ISOPODA. 



ISOPODA. 



293 



knot or joint. Sometimes the sixth segment of the body is sensibly 

 longer than the preceding segments and the succeding one. One of 

 the leaflets of the fins only is projecting. 



Ncfta (Campecopcea, Leach). Sometimes the sixth segment of the 

 body is of the length of the preceding segments, and of the succeed- 

 ing one, as in Ciliccea, where one of the leaflets of the fins only is 

 projecting, the other leaning against the posterior border of the last 

 segment. 



CiltCfca Lati'eilll. 



Cymodtfea (Leach). In this form the leaflets of the fins are pro- 

 jecting an 1 directed backwards. The sixth segment is not prolonged 

 posteriorly, and the extremity of the last segment has a small blade 

 in a notch. 



Cymodocea Laiiiarckii. 



Dynamene. Resembling Cymodocea in the projection and direction 

 of the leaflets of the fins, but having the sixth segment prolonged 

 backwards, and the last with a simple slit only, there being no blade. 



Others again, as Anthura, have a vermiform body, and the antenna;, 

 hardly BO long as the head, consisting of four joints. The leaflets of 

 the posterior fins form by their disposition and approximation a 

 sort of capsule. The anterior feet are terminated by a mouodactyle 

 claw. 



Anthura gracilit, magnified. 



4. IDOTEID.E (Leach). This section consist of laopoda whose 

 antenna; are four in number, but upon the same horizontal and trans- 



Idotea ti icuipidata. 



verse line ; the lateral ones are terminated by a stem ending in a 

 ooint, gradually decreasing and pluriarticulate ; the intermediate 



HAT. HUT. DIV. VOL. 111. 



antennae are short, filiform, or a little the largest towards the end, 

 and 4-jointed, none of the joints beiug divided. The conformation of 

 the mouth is the same as in the preceding sections. The branehia; 

 are in the form of bladders, white in the greater part, susceptible of 

 being blown up, capable of aiding in swimming, and covered by two 

 blades or valvules of the last segment, adhering laterally to its borders, 

 longitudinal, biarticulate, and opening in the middle by a straight 

 line, like a folding door. The tail is formed of three segments, the 

 last of which is much the largest, without appendages at the end or 

 lateral fins. These Crustaceans are all marine. 



Idotea (Fabr.). All the feet strongly unguiculated and identical ; 

 the body oval or simply oblong, and the lateral antennae shorter than 

 the half of the body. 



Stenosoma (Leach). Differing from Idotea in the linear form of the 

 body and the length of the antenna;, which surpasses the half of that 

 of the body. 



Stcnoaoma lineare, natural size. 

 a, lamina; of the under part of the abdomen. 



Arclurui (Latreille). Very remarkable for the form of the second 

 and third feet, which are directed forwards, and terminated by a long 

 bearded joint, unarmed or feebly unguiculate ; the two anterior feet 

 are applied upon the mouth and uuguiciilated ; the last six are strong, 

 ambulatory, thrown backwards, and bidentated at their extremity. 

 In the length of the antenna; and form of the body Arcturns 

 approaches Stenoioma. M. Latreille (1829) says that he never saw 

 but one species, A. tubenulatus, brought home from the North Seas 

 by one of the last English expeditions to the North Pole. 



5. ASELLOTA (Latreille). The fifth section consists of laopoda 

 with four very apparent antennae which are disposed on two lines, 

 and aro setaceous and terminated by a pluriarticulate stem. There 

 are two mandibles, four jaws, ordinarily covered by a species of lip 

 formed by the first jaw-feet. The branchias are vesicular, disposed in 

 pairs, and covered by two longitudinal and biarticulate but free 

 leaflets. The tail is formed of a single segment, without lateral fins, 

 but with two bifid needle-like processes, or two very short appendages 

 in the form of tubercles at the middle of its posterior border. There 

 are other lamellar appendages situated on its inferior base, more 

 numerous in the males than in the females, and these serve to 

 distinguish the sexes. 



Axe/lus (Geoffroy). Two bifid needle-like processes at the posterior 

 extremity of the body ; eyes distant; superior antenna; at least as long 

 as the peduncle of the inferior antenna;. Hooks at the end of the 

 feet entire. 



A . aquaticua is very abundant in fresh stagnant waters, as in the 

 pools about Paris. It moves slowly when not terrified. In the spring 

 it comes forth from the mud, in which it has passed the winter. The 

 male, which is much larger than the female, carries her about for a 

 space of eight days, holding her by means of his fourth pair of feet. 

 When he quits her she is pregnant with a great number of eggs, 

 inclosed in a membranous sao placed under her breast, and opening 

 by a longitudinal slit to give passage to the young. 



Oniacodo, (Latreille). These, the Janirce of Dr. Leach, differ from 

 the Aadli in the approximation of their eyes, in having their superior 

 antenna; shorter than the peduncle of the inferior ones, and in the 

 hooks of the tarsi, which are not bifid. M. Latreille remarks that the 

 only species known (Junira mactdosa of Leach) has been found on the 

 coasts of England among the sea-weeds and Uh'CK. 



Jcera (Leach). This form, in the place of the needle-like processes 

 (stylets) at the end of the tail, has only two tubercles. M. Latreille 

 remarks that only a single species (/. albifrom, Leach) has been found, 



u 



