THE ARTHROPOD A 325 



of its legs when caught in a position where it seems incapable of extri- 

 cating itself. 



At certain parts of the legs, there is a thick diaphragm with a tiny 

 hole through which blood passes, and it is here that the animal breaks 

 off its own leg, tire tiny drop of blood there exposed coagulating almost 



immediately and thus preventing its 

 bleeding to death. 



With an open blood system, 

 such as the crayfish has, bleeding to 

 death would be an easy matter were 

 this special arrangement not made 



Diagram showing antenna-like organ re- in the animal. A new leg, as large 

 generated in place of an eye of Palcemon. ., < -11 j i c 



(From Morgan, after Herbst.) aS the One lost, Will develop from 



the stump thus remaining. 



Reed says, "Autotomy is not due to a weakness at the breaking 

 point, but to a reflex action, and that it may be brought about by a stim- 

 ulation of the thoracic ganglion as well as by a stimulation of the nerve 

 of the leg itself." 



It w r ill be seen quite readily that this power of autotomy is of con- 

 siderable advantage to an animal. 



PARASITIC CRUSTACEA 



Sacculina ( ). (Fig. 212.) 



The young are active free-swimming larvae "much like a young 

 prawn ( )" or young crab. But the adult bears abso- 



lutely no resemblance to such a typical crustacean as a crayfish or crab. 

 The Sacculina, after a short period of independent existence, penetrates 

 to the abdomen of a crab, and completes its development while living 

 as a parasite on the crab. In its adult condition it is simply a great 

 tumor-like sac, bearing many delicate root-like suckers which penetrate 

 the body of the crab host and absorb nutriment. The Sacculina has no 

 eyes, no mouth parts, no legs, or other appendages, and hardly any of 

 the usual organs except reproductive organs. Degeneration here is car- 

 ried very far. 



"Other parasitic Crustacea, as the numerous kinds of fish lice which 

 live attached to the gills or other parts of fish, and derive all their nutri- 

 ment from the body of the fish, show various degrees of degeneration. 

 With some of these fish lice the female, which looks like a puffed-out 

 worm, is attached to the fish or other aquatic animal, while the male, 

 which is perhaps only a tenth of the size of the female, is permanently 

 attached to the female, living parasitically on her." 



PLANKTON 



One may, with a fine-meshed net, sweep in a considerable collection 

 of organisms from the surface of ponds, lakes, rivers, or ocean. There 



