402 



ARTHliOPODA. 



i""^^^^ 



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

 toothed chitin (dgs. 404, 2; 410, II, T'; 

 507). Tlie other joints may entirely 

 disappear. AVhen they persist they form 

 a more or less leg-like ajipjendage, the 

 palpus. Since several appendages may 

 be modified into jaws, the first are called 

 mandibles, the next maxill*, and second 

 maxillse may follow. The maxillipeds 

 may have more the apjjearance of jaws, 

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

 5-7). 



The false feet (p)leopoda) are small 

 and inconspicuous ap23endages which may 

 have various functions: they may serve 

 as gills or suj^ports for the gills, as places 

 for the attachment of eggs, as organs for 

 the transfer of sperm, or as swimming or 

 creeping organs. 



These api)endages have constant po- 

 sitions in the body. First on the head 

 come the antennfe 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 antenna? 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 bod)' which bears, besides an- 

 tennfe 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, referrii]g 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 volimie. A second theory regards the antenna? 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 antenna^ of the Crustacea. Other questions are 

 as to the possible loss of segments and appendages. 



The concentration or fusion of somites to body regions luis had 

 an influence upon the internal structure aiul especially upon the 

 nervous system (tig. 405). A ladder-like nervous system consists. 



-Appendages of the 

 crayrtsh. i, tirst antennee ; :-', 

 /iiandible; o. A, first and 

 second maxiUse; 5^ 6, 7, maxil- 

 lipeds : 6', walking leg ; 

 pleopod. 



