Fig. 193.—A Cluster of Acorn Shells
The Acorn Barnacles, so numerous on our shores, are good types of the Cirripedia, and they are so easily kept alive in the indoor aquarium that their interesting movements may be well observed. A cluster of these animals may be obtained by chipping off a piece of the rock on which they grow; or, instead of this, a few minutes’ searching on a rocky coast at low tide will certainly provide us with a stone of suitable size, or the shell of a mollusc, on which the creatures have found a home.
Fig. 194.—Shell of Acorn Barnacle (Balanus)
Place them in the indoor aquarium, or in any shallow vessel containing just sufficient sea-water to cover them, and carry out your observations with the aid of a hand lens. They will soon open the inner cone of their many-valved shell, and slowly protrude six pairs of gracefully curved and delicately-feathered appendages which, as previously stated, are attached to the thoracic portion of the body. Then, with a much more rapid movement, the appendages will be withdrawn, and the shell closed. These alternate movements are continued incessantly, and are the means by which the animals provide themselves with both food and air. The reader should also obtain some specimens of the larger species for the examination of the shell, the structure of which is interesting and, of course, peculiar to this order.
Fig. 195.—The Acorn Barnacle (Balanus porcatus) with Appendages protruded
In general structure and habits Barnacles are very similar to the acorn barnacles, except that the body is supported on a tough stalk, which, as we have already stated, is the modified anterior portion of the animal. These animals also may be easily kept alive and examined in the indoor aquarium. They are not creatures of the sea shore, but may often be obtained on masses of timber that have been washed ashore, or from the bottoms of ships that have been placed in the dry dock for repairs.