222 



THE SCIENCE OF LIFE. 



ceans pass through a series of strange metamorphoses 

 before reaching their adult form. The Balanus, or 

 acorn-shell, which incrusts the rocks of the sea-coast in 

 great numbers, begins life as an active, one-eyed free 

 swimmer, called a " Nauplius" which after one or two 

 molts becomes a pupa, inclosed in a bivalve shell by a 

 folding of the dorsal portion. Finally it becomes a sed- 



FIG. 124. Development of Balanus balanoides : A. Earliest form. B. Larva after 

 second molt. C. Side view of the same. D. Stage immediately preceding the loss of 

 activity, a. Stomach. (?) b. Nucleus of future attachment. (?) 



entary Cirripede, (cirrus, a curl ; pes, a foot.) (Fig. 124.) 

 It will be convenient to divide Crustaceans into four 

 groups, or orders. 



I.) Cirripeds, distinguished by being fixed, having a 

 shelly covering, and by their feathery arms. Such are 

 Barnacles, (Lepas^) which have a peduncle, or stalk, and 

 are often found on the backs of whales or on ship's bot- 

 toms, and Acorn-shells, (Balanus^) which are sessile. 



