BARNACLES. 
281 
washed into the month that lies below them. The abdomen is undeveloped; but 
the rest of the body is enveloped in a fold, or mantle, supporting the outer shelly 
skeleton. The jaws consist of two pairs of maxillae, and a pair of mandibles, and 
the lower part of the head is inferiorly continued into the stalk, which contains 
the gland secreting the cement. If a barnacle be carefully removed from its point 
of attachment, the remains of the first pair of antennae may be observed on the 
adhesive surface. When first hatched, the young are in the Nauplius stage, being 
furnished with a median eye, and three pairs of appendages, of which the posterior 
two are branched. After swimming for a while by means of these appendages, 
BARNACLES ATTACHED TO PUMICE. 
the larva moults several times, and passes into a second stage, in which, with its 
two eyes and compressed carapace, it resembles a Daphnia. The rudiments of the 
six pairs of thoracic legs appear behind the mouth, and the first pair of swimming 
appendages become antenniform, each being provided with a sucker. By means 
of these suckers the larva fixes itself to its permanent resting-place, and, the cement- 
gland pouring out its secretion, glues the creature firmly to its point of attachment. 
Hence it follows that the fixed end of the stalk is the front extremity of the body. 
I11 the allied stalkless barnacle ( Megalasma ) the shell is attached directly to the 
support. We are thus led on to the acorn-barnacles ( Balanus ), in which the entire 
animal is enclosed in a shell formed originally of six pieces, which grow into a 
tube of variable length. Some of the latter group (. Balanidce ), namely, the genus 
