Fig. 191. The Barnacle
For some time naturalists could not agree as to the proper place of these animals in the scale of life, but the matter was finally settled when some minute creatures only about a twelfth of an inch in length, and closely resembling the early stages of certain crustaceans, were seen to undergo metamorphoses, and finally develop into acorn barnacles. Their position in the animal kingdom was thus determined by their early stages; but these, instead of changing into a segmented and highly organised creature like the typical crustacean, lose some of their appendages, cease to be free-moving animals, and attach themselves to floating bodies by which they are carried about. Thus they are enabled to find the food they can no longer seek without such aid. In their young state they possess not only the means of freely moving in search of their food, but have organs of vision to aid them in the capture of their prey. As they grow, however, the foremost appendages are transformed into a sucking-disc, and the eyes, no longer necessary, disappear. It will thus be seen that the degenerated adult—the product of a retrograde development—is attached by what was originally the front of its body, while the abdomen is undeveloped, and the thorax, with its appendages, forms the summit of the free extremity.
Fig. 192.—Four Stages in the development of the Acorn Barnacle
a, newly hatched larva; b, larva after second moult; c, side view of same; d, stage immediately preceding loss of activity; a, stomach; b, base of future attachment. All magnified
Some of the Cirripedes attach themselves to the bodies of whales and other marine animals. The majority of these are pseudo-parasites—creatures that live on the bodies of other animals, but do not derive their food at the expense of their hosts; others, however, are true parasites, subsisting on the nourishing juices they extract from the animals to which they are attached.
