^04 CHAPTERS ON EVOLUTION. 



in other words, from the days of its infancy to the days of its youth. 

 As the " pupa," its body is enclosed in a bivalve shell or " carapace." 

 Two compound eyes replace the single organ of vision of the Nau- 

 plius-stage ; the first pair of legs (Fig. 120, B a) have become enlarged, 

 .and appear as antennae or feelers provided each with a sucker ; whilst 

 behind the mouth six pairs of " cirri," or small hair-like limbs (/), 

 are developed. The mouth appears to become abortive in this stage, 

 in which the resemblance of the young Barnacle to a Water-flea such 

 as Daphnia (Fig. 1 16, c) or Cypris (B) is sufficiently striking. Darwin 

 remarks, that in the Nauplius- stage the young barnacles feed actively 

 -and increase in size; whilst in the second stage, their function is "to 

 search out by their well-developed organs of sense, and to reach 

 by their active powers of swimming, a proper place on which to 

 become attached and to undergo their final metamorphosis." 



The concluding phases in barnacle-history are not difficult to trace. 

 The body of the young barnacle becomes somewhat flattened and 

 compressed, and, as Darwin remarks, resembles in its shape a mussel- 

 shell or the water-flea known as Cypris (Fig. 116, B). The carapace 

 or shell appears paramount in the final stages of development, the 

 limbs and body being hidden and enclosed by the shell ; and although 

 jaws exist, these organs are covered by integument, and the organism 



is thus deprived of the power 

 of nourishing itself. Certain 

 remarkable glands now begin 

 to be developed in the pupa- 

 barnacle. These organs open 

 by the so-called "cement- 

 ducts," in the suckers of the 

 well-developed first pair of 

 appendages the great feelers 

 or antennae (a) already men- 



FIG. H^-SACCULINA.^ tioned. The pupa in due 



time seeks a location and 

 resting-place, and adheres (Fig. 120. A) to its floating log, or to the side 

 of the ship, by means of its feelers. Thereupon the cement glands 

 pour out their secretion, which acts as a veritable " marine glue," 

 defying the solvent action of the water, and fastening the barnacle 

 head downwards to the place of attachment. Then the compound 

 eyes disappear, leaving the future existence of the barnacle sightless. 

 The characteristic limy formations or plates seen in the "shell" of the 

 adult barnacle (Fig. 117) are developed. The six pairs of swimming 

 feet become the plumes, " cirri," or " glass hand " of the barnacle, 

 and by their incessant waving draw food particles into the mouth. 

 With the production of the characteristic fleshy stalk or "peduncle " 

 * of the full-grown form which grows from the front part of the body 



