ARTHROPODA 



353 



are innervated from the ventral chain. In their elongate shape antennae 

 are not unlike legs, but they lack the terminal claws. 



The form of the jaws is strikingly modified. One or two basal joints 

 serve for the comminution of food, and these parts are strong and are 

 covered, especially on the medial side with a 

 hard, toothed chitin (figs. 369, 2 ; 374, ///, I'). 

 The other joints may entirely disappear, or 

 may form a more or less leg-like appendage, 

 the palpus. Since several appendages may be 

 modified into jaws, the first are called mandi- 

 bles, the next maxilla?, and second maxilla? 

 may follow. The maxillipeds may have more 

 the appearance of jaws, at other times are 

 more leg-like (tig. 369, 5-7). The false feet 

 (pleopoda) are small and inconspicuous ap- 

 pendages which have various functions: they 

 may serve as gills or supports for gills, places 

 for the attachment of eggs, organs for the 

 transfer of sperm, or as swimming or creep- 



ing organs. 



FIG. 369. Appendages of 

 the crayfish, i, first antenna; 

 2, mandible; 3, 4, first and 

 second maxillce; 5, 6, 7, maxil- 

 lipeds; 8, walking leg; 9, 

 pleopod. 



These appendages have constant positions 

 in the body. First on the head come the an- 

 tenna? and then, in the region of the mouth, 

 the jaws and, so far as they are present, the 

 maxillipeds. Third come the true feet, and 

 lastly, when they exist, the false feet. Those 

 somites which bear antennae or jaws belong to 

 the head, those bearing walking feet to the 



thorax, while the somites of the abdomen bear either false feet or lack 

 appendages. As a sequence the cephalothorax is that region of the 

 body which bears, besides antenna? and jaws, legs as well. 



The somites of Arthropoda have given rise to various disputes. Many zoolo- 

 gists speak of a pre-antennal somite and a pre-antennal appendage, referring to 

 the eye stalk of some Crustacea, which, however, differs markedly in its develop- 

 ment from the true appendages. Those who accept an ocular somite must u<l<l 

 one to the number of somites as stated in this volume. A second theory regards 

 the antennas as ventral appendages innervated from the ventral chain which 

 secondarily become dorsal and receive their nerves from the brain. This view 

 is firmly grounded for the second antennas of the Crustacea. Other questions 

 are as to the possible loss of both segments and appendages. 



The union of somites to body regions has had an influence upon the 

 internal structure and especially upon the nervous system (fig. 370). A 



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