112 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



straighten the abdomen. After the removal of the whole dorsal exoskeleton, 

 and the muscles attached to them, notice the large masses of ventral abdominal 

 muscles, the flexors of the abdomen. Why are they more powerful than the 

 extensors? Remove the left thoracic wall, and observe the forward extension 

 of the flexor muscles to their origins from the lateral and ventral thoracic walls. 



To illustrate the arrangement of muscles in the appendages, examine those 

 of the chela. Cut off the chela from the first pereiopod and then remove the 

 shell from one surface of it. Find within the two muscles, one much larger than 

 the other, and find by pulling upon them their method of attachment and action 

 on the movable part of the pincer. Pick away the muscle fibers and find the 

 strong tendon in the center of the mass. Each joint of each appendage is simi- 

 larly provided with an extensor and a flexor muscle for moving the next joint 

 (see Hegner, p. 209 and Fig. 105). 



We observe that the muscular system of the lobster is highly developed as 

 compared with that of the lower invertebrates which we have studied. In those 

 forms there exist simple cylindrical tubes of muscle fibers, extending lengthwise, 

 and producing merely extension, contraction, or bending of the body. But in 

 the lobster as in the frog separate definite muscles exist having specific actions 

 on various parts of the body, permitting much greater variety and exactness of 

 movement. They have definite origins and insertions on the exoskeleton. Are 

 the abdominal muscles segmentally arranged? 



Place a small piece of muscle on a slide in a drop of water, tease into fibers, 

 cover, and examine. The fibers will be found to be cross-striated, like the volun- 

 tary muscles of the frog. On the other hand, the muscles of the lower inverte- 

 brates are like the smooth involuntary muscles of the frog. 



b) The circulatory system: Your specimen should now have the dorsal and 

 one lateral wall cleaned away so as to expose the viscera. The conspicuous 

 organs are the stomach anteriorly, the heart posterior to this, usually injected 

 with yellow material and with yellow vessels springing from it, and a large white 

 organ, the digestive gland, occupying the sides of the space within the thorax 

 and extending back into the abdomen. 



The space around the heart and between the viscera is not a coelome, as the 

 student might expect, but it is an enormous blood space, or blood sinus, filled 

 in life by blood. The coelome has been greatly reduced, in fact, is practically 

 absent through this great development of blood sinuses. The studies which we 

 have made on the coelomes of the earthworm and the frog should convince the 

 student that this space in the lobster cannot be a coelome because mesenteries 

 are completely lacking. Note that none of the organs are supported by mesen- 

 teries and that there is no peritoneal lining. 



The heart lies free in a large sinus, the pericardial sinus. The following 

 arteries arise from it. From the anterior end of the heart in the median line 

 is the single ophthalmic artery. Trace it forward along the dorsal surface of the 



