Structure of the Mouth in Sucking Crustacea. 261 



mode of cooperation, which depends upon the latter, has at 

 present given me the following results. 



The structure of the mouth in masticating Amphipoda pre- 

 sents three principal types, the connexions and mode of 

 movement of the mandibles offering the best distinctive marks 

 for them. To the first type belong the great bulk of those 

 genera which are known as Gammarina and Caprellina^ and 

 which have been subdivided into numerous families according 

 to their general appearance, and mixed up with Amphipoda pre- 

 senting other types of oral structure. In reality, all those 

 presenting this first tjpe form one continuous series commen- 

 cing with genera like Gammarus, having the body compressed, 

 the epimera expanded into large branchial opercules, and a 

 fully developed tail, from which we are gradually led on to 

 the' slender and cylindrical forms like Caprella and the flat 

 form represented by Cyamus, having small or no epimera and 

 more or less reduced tail — the modifications in the general 

 form of the body being determined by the gradual substitution 

 of an equipment for crawling and climbing in various combi- 

 nations, instead of an equipment for jumping and swimming, 

 and by a corresponding reduction in the size of the respiratory 

 surface and the strength of the respiratory mechanism. With 

 regard to La3modipoda it must be observed, however, that 

 Kroyer, in discussing the systematic rank of this division 

 (' Naturhistorisk Tidsskrift,' iv. pp. 490-496), pays too little 

 attention to the cardinal point to which Latreille attributes 

 so much importance (' R^gne Animal,' 1829, iv. p. 126), viz. 

 that the second segment of the thorax is joined to the head, 

 as well as the first segment, and that its limbs almost assume 

 the character of maxillipeds. 



It has been indicated already in the proper place, in the 

 foregoing division of this treatise (Naturh. Tidsskr. 3 ser. iv. 

 p. 177*) that the present series of Amphipoda entirely corre- 

 ponds to the series of Onisci amongst Isopoda ; but comprising, 

 as it does, animals of more varied habitats than the latter, it 

 is richer in modification of its type, and is in that respect most 

 strikingly illustrated by comparison with several of those 

 Ulonata, which amongst Insecta approach nearest to Crustacea, 

 especially Mantidae, Phasmidse, Acridii, Locustidaj, and Gryl- 

 lidge, where we meet with quite corresponding series of forms, 

 representing a corresponding gradual transition from flight to 

 walking, and finally digging, as a condition of life. 



The mandibles in tliis type are short, trilateral, with a broad 

 triangular base, of which the outer corner with a short protu- 



* Ann. & Mag. Nat. Hist. ser. 4, vol. i. p. 7. 

 Ann. & Mag. N. Hist. Ser. 4. Vol. xviii. 18 



