402 



ARTHROPOD A. 



are strong and are covered especially on the medial side with a hard, 

 toothed chitin (figs. 404, 2; 410, //, F; 

 507). The other joints may entirely 

 disappear. When they persist they form 

 a more or less leg-like appendage, the 

 palpus. Since several appendages may 

 be modified into jaws, the first are called 

 mandibles, the next maxillae, and second 

 maxillae may follow. The maxillipeds 

 may have more the appearance of jaws, 

 at other times are more leg-like (fig. 404, 

 5-7). 



The false feet (pleopoda) are small 

 and inconspicuous appendages which may 

 have various functions: they may serve 

 as gills or supports for the gills, as places 

 for the attachment of eggs, as organs for 

 the transfer of sperm, or as swimming or 

 creeping organs. 



These appendages have constant po- 



Fio. ^.-Appendages of the sitions in the bod y- First n the head 



maSdfbie^'f^^fl^^nd come the antennae and then, in the region 

 ffpeds d - m J"waii'fng ^leg^" of ^e mouth, the jaws and, so far as 

 pieopod. ' 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 abdo- 

 men bear either false feet or lack appendages. As a sequence the 

 cephalothorax is that region of the body which bears, besides an- 

 tennas and jaws, legs as well. 



The extremities of Arthropoda have given rise to various disputes. 

 Many zoologists speak of a pre-antennal somite and a pre-antennal ap- 

 pendage, referring to the eye stalk of a part of the Crustacea, which, how- 

 ever, differs markedly in its development from the true appendages. 

 Those who accept an ocular somite must add one to the number of somites 

 as stated in this volume. A second theory regards the antennae 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 antennae of the Crustacea. Other questions are 

 as to the possible loss of segments and appendages. 



The concentration or fusion of somites to body regions has had 

 an influence upon the internal structure and especially upon the 

 nervous system (fig. 405). A ladder-like nervous system consists, 



