ISOPODA 373 



lamellar and functioning as gills or lungs, while the last pair, which 

 are called uropods (Fig. 602) and have a distinct exopodite and endopo- 

 dite, are elongated and function as feelers or as swimming fins: about 

 18 families and over 2,300 species, which are small, creeping or swim- 

 ming animals, most of them marine, living under stones along the sea- 

 shore and among seaweed or parasitic on fish; a few live in fresh water, 

 while several very familiar species are terrestrial. 

 Key to the families of Isopoda here described : 



a, First pair of legs chelate 1. TANAIDAE 



a, First pair of legs not chelate. 

 Z>! Uropods lateral. 

 c x Uropods with telson forming a caudal fin ; pleopods mostly natatory. 



d x Exopodite of uropod arches over base of telson 2. ANTHURIDAE 



d, Exopodite of uropod does not thus arch. 

 ^ Abdomen composed of 6 segments. 

 /! Both branches of uropod well developed. 

 ffi Not parasitic ; body more or less cylindrical ; eyes usually small. 



3. ClROLANIDAI 



g t Parasitic on fish. 

 h t Body broad and flattened ; first 3 pairs of legs prehensile . . 4. MGID\E 



h t Eyes large ; legs all prehensile 5. CYMOTHOIDAE 



/, The 2 branches of uropod not of same length, the exopodite being 



minute ; animal bores in wood 6. LIMNORIIDAE 



e. Abdomen composed of 2 segments. . 7. SPH^ROMIDAE 



c, Uropods arch over the other pleopods, covering them 8. IDOTHEIDAE 



&, Uropods terminal. 

 Cj Animals aquatic. 



d x First antennae much smaller than the second, but not minute. 

 6j Animals not parasitic. 



/i Fresh-water forms 9. ASELLIDAE 



/ 2 Marine forms 10. JANIEIDAE 



e a Animals parasitic on decapods 15. BOPYRIDAE 



d 3 First antennae minute and not easily seen 13. LIGYDIDAE 



c, Animals terrestrial (occasionally aquatic). 

 d t Cannot roll itself into a ball (except CyUsticus convexus). 



i End segment of abdomen pointed or angular 11. ONISCIDAI 



e 2 End segment truncate or indented ... 14. TRICHONISCIDAE 

 d 2 Can roll itself into a ball 12. ARMADILLIDIDAE 



FAMILY 1. TANAIDAE. 



Body more or less cylindrical, with 6 free thoracic 

 segments; first pair of legs chelate; abdominal append- 

 ages, when present, natatory; gills on the thoracic append- 

 ages: about 16 American species; marine. 



1. TANAIS Audouin and Edwards. Only 3 pair of 



pleopods present; uropods uniramose and short: 5 Amer- Tanaiscavolini 



(Harger). 

 ican species. 



T. cavolini Milne-Edwards (Fig. 591). Body slender, abdomen with 

 5 segments; uropods 3-jointed; length 4 mm.; width 1 mm.: Greenland 

 to Long Island Sound ; in shallow water on piles and among seaweed. 



