80 SEA-SHORE LIFE 
The eggs of crustaceans are often carried about attached to the 
abdominal appendages of the female. In the lower forms the egg 
usually develops into a more or less oval-shaped embryo called a 
nauplius, which has a single eye and three pairs of appendages. 
The first pair is simple and becomes the front feelers of the adult, 
while the other two pairs are forked, and become the second pair of 
feelers and the mandibles. In the higher crustaceans the eggs often 
coutain so much yolk that the embryo is not set free until its devel- 
opment has gone farther than the nauplius stage. Good general 
descriptions of our crustacea are given in Arnold’s “Sea Beach at 
Ebb-Tide;” Stebbing’s “Crustacea,” in The International Scientific 
Series, 1893; and Volume II of “The Riverside Natural History,” 
edited by Kingsley. 
THE BARNACLES 
The older naturalists believed that barnacles were mollusks, but 
a study of their development showed that they are crustaceans re- 
lated to the water fleas. 
The egg of the barnacle is set free in the water and develops 
into a minute larva with a triangular shield over the back, a single 
eye immediately above the brain, a mouth, intestine and three pairs 
of appendages. The larva then moults a number of times, acquir- 
ing a pair of stalked eyes, and a pair of shells hinged along the back 
and projecting over the sides of the body. The first pair of append- 
ages have now changed into organs of attachment which enable 
the little creature to fasten itself head-on to some rock or other suit- 
able anchorage where it is destined to pass the remainder of its life. 
A considerable change then comes over the creature. It re- 
mains without food while it develops a shell with hinged lids which 
may close or open the aperture. The barnacle has been described 
as a crustacean which is fastened by its head, lies on its back, and 
kicks its food into its mouth. If one watches a barnacle one will 
see how the feathery jointed legs are thrust out at regular intervals, 
and wave gracefully through the water to aerate the blood, and to 
set up currents which drive small creatures into the mouth of the 
barnacle. 
Most of the barnacles are hermaphrodites, but in some genera 
