CLASS IV. CRUSTACEA: 
ORDER 8. ISOPODA. 
611 
SQtJILLA, 
mouth, and a second piece, composed of the thoracic segments, which bears seven or eight pairs 
of long, slender feet on its margins. The abdomen is 
very small. These animals are oceanic in their hab- 
its, and are generally found in the southern seas. 
In the second family, the Squillidce, the body is 
elongated, and bears a considerable resemblance to 
the well-known insect, the Mantis ; hence the typical 
genus Squilla is frequently called the Sea-Mantis. 
Some of them attain the length of a foot or more, but 
their average size is about three or four inches. They 
are esteemed for food. The eyes are mounted on short footstalks. The antennse are of moderate 
length, and the outer pair have an oval plate at the base. The Squilla empusa, three to four 
inches long, is frequently taken on our coasts, but it appears to have no popular name. 
The species of the third family, the Mysidm^ closely resemble the shrimps, the thoracic seg- 
ments being completely inclosed in a carapace, and the abdomen bowed and furnished at its 
extremity with a caudal fin of five plates. They have received the name of Opossum- Shrimps 
from the curious pouch, formed of plates attached to the abdominal legs, in which the female 
protects both her eggs and young until the latter have attained a considerable development. 
They are not common in the European seas, but swarm in profusion in some parts of the 
world, especially in the Arctic Ocean, where they are said to constitute an important portion 
of the diet of the whale. The Mysis spinulosus is abundant on our coasts in winter. Other 
species are known on the coasts of Georgia and Florida. 
ORDER 3. ISOPOBA. 
This order includes the greater part of the JEdrioiyhthalmata, that is, Fixed-eyed Crustacea. ; 
the animals composing it exhibit a great varity of form and structure. TTie body is sometimes 
of an oval, sometimes of an elongated form, convex above and flat beneath ; the head is small, 
distinctly separated from the first thoracic segment, and bears a pair of round eyes, usually 
formed of a collection of simple eyes, but sometimes truly compound. They are divided into 
three sections, denominated, from their habits, Cursorial, Natatorial, and Sedentary Isopods. 
The latter comprises those species which are fitted for a strictly parasitic existence, being fur- 
nished only with chnging feet. We include only a single family in this section, the Bojyyridce^ 
which live in the branchial cavity of shrimps. 
The Natatorial Isopoda include the CymotJwidce, which are parasitic upon fishes, apparently 
having an especial predilection for their tails ; they have small heads, with short antennas, and 
the legs are short and terminated by hooks : also the Sphceroinidce, which have a nearly hemi- 
spherical body and live in the sea, particularly on rocky coasts. 
The Cursorial or Walking Isopods, include the Idotheidce, which aU live in the sea ; they are 
of an elongated form, and the antennse are usually of great length. The Idotea coica, one-third 
of an inch long, is known on our shores, and makes the serpentine tracks often noticed in the sand. 
The Asellidce resemble the preceding in many respects. One species, the Limnoria terebra7is, about 
one-sixth of an inch in length, is exceedingly destructive to wood- 
work immersed in the sea. It bores into timber in every direc- 
tion, apparently for the purpose of feeding upon it, and has often 
produced great alarm by its ravages. Some species of this family 
also live in fresh water. They are common on our shores. 
The last family, the Oniscidce, including the well-known Wood- 
Louse, Oniscus murarius, and many similar animals, are character- 
ized by the adaptation of their members to a terrestrial existence. 
THB WOOD-LOUSE MAGNIFIED. Nearly all these animals live on land, in damp places, under stones, 
dead leaves, and moss; some of them, as the Common Sow-Bug, 
0. asellus, are not uncommon in cellars. T^Hien alarmed, they roll themselves up into a ball, 
