96 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. ¢.s., eggs; @, intestine ; 02, 
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. 
