The Arthropoda 317 



in the midline through which sperm may be inserted. Immediately be- 

 hind the left walking leg, on the first abdominal segment, a peculiar 

 atrophied ( ) pair of appendages is found. 



In the male, however, these appendages on the first and second ab- 

 dominal segment are wide, and the left walking leg possesses a small 

 opening through which the sperm are ejected. In the male the first pair 

 of swimmerettes is also transformed into "copulating organs." The 

 anal opening is found on the ventral surface in the midline of the telson. 



SERIAL HOMOLOGIES AND ADAPTATIONS 



When two parts of an organism develop alike as to structure, for 

 example the femur in the thigh and the humerus in the upper arm, we 

 call such bones or parts homologues ( ). 



And when two parts function similarly, regardless of whether they 

 are alike structurally, we call such organs or parts analogues ( ). 



While if any organ or part of an organ changes, due to a change of 

 environment so as to better or benefit an organism, we call such change 

 an adaptation. In the crayfish there is what is called a serial homology. 



This type of "homology" is characteristic of the group of the higher 

 Crustacea known as the sub-class Malacostraca ( ), 



and this group well illustrates how a single plan of structure may run 

 through a series of forms of the utmost diversity in appearance, and how 

 parts essentially alike may be adapted to the most diverse ends. 



According to Lohr the "malacostracan body, be it an amphipod 

 ( ), an isopod ( ), a decapod ( ), 



or what not — is composed of a series of twenty 1 segments, each of which 

 is essentially of the skeletal plan shown in the diagram, except that the 

 appendages of the foremost segment are typically unbranched and the 

 hindmost segment (the telson) is rudimentary and bears no appendages 

 at all. Some of these segments may become fused together and con- 

 solidated on the dorsal side, only the appendages and ventral margins 

 remaining free. This may occur at either end of the body, but it occurs 

 constantly in the five front segments, these by fusion forming the head. 

 The appendages of these five segments always consist of two pairs of 

 antennae at the front, one pair of mandibles beside the mouth, and two 

 pairs of maxillae following the mandibles." These parts and their func- 

 tions will readily be understood a little later because of their likeness 

 to the parts bearing the same names in the insects shortly to be studied. 

 "Immediately following the maxillae are one or more pairs of maxilli- 

 peds, likewise directed forward beneath the mouth to assist in the manip- 

 ulation of the food. Then follow legs and swimmerettes in more or less 

 variety, the terminal joints of some of the legs being modified in many 

 cases into highly specialized grasping organs called pincers, or chelipeds, 

 and the swimmerettes being frequently modified to serve reproductive or 



1 This is not counting a vestigial segment in the head region, that is discoverable only during 

 embryonic life. 



