238 CO]MPAEATIV£ ANATOMY. 



The serial similarity^ which accompanies the lower grade o£ 

 structure of the parapodia in Annelids, is also seen in the lower 

 types of the Arthropoda^ as for example in PerijDatus, the Myria- 

 poda, and in many Crustacea (Phyllopoda, etc.). In Peripatus the 

 appendages retain their lowest condition, and resemble the parapodia 

 of Vermes ; their resemblance indeed to the appendages of the 

 Tracheata is merely due to the possession of a movable terminal 

 segment, which carries a pair of claws. Two phtenomena, which may 

 be recognised in these appendages of the Arthropoda, tend to convert 

 the multifid Annelid-like organism into a more compact one. 



The first of these is the metamorphosis of the appendages 

 into a series of different structures having the most varied functions. 

 As the appendages come to differ in function they change in form, 

 and become adapted to their new duties. 



The second phaenomenon is the limitation of the number 

 of the appendages, which obtains in the higher divisions, con- 

 currently with the greater development of heteronomous metameres, 

 or with the foi'mation of larger divisions of the body by the fusion 

 of its separate groups of metameres. 



Appendages of' the Branchiata. 



§ 185. 



The appendages ai'e seen at their simplest in the NaupHus form 

 of the Crustacea. At first two, and afterwards three pairs of 

 jointed appendages appear in the unsegmented body. They all 

 function as locomotor organs (swimming-feet), and are beset with 



sette, which are often arranged in large 

 bundles. The first pair of these appen- 

 dages (Fig. 121, a) is sim^Dle, the second 

 and third pairs forked, and this forked 

 character obtains in all the succeed- 

 ing appendages of the Crustacea. The 

 first two pairs are distinguished from the 

 third pair, and from those which appear 

 later on, by their connection with the nerves, 

 which arise from the supraoesophageal gan- 

 Fig. 121. Nauplius of a „i^q^ ^^^j^g the third and all succeeding 



Copepod (Cyclops), a h c ° ■ tti ji t i- 



Appendages. pairs are supplied by the lower ganglia. 



With this is connected a difference in 

 function, for the two anterior pairs are ordinarily developed into 

 antennae. In many Copepoda they still function as locomotor 

 organs; they do so most completely in the Ostracoda. In the 

 Cladocera also the second antenna is still developed as a swimming 

 organ, and this stage is retained by the Phyllopoda during a very 

 long period of development. This ju^stifies us in reckoning the 

 dorsal outgrowths (the first two pairs) as strictly belonging to the 



