GROUP OF A COMMON SPECIES OF ACORN-SHELL OR ROCK BARNACLE (Balanus Balanoides) (NATURAL SIZE)

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The Stalked Barnacles, like Lepas (suborder Pedunculata), and the Sessile Barnacles, or Acorn-shells, like Balanus (suborder Operculata), together form the order Thoracica. Of the other orders which compose the subclass Cirripedia, the only one that need be mentioned here is the Rhizocephala, which comprises strangely degenerate parasites living on other Crustacea.

The Cirripedia are unlike nearly all other Crustacea in the fact that, with few exceptions, they are hermaphrodite, having both sexes united in each individual. In certain species of the Stalked Barnacles, however, there are minute male individuals that are attached, like parasites, to the large hermaphrodites. In a few species the large individuals only possess female organs, so that the separation of the sexes is complete.

The remarkable larval metamorphoses of Cirripedes and the modifications of structure presented by some parasitic forms will be described in later chapters.

The fifth and last subclass, that of the Malacostraca, is by far the largest and most important, and will require to be considered in more detail than any of the others. The animals composing the various orders into which the subclass is divided differ very greatly in structure, but they all agree in having typically the same number of appendages as the Lobster—namely, nineteen pairs (or twenty, if the eye-stalks be included). They also agree in the very important character that the trunk limbs are divided into two sets, thoracic and abdominal, the former of eight, and the latter of six pairs.

Fig. 15—Nebalia bipes. Enlarged. (From Lankester's "Treatise on Zoology," after Claus.)