326 General Biology 



At certain points in 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, the 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 

 in the animal. A new leg, as large 



Diagram showing antenna-like organ re- ,« i , -ii j i r 



generated in place of an eye of Palaemon. aS the One lost, Will develop from 

 (From Morgan, after Herbst.) ^ stump thus remaining> 



Reed declares that "autotomy is not due to a weakness at the break- 

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

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

 the nerve of the leg itself." 



It will 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. The adult bears abso- 



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

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

 abdomen of a crab, and completes its development while living as a 

 parasite in 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. 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 carried very far. 



There are various other parasitic Crustacea, such as the numerous 

 kinds of fish lice which live attached to the gills or other parts of fish, 

 and derive their nutriment from the body of the fish. These also show 

 various degrees of degeneration. With some of the 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 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 

 will be thousands of minute creatures of varying shape and size. Some 

 of them are too small to be seen with the naked eye, while others are 



