NATURAL. HISTORY OF AMERICAN tOBSTER. 265 



dactyl 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 lasrgest tendons. The tendon of the flexor (t. fi.^) is a broad leaf- 

 shaped plate, keeied 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. 1,1 p, pi. xuii.) 



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. xi<). 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 series of narrow rings as if it were a piece of candy, the blood is of necessity withdrawn 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 Che 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 cuiVes 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. xi.) consist of two main bundles («.* and vF), 

 made up of a number of closely related strands. In the basal segments of the limb the 

 larger and more complex bundle (w^) is anterior while the smaller bundle (n^), 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. 



