354 THE INVERTEBRATA 



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 can form a siphon. 



Diastylis (Fig. 259) is a British genus. 



car. 



^an'. 



'Th.4 



Fig. 259. Female Diastylis stygia. After Sars. The carapace is represented as 

 transparent to show the gill. car. carapace; an.' first antenna; Th.^, fourth 

 thoracic limb; gill, gill borne on first maxilliped; Th.s-Th.8, fifth to eighth 

 thoracic limbs ; Th. free part of thorax ; ab. abdomen ; ab.6, appendage of the 

 last segment of the abdomen. The male has pleopods. 



Order TANAIDACEA 



Peracarida with a very small carapace, covering only the ist thoracic 

 somite, 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. 



Tanais and Apseudes 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. 260), found just 

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



