THE DECAPODA 273 



this is only the case in the Eryonidea. In the Astacura the 

 first pair, and in the remaining lieptantia all the five pairs, of 

 legs have the basipodite and ischiopodite immovably united. 

 Perhaps correlated with this fusion is the presence of a "fracture 

 plane " in the basipodite, at which separation of the limb takes 

 place in autotomy in many lieptantia. 



In a few Decapoda some of the legs become quite vestigial or 

 even disappear altogether. In the Sergestidae the last tAvo pairs 

 are reduced, and in Acetes the last pair and in Leucifer the last two are 

 quite absent. In the Pinnotheridae (Brachyura) the last pair may 

 be rudimentary or absent. In many Crangonidae the second pair are 

 smaller than the others, and in Paracrangon they disappear entirely. 

 This case is especially noteworthy since the suppression of members 

 of a meristic series rarely occurs except at one end of the series. 



The epipodites and associated structures of the thoracic limbs 

 will be described below in connection with the branchial system. 



The pleopods of the Decapoda present typically the same structure 

 as those of the Euphausiacea. Of the two segments composing the 

 protopodite the first is usually small, often apparently absent, the 

 second elongated and often stout. The two rami may be multi- 

 articulate and nagelliform, more often flattened and unsegmented, 

 and bear a marginal fringe of natatory setae. The endopodite may 

 bear on its inner margin an appendix internet tipped with a group of 

 coupling-hooks. 



It is interesting to note that the Penaeidea and Stenopidea, 

 which, on the whole, take the lowest place among the Decapods, 

 never possess an appendix interna (except in so far as an element 

 derived from it may possibly share in forming the copulatory 

 appendage on the first pair of pleopods in the male), though the 

 presence of that organ in the Leptostraca, Euphausiacea, and 

 Stomatopoda shows that its possession must be reckoned a 

 primitive feature among the Malacostraca. 



The pleopods are most strongly developed in the Natantia, 

 where they form the chief swimming-organs. In the Reptantia the 

 natatory function is less important and the pleopods are generally 

 feebler, though in some fossorial Thalassinidea they are of consider- 

 able size. An appendix interna is wanting except in some Thalas- 

 sinidea and in the Scyllaridea, where the pleopods are peculiarly 

 modified. In the Anomura, excluding the Thalassinidea, the 

 pleopods are generally feeble, often uniramous, and are sometimes 

 absent from the first somite, as they are also in the Scyllaridea 

 and Parastacidae. They are absent altogether in the males of 

 Hippidea, Lithodidae, and of some other Paguridea ; when present 

 in the Paguridea, they are, as a rule, developed only on one side 

 of the body and an appendix interna is sometimes present. In 

 the Brachyura the first and second pairs (which are specially 



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