CHAPTEK XII 



THE ISOPODA 



Order Isopoda, Latreille (1817). 



Sub-Order 1. Asellota. 



„ 2. Phreatoicidea. 



„ 3. Flabellifera. 



„ 4. Valvifera. 



„ 5. Oniscoidea. 



„ 6. Epicaridea. 



Definition. — Peracarida in which there is no distinct carapace,, 

 but the first thoracic somite (rarely also the second) is coalesced 

 with the head ; the telson is rarely defined from the last somite ; 

 eyes sessile or set on immovable processes of the head ; antennules 

 uniramous (except in Bathynomus) ; antennae sometimes with a 

 minute exopodite ; thoracic limbs without exopodites ; first pair 

 modified as maxillipeds, the epipodite, when present, not enclosed 

 in a branchial cavity ; remaining pairs all similar or variously 

 modified, coxojDodite always short, often fused with the body and 

 expanded laterally ; pleopods typically biramous, with lamellar, 

 branchial rami, generally the second and sometimes also the first 

 pair modified in the male ; heart lying wholly or partly in abdomen ; 

 the young leave the brood-pouch before the appearance of the last 

 pair of thoracic limbs. 



Historical. — The terrestrial habits of the more familiar members 

 of this order, and the close resemblance which some of them bear to 

 certain Diplopoda (Oniscomorpha), led to their being widely separated 

 from the other Crustacea in many of the eai'lier systems of classifi- 

 cation. Even Latreille, to whom the name Isopoda is due, placed 

 them at first among the Insecta. Leach ranked them along with 

 the Amphipoda in his group Edriophthalma, thus giving them the 

 position which they occupy in most modern systems. Our know- 

 ledge of the morphology and classification of the Isopoda is largely 

 due to the work of Scandinavian naturalists, especially to the 

 monographs of Schiodte, Meinert, G. 0. Sars, Bovallius, Budde- 



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