ACORN-SHELLS. 179 



consisting of two joints and terminated by four bristles, which 

 constitutes an additional apparatus of propulsion. Thus endowed, 

 they swim along in a series of bounds, the oars and tail giving, 

 in measured time, successive impulses. They have, moreover, 

 large lateral eyes, and the body is covered with a sort of shell, such 

 as we see in the early state of certain Entomostracans (Cyclops), 

 which they closely resemble. Mr. J. V. Thompson, believing the 

 little creatures so constructed to be larvae of some Crustaceans, 

 kept some of them in a glass vessel, covered to such a depth with 

 sea-water, that they could be examined at any time by means of a 

 common magnifying-glass, and, to his great surprise, in the course 

 of a few days they threw oft' their larva skins, and became firmly 

 adherent to the bottom of the vessel, changed into young Bar- 

 nacles, such as are usually seen in the spring-time intermixed 

 with grown specimens on rocks and stones. In this stage the 

 valves of the shell and of the operculum were visible, as well as 

 the movements of the arms of the contained animal, although 

 these last were not yet completely developed. The eyes, also, 

 were still perceptible, but these gradually disappear with the in- 

 creasing opacity of the shell, and the animal becomes blind for 

 the remainder of its life. Thus, then, a creature originally free, 

 capable of swimming about, and furnished with distinct organs of 

 sight, becomes permanently and immoveably fixed, and its optical 

 apparatus obliterated. 



The Cirripeds are divided into two families. 



The Barnacles (Lepas] (Fig. 181) are always found attached to some 

 foreign substance by a long flexible peduncle, -which possesses great power of 

 contraction. Each valve of their shell is usually composed of two triangular 

 pieces, and is closed at the back by an elongated plate, so that the whole shell 

 consists of five pieces. They are very widely disseminated, and adhere to sub- 

 marine bodies in considerable numbers. They are found not only on floating 

 wood, the hulls of ships, bottles, and other articles floating about, but on shells, 

 on turtles, whales, and even sea-snakes. Large logs of timber are sometimes 

 completely covered with them, compacted in close array, writhing and twisting 

 about like the serpents on Medusa's head, and presenting a most remarkable 

 spectacle. 



The Acorn-Shells (Balani] (Fig. 182) in their general structure and 

 habits resemble the barnacles ; they have, however, no pedicle or footstalk, 

 but are sessile that is, are fixed immediately on the substances upon which 

 they attach themselves, or in which they are more or less embedded. The 

 common species is often found in great numbers covering the shells of mussels 

 and oysters, and may thus be easily procured for examination. Taking one 

 of these as an example of the group, we find them to consist of a shelly cone 

 composed of various pieces, accurately fitted together, and capable of enlarging 

 with the growth of the animal. A thin calcareous base or closing plate fixes 

 the cone to the substance on which it rests, while its apex presents four pieces, 



