414 INVERTEBRATE MORPHOLOGY. 



covered in at the sides by duplicatures of the body-wall, 

 which enclose a small respiratory cavity. 



The antennules and antennae are uuiramous except in 

 Apseudes in which the autenuules carry two terminal flagella. 

 The palps of the anterior maxillae project into the respiratory 

 chamber and serve for cleansing it, and the first thoracic 

 limbs, the maxillipeds, bear each an epipodial branchial ap- 

 pendage lying in the respiratory chamber. This limb and 

 the succeeding one are chelate, the inner angle of the penul- 

 timate joint being prolonged into a process against which the 

 terminal joint may be apposed. The abdominal appendages 

 are biramous swimming-feet in Tanais and Apseudes, the last 

 pair being in Anthura especially enlarged to form with the 

 telson a terminal finlike structure. 



2. Order Isopoda. 



The majority of the Isopoda are marine, the genus Asellus 

 (Fig. 189), however, occurring in fresh water, while Onisciis, 

 Porcellio, and Armadillidium are terrestrial, being commonly 

 known as Wood-lice or Sow-bugs. The body in all forms is 

 more or less flattened dorso-veutrally and only the anterior 

 thoracic segment is fused with the head, the remaining seven 

 remaining perfectly distinct. There is no trace in the adult 

 of a carapace, and the abdominal segments are usually small 

 and may be fused more or less completely. 



The maxillae are destitute of palps and the maxillipeds 

 (mxp) usually fuse together to form a sort of lower lip. The 

 remaining thoracic appendages are limblike and do not bear 

 any respiratory appendages, though lamellae are attached to 

 the basal joints of several of them in female individuals, 

 serving to form a brood-pouch. The five posterior abdominal 

 appendages are biramous and lamellar (ab), serving both for 

 swimming and for respiration, the anterior pair (op) usually 

 becoming hard and forming an operculum which covers in 

 the posterior more delicate appendages and in the terrestrial 

 forms may have branching spaces containing air (tracheae) 

 ramifying through them. 



The heart (Jit), in conformity to the position of the respira- 

 tory organs, is situated principally in the abdomen, extending 



