CLASS IV. CRUSTACEA: ORDER 3. ISOP(U)A, 



Gil 



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 arc 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. 



sQuiLLA. '1^:' 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 antennae are of moderate 

 length, and tlie outer pair have an oval plate at the base. The Squilla eiwpusa^ 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 MysidcB, 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 liave received the name of Oj^ossum-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. ISOPODA. 



This order includes the greater part of the Edriophthalmata, that is. Fixed-eyed Crustacea ; 

 the animals composing it exhibit a great varity of form and structure. The 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 clinging feet. We include only a single family in this section, the Bopyridm^ 

 which live in the branchial cavity of shrimps. 



The Natatorial IsojJoda include the Cymothoida;, which are parasitic upon fishes, apparently 

 having an especial predilection for their tails ; they have small heads, with short antennae, and 

 the legs are short and terminated by hooks : also the Sphcei-omidce, which have a nearly hemi- 

 spherical body and live in the sea, particularly on rocky coasts. 



The Cursorial or Walking Isoj^ods, include the Idofheidce, which all live in the sea ; they are 

 of an elongated form, and the antennae are usually of great length. The Idotea cceca, 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 terebrans, 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 Oniscida;, 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. 

 THK 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-Lug, 

 0. asellus, are not uncommon in cellars. ^Vhen alarmed, they roll themselves up into a ball, 



