216 THE RISE OF ANIMAL LIFE 



nearly hatched eggs) and usually do not increasing reluctance, until but three re- 

 hatch until the summer following that in main. Then it must be stimulated rather 

 which they were laid. This condition has drastically before it will leave these behind, 

 resulted in much confusion concerning the It seems to sense the danger of being leg- 

 laying of eggs by the lobster. less, even though it will later acquire a new 



When the lobster hatches as a free-swim- set through regeneration. The value of au- 



ming larva, the period of fosterage is over totomy is obvious; it enables the animal to 



for the female lobster. By fanning move- escape with the loss of a single appendage, 



ments of the swimmerets the young are whereas without this ability it might not 



driven away from the body of the mother. escape at all. The break always occurs at 



Young lobsters, however, tend to keep to- a certain place near the base of the leg. 



gether in a cluster. There is a constriction of the wall at this 



Growth. Molting is an important process point so that the loss of the appendage is 



for the crayfish and lobster, as it is for all followed by only a minor loss of blood, 

 of the arthropods. This is necessary in order The lost parts are replaced by the slow 



to provide for increase in the animal's size, regeneration of a new appendage. This oc- 



although a small amount of growth can oc- curs more readily in young animals than in 



cur under the rigid exoskeleton. Just before older ones, although the ability to regener- 



the molting process begins, some of the ate seems to remain throughout life. Per- 



lime is withdrawn from the exoskeleton, haps due to the greater specialization of 



softening it somewhat. Simultaneously, a these animals regeneration is limited to the 



new skeleton is secreted beneath it, arising appendages and to the eyes, and thus much 



from the epithelium, called the hypodermis. less pronounced than in the lower forms of 



The muscles and body bulk then shrink a animals. This is as might be expected, since 



little and the old skeleton splits on the the greater the integration of parts, the 



dorsal side, between the abdomen and more dependent each one is on the other, 



carapace. The animal backs out slowly, and the less possibility there is for any 



leaving behind a replica of itself, complete part to replace the whole animal. The abil- 



in all details, except for the actual body, ity to regenerate is reduced still further in 



This shedding is so thorough that even the the vertebrates. 



facets of the eyes, and the lining of a part The nervous system. In contrast to the 

 of the gut are included. Once the old skele- rather meager sensory equipment of flat- 

 ton is cast off, the animal grows rapidly for worms and annelids, the lobster and cray- 

 a period of time as a result of taking in a fish are well supplied with sense organs. If 

 great deal of water. For several days dur- a small bit of liver is dropped in one end 

 ing the hardening period the animal re- of an aquarium containing a crayfish, very 

 mains in hiding, and it becomes aggressive shortly the animal will orient itself in the 

 again only after its skeleton is well hard- proper direction and move directly toward 

 ened. the food. The soluble parts of meat dif- 



Another interesting characteristic of fuse through the water and touch tiny hairs 



many crustaceans, associated with growth on the chelipeds, antennules, antennae, and 



and reproduction, is autotomy, the power mouth parts, causing impulses to pass to 



to throw off appendages at will. The fiddler the central nervous system. This sensitivity 



crab affords a most striking example of this to chemicals is equivalent to man's senses 



phenomenon. If one of its posterior legs is of taste and smell. 



held with a forceps, it will drop it off The animal's ability to move about satis- 



readily (Fig. 11-15). As other legs are factorily in total darkness indicates that it 



pinched, they too are dropped but with must have senses that guide it under such 



