NO, lO CRUSTACEAN METAMORPHOSES — SNODGRASS 73 



fied, but the swimming appendages (E) clearly retain the structure 

 of the second maxilliped. In other branchiopods the appendages may 

 be variously reduced (F, G, H, I) obscuring the basic leg structure. 



The segmentation of the arthropod legs is surprisingly constant; 

 variations result from the elimination of segments, seldom from addi- 

 tion, though the propodite (tarsus) is generally rendered flexible by 

 subdivision. If all the podomeres in the legs of the trilobite (fig. 

 25 A) and Marella (B) are true musculated segments, the ancient 

 arthropods had eight limb segments, including a small apical dactylopo- 

 dite, or pretarsus, and thus possessed all the segments that are present 

 in any of the legs of modern arthropods. Among the latter, eight 

 segments are present in the Pycnogonida and in some of the legs of 

 the arachnid Solpugidae (D), but in most of the arachnids the leg 

 has only seven segments by the elimination of the third segment from 

 the base. The segment beyond the knee bend (D, Pat), which is the 

 fifth segment in the trilobite leg (A), is called the patella, though it 

 might appear to correspond with the carpopodite (tibia) in the leg 

 of a centipede (E) or a decapod (F). Yet there are three segments 

 beyond it in the spider leg, and only two in the other arthropods. In 

 the latter, therefore, either two original segments in the distal part 

 of the leg are united, or one has been eliminated. The legs of the 

 chilopods and the decapods (E, F) have seven segments; the insect 

 leg has only six segments because of the apparent union of the 

 ischiopodite (second trochanter, or pre femur) with the meropodite 

 (femur). 



Though the primitive arthropods (fig. i C) undoubtedly were 

 aquatic, they were walking animals provided with jointed limbs, and 

 probably lived on plants in shallow water near the shore. Their habits 

 may have been similar to those of the modern Anaspides (D). The 

 typical jointed ambulatory leg has been retained in all modern arthro- 

 pods, except in those crustaceans in which it has been modified for 

 swimming, but even the phyllopodium preserves evidence of the seven- 

 segmented structure of a walking leg. It would appear that the primi- 

 tive arthropods had more legs than they needed for walking, and 

 because of this fact their descendants have been able to reconstruct 

 many of them into the great variety of appendicular organs possessed 

 by modern forms. The arthropods owe what they are, as well as their 

 name, to their jointed appendages. 



REFERENCES 

 Alikunhi, K. H. 



1952. An account of the stomatopod larvae of the Madras plankton. Rec. 

 Indian Mus., vol. 49, pp. 239-319, 25 figs. 



