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L.M.B.C. MEMOIRS. 



No. XII. GAMMABUS. 



BY 



MARGARET CUSSANS, B.Sc. 



Zoological Department, University of Liverpool. 



Introduction. 



The Amphipoda and Isopoda form the two sub-orders 

 of the Edriophthalma, or Sessile-eyed Crustacea, which 

 are characterised by the possession of seven, or rarely 

 fewer, free thoracic segments, each bearing a pair of legs. 



The Amphipoda are distinguished from the Isopoda, 

 which are depressed dorso-ventrally and bear lamelliform 

 appendages functioning as gills on the frequently reduced 

 abdomen, by their laterally compressed body with an 

 elongated abdomen, and gills borne on the thoracic legs. 

 Usually quite small, being only in rare cases as much as 

 three inches long, the Amphipoda abound in both salt and 

 fresh water, through which they move by springing and 

 swimming. They form an important factor in the food- 

 supply of the sea. 



They may be divided into three groups : — 



(1) The Ljehodipoda, including Cyamns ceti, which 

 is parasitic on the skin of whales, and the thin, curiously 

 elongated Caprellidae, mostly found living among Polyzoa 

 and Hvdroids. 



