STOMA TO POD A. 2 7 3 



mediate between the typical members of the last and present series, the thorax being 

 larger and broader than the abdomen, while the carapace covers all but the last 

 five segments. The front angles of the carapace are produced to meet in a kind 

 of beak in front of the head, and the eyes are generally united in a single cluster 

 of ocelli. None of the thoracic limbs are prehensile or chelate. The first five 

 segments of the abdomen have no appendages in the female, although such limbs 

 are present in the male. In the sixth segment appendages are present in both 

 sexes, and form a fork-like termination to the body. Two of the best known 

 genera of the group are Cmna and Diastylis. The order, however, is relatively 

 a small one, containing only a little over a hundred species. It has, nevertheless, 

 a wide distribution, forms being met with in shallow and deep water in all seas, 

 although the Arctic Ocean produces individuals of the largest size and in the 

 greatest abundance. 



Order Isopoda. 



Unlike the last, this second order of the series exhibits great diversity of 

 structure. As a rule, the posterior seven segments of the thorax are free, and 

 at least the first three, and generally the first five segments of the abdomen are 

 short and sometimes fused together, while the sixth is the largest, and bears the 

 telson and a pair of appendages. The other abdominal appendages usually 

 overlap, and are modified to act as gills. The seven thoracic limbs are generally 

 large, and perform the function of walking or swimming organs, while the 

 posterior five pairs at least have no exopodites. There are four pairs of jaws, 

 namely, the mandibles, two pairs of maxillae, and one pair of maxillipedes. 

 Although there are many exceptions, it may be said that, as a rule, the body is 

 broad, short, and flattened. Corresponding to the structural variations, the 

 Isopods exhibit great diversity of habits and habitat. Most are marine, occurring 

 in shallow waters or at great depths. Some live in fresh water, others on land, 

 while others have taken to a parasitic life, and have thus to a great extent lost the 

 characters of the order. 



Of the five tribes, the Valvifera have the posterior pair of abdominal 

 appendages, or uropods, transformed into valves or flaps, covering part of the 

 lower surface, and constituting a chamber for the gills. The tribe contains two 

 families, the Arcturidce and Idoteidce. The former are distinguished by their 

 slender cylindrical shape, long lower antennas, and the length of the fourth 

 thoracic segment, which separates the posterior three pairs of thoracic legs from 

 the anterior pairs by a wide space. The anterior thoracic feet are slender and 

 hairy on the inner side, while the posterior feet are strong and prehensile, and 

 enable the animal to fix itself to the branches of corallines. In the Idoteidce the 

 body is longish and narrow, the thoracic segments being all of about the same 

 size and shape, and their appendages short, stout, and used for walking. The 

 anterior segments of the abdomen are short, and the posterior fused into a 

 caudal shield. The species of the -vims Tdotea live in shallow water, and 

 frequent places where there is an abundance of decomposing seaweed. They are 

 essentially carnivorous, feeding on dead fish, worms, and molluscs. 



vol. vi. — 18 



