242 BULLETIN OF THE BUREAU OF FISHERIES. 



twisted rope-like mass is thus formed, the forward strands of which are attached to the 

 linkwork of hard tendons in the thorax. There are also in the thorax, rotator abdom- 

 inis, ventral thoracico-abdominis and tergo-epimeral muscles, as well as flexors of the 

 telson and tail fan in the abdomen. 



The weaker dorsal muscles (pi. xxxni) form a pair of segmented strands overl)dng the 

 Alimentary canal and dorsal blood vessel. They are inserted into the anterior border 

 of each abdominal somite and diverge as extensor abdominis muscles in front, where 

 they are attached to the walls of the thorax below the cervical groove. When the 

 .ventral muscles suddenly contract at the command of the nervous system, the combined 

 pulls on successive joints bring the tail with expanded tail fan quickly and violently 

 down upon the thorax, and the animal shoots backward through the water. By the 

 contraction of the weaker extensor muscles the body is again brought into a horizontal 

 position, and ready for another downward stroke. Raising the abdomen tends to send 

 the animal forward, but owing to the obliquity and slowness of the stroke after closure 

 of the tail fan the speed is but little checked. The muscular equipment of the great 

 claws and legs are described in chapter vii. 



Two prominent light spots are conspicuous on either side of the carapace of an 

 adult lobster, one at a point about an inch behind the base of the large "feelers," and 

 the other about as far behind the first, close to the irregular depression known as the 

 cervical groove. (See p. 220.) The first, which is large and very conspicuous at the 

 sixth stage, when the animal is barely five-eighths inch long, is the mark of a straight 

 rod-like tendon which binds the carapace firmly to the internal skeleton below. The 

 latter was without doubt originally a tendon-mark also, but in place of a distinct tendon, 

 short muscle fibers issue from its margin, and from the groove in front, to be attached 

 to the wall of the gill chamber. The scar-like impression conforming to the groove and 

 immediately in front of it marks the attachment to the shell of the posterior suspensory 

 muscles of the stomach sac. The powerful adductor of the jaws, by the contraction of 

 which their cutting surfaces are brought to bear on the food, divides to give passage to 

 this gastric muscle, one section of which is attached to the carapace in front of the 

 groove, and the other just behind it on the endotergites, which as stated above are 

 tendinous ingrowths from the fold itself. The anterior gastric muscles are inserted on 

 the procephalic plates. 



Some fourteen pairs of extrinsic and intrinsic gastric muscles have been described 

 by Williams {2']()) . These serve either to suspend the stomach sac to the inner wall of 

 the carapace (anterior gastric, anterior dilators, and posterior and lateral gastrics) or to 

 move its nicely articulated framework, bring the food to mill, work the grinding teeth, 

 and to effect in some measure the sorting and straining of the comminuted food particles. 



THE BLOOD AND ORGANS OF CIRCULATION. 



The blood of the lobster when freshly drawn is quite colorless, leucocytes or white 

 blood cells being the only corpuscles present, but after exposure to the air for a few 

 minutes it becomes tinged with blue, and thickens or coagulates. The bluish color is 

 imparted by a respiratory pigment called haemocyanin, which like the haemoglobin of 



