APPENDAGES OF ARTHROPODA. 



230 



ffl.C 



category of appendages even on functional grounds. In the Mala- 

 oostraca the two pairs of antennas have no relation to locomotion, 

 as may indeed be always seen from their form. The hinder pair 

 (Fig. 123, at 1 ) is ordinarily placed beside, and often is larger than, 

 the anterior pan (at). (Cf. also Fig. 128, a' a".) 



All the other appendages are ventral in position. When the 

 metameres begin to be formed they make their appearance following 

 on the first pair of swimming-feet, found in the 

 Nauplius ; one pair is distributed to each seg- 

 ment. Like this swimming-foot, and like the 

 second pair of antennas they have two ter- 

 minal branches; these are not as a rule dif- 

 ferentiated to an equal extent, for one branch is 

 greatly developed and forms the principal part 

 of the appendage, while the other forms an 

 appendage to it. It sometimes, however, has 

 relations to the respiratory function, and is then 

 largely developed. The appendages have very 

 different functions, and are so metamorphosed 

 as to adapt themselves to these functions. 



Such of these more anterior ventral appen- 

 dages as lie near the mouth are converted into 

 mouth-organs, and either form jaws 'and jaws 

 only, or foot -jaws. The relation between this 

 arrangement and the concrescence seen in the 

 cephalothorax, has been already referred to. A 

 few pairs only are converted into gnathites in 

 the Branchiopoda, and the remaining appendages, 

 of which there is a veiy large number in most 

 of the Phyllopoda, have very much the same 

 characters as swimming-feet. The same thing 

 happens in the Ostracoda, Copepoda and Cirri- 

 pedia. In the last-mentioned group the posterior 

 appendages are converted into the characteristic 

 cirri (Fig. 119, bb'). The metamorphosis of the 

 appendages is seen most completely in the Mala- 

 costraca ; let us examine more closely the arrange- 

 ment seen in one of the Decapoda. We here find 

 six pairs of appendages converted into gnathites, 

 in the more anterior of which the form of the 

 Phyllopod foot is retained almost unmodified. A 

 pair of strong jaws (Fig. 122, md), are succeeded by two pairs of 

 maxillas (mx mx'), and these by three pairs of maxillipedes (mp 

 mp' mp"). The latter gradually pass into the locomotor appen- 

 dages ; of which there are five pairs (Fig. 123, P 1 — P 3 ), which are 

 attached to the cephalothorax, and together with the maxillipedes 

 and maxillos indicate the number of metameres of which it is built up. 

 The terminal joint of most of the ambulatory feet forms a scissor- 

 shaped termination, owing to the great size of the penultimate 



Fig. 122. Gnathites 

 of Astacus fluvia- 

 tilis. md Mandible. 

 mx First ; mx ' Second 

 maxilla, mp mp' mp" 

 Foot-jaws, a Endo- 

 podite. c Exopodite. 

 d Epipodite. 



