PERACARIDA 395 



peds; a large epipodite, bearing a gill, on the ist thoracic limb and 

 natatory exopodites on some of the others; and slender uropods, 

 which do not form a tail fan. 



Small, marine organisms which live in mud or sand and are highly 

 specialized, especially in their respiratory mechanism, for that 

 habitat. The first thoracic exopodites form a valved exhalant siphon 

 with the carapace lobes which lodge them. 



Diastylis (Fig. 272) is a British genus. 



Order TANAIDACEA 



Peracarida with a very small carapace, covering only two thoracic 

 somites, with which it fuses; eyes (if present) on short, immovable 

 stalks; a small scale, or none, on the antenna; thoracic exopodites 

 absent or vestigial, a branchial epipodite on the maxilliped; and 

 slender uropods, which do not form a tail fan. 



Small, marine crustaceans, usually inhabiting burrows or tubes, 

 which are in an intermediate condition between the Cumacea and 

 Isopoda in respect of the loss of the caridoid facies. 



Apseudes (Fig. 273 A), and Tanais, which differs from it in having 

 short, uniramous antennules and liropods and no antennal scale, and 

 lives in a mass of fibres it secretes, are British genera. 



Order ISOPODA 



Peracarida without carapace ; with sessile eyes ; the body usually de- 

 pressed; the antennal exopodite absent or minute, the thoracic limbs 

 without exopodites, the first pair modified as maxillipeds, the re- 

 mainder usually alike ; the pleopods modified for respiration, and the 

 uropods usually not forming a tail fan. (Any of these features may 

 be absent in the adults of parasitic forms.) 



The Isopoda are a large group and exhibit much variety. We will 

 study as an example Ligia, the shore slater (Fig. 274), found just 

 above tidemarks in Britain and most parts of the world. This creature 

 has a depressed, oval body, the cephalothorax, formed by fusion of 

 the ist thoracic somite with the true head, lying in a notch on the 

 anterior edge of the 2nd somite of the thorax. Two large, sessile com- 

 pound eyes take up the sides of the head. The abdomen continues the 

 outline of the thorax, and its 6th SQmite is fused with the telson. The 

 anteiiniiles , which are usually short in isopods, are here minute. The 

 antennae are of a good length, which is due to the elongation of the 

 two joints which precede the flagellum. The mandibles, unlike those 

 of most isopods, lack the palp, but otherwise they are complicated, 

 having between the incisor and molar processes a row of spines and 



