CHAPTEE XI 



THE TANAIDACEA 



Order Tanaidacea, Hansen (1895). 



De/initio7i.—PeriXGa.ridix in Avhich the carapace coalesces dorsally 

 with the first two thoracic somites, and overhangs on each side to 

 enclose a branchial cavity ; the telson is not defined from the last 

 somite ; the eyes, when present, are usually set on short, im- 

 movable peduncles ; the antennules may be biramous ; the antennae 

 may have a small exopodite ; vestigial exopodites are sometimes 

 present on the second and third pairs of thoracic limbs ; the first 

 pair of thoracic appendages are modified as maxillipeds ; they have 

 an epipodite lying in the branchial cavity, but no exopodite ; the 

 second pair are chelate ; the pleopods are usually present and 

 biramous ; the uropods are slender ; the young leave the brood- 

 pouch before the appearance of the last pair of thoracic limbs. 



Historical. — The first known member of this order Avas that 

 described by Montagu in 1808 as Cancer gammarus falpa, now placed 

 in the genus Apseucles. Leach, Latreille, and others ranked the 

 species known to them among the Amphipoda, while H. Milne- 

 Edwards placed them among the Isopoda, remarking, however, that 

 they established a transition to the Amphipoda. Dana (1852) 

 united them with certain parasitic and other Isopoda in a group 

 Anisopoda interposed between Isopoda and Amphipoda, and Spence 

 Bate in 1868 combined them with the Isopod Anthuridae and 

 Anceidae (Gnathiidae) in a no less heterogeneous group of " Isoi^oda 

 aberrantia." Van Beneden (1861) called attention to the im- 

 portance of the carapace of Tanais as a systematic character, and 

 approximated that genus to Cnma and Mysis. F. Midler (1864) 

 also attached great weight to the same character. Gerstaecker 

 (1886) once more included the Tanaidacea among the Amphipoda; 

 but this retrograde step has met with little support, and most 

 modern writers follow Sars in ranking them as one of the tribes of 

 Isopoda (Chelifera). Claus, however, in 1888 placed them in an 

 independent order, for which he adopted Dana's name Anisopoda, 



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