PHYLUM ARTHROPODA 



215 



tinct advantage since it places the animal in 

 a position of safety. 



Light. Light of various intensities in the 

 majority of cases causes the crayfish to re- 

 treat. Individuals prefer colored lights to 

 white. Negative reactions to light play an 

 important role in the animal's life, since 

 they influence it to seek a dark place where 

 it is concealed from its enemies. 



Chemicals. The reactions of the crayfish 

 to food are due in part to a chemical sense. 

 Positive reactions result from stimulation by 

 food substances. For example, if meat juice 

 is placed in the water near an animal, the 

 antennae move slightly and the mouth parts 

 perform vigorous chewing movements. The 

 meat juice causes general restlessness and 

 movements toward the source of the stimu- 

 lation, but the animals seem to depend 

 chiefly on touch for the accurate localization 

 of food. Acids, salts, sugar, and other chem- 

 icals produce a sort of negative reaction, 

 indicated by the animal scratching the cara- 

 pace, rubbing the chelae, or pulling at the 

 part stimulated. 



Habit forming 



It has been shown by certain simple ex- 

 periments that crayfishes are able to form 

 habits and to modify them. They learn by 

 experience and modify their behavior slowly 

 or quickly, depending upon their familiarity 

 with the situation. The chief factors in the 

 formation of such habits are the chemical 

 sense (probably both smell and taste), 

 touch, sight, and the muscular sensations re- 

 sulting from the direction of turning. Experi- 

 ments show that the animals are able to 

 learn a path even when the possibility of 

 following a scent is excluded. 



Cave crayfishes 



There are at least 12 different species of 

 cave crayfishes in the United States; some 

 are restricted to the waters of a single cave, 

 such as Mammoth Cave, Kentucky. 



Cave species are interesting because of 



their striking modifications. All are blind, 

 the eyes are atrophied and the eye stalks are 

 more or less undeveloped. Pigmentation is 

 absent, and the body is light-colored. They 

 are mostly small species; the chelae are not 

 well developed. The antennae are long and 

 highly specialized as tactile organs. 



OTHER CRUSTACEA 



Almost every pond, lake, or stream con- 

 tains crustaceans of many species, and salt 

 water is likewise inhabited by a large variety 

 of forms. A few live on land. Often crusta- 

 ceans are very abundant. Only a few of the 

 more common or more interesting species 

 can be mentioned here. 



Fairy shrimps, Eubranchipus, reach a 

 length of about one inch, live in fresh-water 

 pools, and are common. They are semitrans- 

 parent, pinkish, and swim on their backs 

 (Fig. 110). Eggs laid in the summer be- 

 come buried in the mud and are able to 

 withstand dr}ang and the winter cold. They 

 hatch the following spring. One species of 

 fairy shrimp lives in water more salty than 

 that of the sea. 



Water fleas, Daphnia, are oval, laterally 

 compressed crustaceans with a prominent 

 beak on the under side of the head, and a 

 sharp caudal spine (Fig. 110). They are 

 about 2 mm. long and are very common in 

 fresh water. The soft body is enclosed in 

 a bivalve shell through which can be seen 

 a regularly beating heart, in front of which 

 is a single eye, and behind which is a brood 

 chamber full of eggs. 



The ostracods are common in fresh and 

 salt water and widely distributed. The bi- 

 valve shell (Fig. 110) makes it appear like a 

 microscopic clam if the appendages are not 

 seen. They swim with the first pair of legs. 

 Only females are known in certain genera; 

 these lay eggs which develop without fertili- 

 zation, that is, they are parthenogenetic. 



The modern cyclops is not a giant with a 

 single eye in his forehead like the Cyclops of 



