n 2 ) is anterior while the smaller bundle ( n 1 ), which is 
double, follows it closely on its posterior or outer side. 
The nerves usually enter the claw in three closely related strands, one of which, sup- 
plies chiefly the extensor, one the dactyl and flexor, while the outermost branch is dis- 
tributed to the flexor and large “finger” of the claw. Both arteries and nerves regularly 
divide and subdivide in the terminal parts of the claw to form a very complicated 
system. 
