572 ARKACHNIDA, .- 
their food, The common acorn barnacle of. our coasts affords a famiiiar and 
beautiful example of the mode. by which this structure is made subservient to 
procuring a supply of food. The closed valves at the upper part of the sheil 
are seen to open slightly, a kind of fairy-like hand is thrust out, the fingers 
expanded, a grasp made at the water, and the closed member then withdrawn 
into the shell. 
This hand-like object is in fact the aggregated mass of legs with their fila- 
ments. As the limbs are thrust forward, they spread so as to form a kind of 
casting net ; and as they return to the shell, they bring with them all the 
minute organisms which were swimming in the water. This movement con- 
tinues without cessation, as long as the barnacles are covered with water, 
and appears to be as mechanically performed as the action of breathing is 
performed by the higher animals. 
In the illustration is seen a group of the common GOOSE MUSSEL, or DUCK 
BARNACLE, so called on account of the absurd idea that was once so widely 
entertained, that this species of barnacle was the preliminary state of the 
barnacle-goose, the cirrhi representing the plumage, and the valves doing 
duty for the wings. 
This barnacle is tolerably universal in its tastes. It clings to anything, 
whether still or moving, and is the pest of ships on account of the pertinacity 
with which it adheres to their planks. Its growth is marvellously rapid, and 
in a very short time a vessel will have the whole of the submerged surface 
coated so thickly with these cirrhipedes that her rate of speed is sadly 
diminished by the friction of their loose bodies against the water. 
A good example of these creatures is afforded by the 
well-known ACORN BARNACLE, so plentiful on our coasts. 
They have no necks like those of the Goose Barnacle, 
but are sessile on the rocks. Spots over which the 
tide only runs for a few hours are thickly studded with 
these Barnacies, and it is interesting to see how quickly 
they open their valves and fling out their arms as soon 
as the water covers them at each returning tide. When 
the sea withdraws, they close their shells firmly, and 
.retain within their interior a sufficiency of water wherewith to carry on the 
business. of respiration until the next tide brings a fresh supply. Total 
submersion seems to be hurtful to them. 
ACORN BARNACLE. 
(Balanus b,lanoides.) 
ARACHNIDA. 
ANOTHER class of animated beings now comes before us, which, under the 
general term of Arachnida, comprises the Spiders, Scorpions, and Mites. 
These beings breathe atmospheric air, have no antennz, and have four 
pairs of legs attached to the fore parts of the body. 
In some of the higher Arachnida there is a bold division into thorax and 
abdomen, and the former portion of the body is clearly divided into separate 
segments. By the earlier naturalists the Arachnida were placed among the 
insects, but may readily be distinguished by several peculiarities. In the 
first place, they have more than six legs, which alone would be sufficient to 
separate them from insects. They have no separate head; the head and 
thorax being fused, as it were, into one mass, called the cephalothorax. 
In many of the lower species there is not even a division between the thorax 
and abdomen ; and the body, thorax, and abdomen are merged into one uni- 
form mass, without even a mark to show their several boundaries. They 
undergo no metamorphosis, like that of the insects, for, although the young 
Spiders change their skins several times, there is no change of form. 
