ISOPODA. 433 



The branchiae are in the form of bladders, white in the majority, capable of being puffed up and 

 used in s\Yimming, and covered by two plates or valves of the last segment, laterally adherent to its 

 sides, longitudinal, biarticulate, opening in the middle in a straight line, like a pair of cupboard doors. 

 The tail is formed of three segments, of which the last is the largest, with neither appendages nor 

 lateral swimmerets. All these Crustacea are marine. 



Idotea, Fab., have the legs strongly hooked, and all of the same form, and the lateral antenns are shorter than 

 half the body. (Onisciis Entomon, Linn.) 



iSteiiosoma, Leach, has the body hnear [and depressed], and tlie [lateral] antenna; nearly equal to the body in 

 length. (Stenosoma lineare, Leach.) 



Arcturus, Latr., is very remarkable in the form of the second and third pairs of legs, which are directed for- 

 ward, and terminated by a long hirsute joint, and unarmed or feebly-hooked : the two anterior are applied to 

 the mouth ; the six posterior legs are long, formed for walking, directed backwards, and bifid at the tip. In 

 the length of the antennae and form of the body they approach Stenosoma. I have only seen one species (A. tubercu- 

 latus), brought from the North Seas in one of the late English expeditions to the Arctic Pole. [This species was 

 published by Sabine under the name of Idotea Baffini, but a second species exists in the north of our coast, which 

 I have described in detail, with figures, in the first volume of the Transactions of the Entomological Society, under 

 the name of Arcturus longicornis.'] 



The fifth section, Asellota, Latr., is also formed of Isopods, having four very distinct antennae 

 arranged in two lines ; they are setaceous, and terminated by a multiarticulate filament, two mandibles, 

 four maxillae, covered in general by a kind of lip formed of the first pair of foot-jaws ; vesicular branchiae 

 disposed in pairs, and covered by two longitudinal, biarticulated, but free plates : the tail is formed of 

 a single segment, and without lateral swimmerets, but with two bifid styles, or two very short ap- 

 pendages in the form of tubercles in the middle of the posterior margin. 



Asellus, Geoffroy, has two bifid styles at the extremity of the body, the eyes distinct, the superior antennas as 

 long as the basal joint of the inferior, and the hooks at the tips of the legs entire. Tlie only species of this genus 

 is the Idotea aquatica, Fab. (Squilla asellus, De Geer), which is very abundant in fresh and stagnant water. It 

 crawls slowly, at least, when not alarmed. In the spring it creeps out of the mud in which it had buried itself 

 during the winter. After impregnation the female carries her eggs, in great numbers, inclosed in a membranous 

 sac, placed beneath the breast, and opening by a longitudinal slit, in order to allow the young ones to escape. 



Oniscoda, Latr. (laiiira, Leach) have the eyes contiguous, and the hooks of the tarsi bifid at the tips. (lanira 

 maculosa, Leach, found on the coast of England amongst the sea-weeds.) 



lara, Leach, has only two tubercles at the extremity of the body. (/. albifrons. Leech, also found on the coast 

 of England.) 



The sisth and last section of the order Isopoda, or the Oniscides, Latr., have also four antennae, but 

 the intermediate pair is so minute as to be scarcely apparent, and never consists of more than two joints ; 

 the lateral are setaceous. The tail is composed of six segments, with two or four style-like appendages 

 at the posterior margin of the hind segment, and destitute of lateral swimmerets : some species are 

 aquatic, but others are terrestrial. In the latter the anterior plates of the under-side of the tail exhibit 

 a row of small holes, through which the air penetrates, and is brought into contact with the respiratory 

 organs, which are inclosed beneath. 



Some of these are marine, and have more than nine joints in the antennfe, (including the terminal annub). 



Tylos, Latr., appears to have the power of rolling itself into a ball ; the posterior segment is semicircular, and 

 exactly fits the incision made by the preceding ; the posterior appendages are very minute ; the antennae have only 

 nine joints. 



Ligla, Fab., have the terminal annuli of the antennae very numerous, and the body is terminated by two styles, 

 divided at the tip into two branches. 



The type, Oniscus oceanicus, Linn., is about an inch long, of a gray colour, with two large yellow patches on the 

 back. The lateral antenns are about half the length of the body, the terminal filament being composed of thirteen 

 joints. The terminal styles are as long as the tail itself. It is very common on the coast, chnging to the rocks and 

 to the parapets of maritime erections. When it is attempted to be seized it immediately folds up its legs, 

 and drops. Another species, Oniscus hypnorum. Fab., has the terminal division of the antennae 10-jointed, and 

 the basal part of the anal styles armed with a tooth on the inside. 



The other Oniscides are terrestrial, and the lateral antenna; have not more than eight joints, of which the propor- 

 tions towards the extremity gradually diminish, none of them appearing to be divided into annuli. 



Philoscia, Latr., has the lateral antennae 8-jointed, and exposed at the base ; the four exterior posterior appen- 

 dages are nearly equal. They are always found in moist situations. (Oniscus syloestris, Fab. ; O. muscorum, Cuv.) 



Oniscus, proper, Linn., have also 8-jointed lateral antenna;, but the base is concealed, and the two outer appen- 

 dages at the tip of the tail are larger than the two internal. The animals of this and the two following genera 

 are called wood-lice, St. Anthony's hogs, &c. They frequent dark and concealed places, such as cellars, caves, 



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