ANIMAL FORMS 



ring in such vast numbers that they impart a reddish tinge 

 to the water over wide areas, and at night are largely re- 

 sponsible for its phos- 

 phorescence. Many oth- 

 ers are parasitic in their 

 habits, and scarcely a 

 salt-water fish exists but 

 that at one time or an- 

 other suffers from their 

 attacks. On the other 

 hand, many fresh- and 

 salt-water fishes depend 

 upon the free-swimming 

 forms for food, and 

 hence, from an economic 

 point of view, they are 

 highly important organ- 

 isms. 



97. Barnacles. The 

 parasitic habit and the 

 lack of locomotion has 

 also produced marvelous 

 changes among the bar- 

 nacles, so great that 

 originally they were 

 placed among the mol- 

 lusks ; and as with the parasitic copepods, their true posi- 

 tion was only known after their life-history had been de- 

 termined. In the goose-barnacles * the body, attached by 

 a fleshy stalk to foreign objects, is enclosed by a tough 

 membrane, corresponding to the carapace of other Crus- 

 tacea, in which are embedded five calcareous plates. This 



FIG. 54. Cyclops, e. s., eggs ; i, intestine 

 reproductive organ. 



* So called because of the belief, which existed for three hundred 

 years prior to the present century, that when mature these animals 

 give birth to geese. 



