Amphipoda 3 



and tlie 1**^ and 3'' are each expanded to a plate, provided with denticles 

 and spines. 6) Maxilliped (Fig. 49 p. 186) 7-jointed; first Joint, sometimes 

 also the 2'^ of tbe one maxilliped fused with that of the other maxilliped; 

 4*'* — 7^^ joints forming tbe palp; 1 or 2 or 3 joints of.the palp sometimes 

 wanting; articulation still more reduced in the Hyperiidea. 



Each Segment of peraeon commonly carries one pair of legs. The 

 7 legs of peraeon are divided into 2 groups-: tbe 1*** — 4*^ leg, and the 

 5th — 7th Jgg.. tbe former forming an angle open to the front, the latter 

 to tbe rear; the 3*^ Joint in all of them acts as the knee of the angle. 

 The first 2 legs are called guatbopods (not so much on account of their 

 own function, as from their bomology with the 2'^ and 3<^ maxillipeds of 

 Decapoda); so legs of peraeon are: 2 gnatbopods and 5 peraeopods. 

 All are uniramous; tbe normal number of joints is 7. The first Joint, greatly 

 enlarged (or perbaps soldered) to the so-called epimera or side-plates in 

 tbe Gammaridea, is iiable to evanescence in tbe Hyperiidea, and wanting 

 in tbe Caprellidea; the last joints of gnatbopod 1 and 2 usually, of other 

 peraeopods very rarely (except in some genera of Hyperiidea), form a grasping 

 organ. Close at the base of gnatbopod 2 and of the peraeopods there are 

 the branchial vesicles, 2 — 6 on each side. 



Tbe legs of tbe first 3 segments of pleon are called pleopods, tbose 

 of the 4*^ — 6^'" Segments uropods. Tbe pleopods consist of a basal Joint 

 and almost always 2 many-jointed, often flagelliform rami, beset with swimming- 

 setae; the basal joints of each pair of pleopods are often coupled by booked 

 spines (Fig. 120 p. 696). The rami of the m-opods are sometimes 2-, usually 

 1-jointed; they usually bear spines, which are sometimes uncinate. 



External sexual characters are found especially in antenna 1 and 2, 

 in tbe grasping hands of gnatbopods and peraeopods. and in tbe marsupial 

 plates of Q. The ducts of ovaries open in the 5^'\ tbose of testes in tbe 

 7*^ Segment of peraeon; no proper copulatory organs observed. 



x4raong the anatomical characters especially of value for Classification are 

 tbe number. form and structure of tbe paired eyes, the number of tbe long 

 midgut-glands (hepato-pancreatic tubes) flanking the intestinal canal, and 

 of the so-called rectal glands; also the number of tbe venöse ostia of 

 the heart and of tbe un-fused ganglia of the venti*al nerve-chain. 



The eggs are laid in the raarsupium, formed by tbe marsupial plates; 

 tbe young, when batching, resemble tbe adult; only in tbe Hyperiidea some- 

 times pleon and pleopods are at fii-st scarcely developed. 



The great majority of the Amphipoda are marine; only among the (Tammaridea 

 are there species living in fresh or brackish water, or near the sea above highwater- 

 mark, or eyen, though exceptionally, far from water in damp places. The Gammaridea 

 and Caprellidea are extensively litoral, the Hyperiidea pelagic; also in each legion 

 there are species living on swimming animals or floating objects. The geographical 

 distribution is cosmopolitan, ascertained to within less than 600 km of the north 

 pole, to a depth of SBIO m in the ocean, and in fresh water to a height of 4054 m 

 above sea-level. 



Their diet is chiefly of animal substances. Some Gammaridea construct dwelling 

 tubes, some live in grooves and hollows, which they burrow in ascidians, sponges or 

 wood. The species, living on or in coelenterata, sponges, tunicates, moUusca, are 

 perhaps commensals, but at least sometimes feed on the body of their host, sometimes, 

 as Phronima, unite feeding with housebuilding. True parasites are the Cyamidae. 

 Parasites of the Amphipoda are Gregarinida, Trematoda and (^opepoda. 



Long before J. C. Fabricius' establishment of the Genus Gammarus, Amphipoda 

 were described under the generic designations of Cancer, Oniscus, Astacus and Squilla. 



