TAXONOMY, ANATOMY, EMBRYOLOGY 



terminating in a pincer. The first pair of walking legs forms the major 

 pincers ( or chelae ) of the organism, serving as defensive and food-procur- 

 ing organs. All but the last pair of walking legs bear gills. There are six 

 pairs of abdominal appendages. The first is often much reduced or even 

 absent in females, while in males it is modified together with the second 

 abdominal appendage to form a copulatorv organ. In females, the second 

 appendage forms a swimmeret or pleopod in which there is a short 

 protopodite followed by a more or less equally developed, filamentous 

 exopodite and endopodite. In both sexes, the third, fourth, and fifth ab- 

 dominal appendages follow this pattern, which appears to be the primitive 

 appendage pattern. The final appendage consists of a short protopodite 

 and a verv broad, flat endopodite and exopodite, forming the telson, or 

 terminal fan of the crayfish. 



Thus, \\'ithin a single organism, the basic crustacean appendage is modi- 

 fied to serve no less than six to ten difterent functions ( depending on how 

 one may wish to classify them). If each of these appendages had been 

 originally created for the fimction which it now serves, then it would be 

 a strange circumstance that all of them are built upon the same pattern 

 as are the legs. For most of these functions are subserved in other groups 

 by organs having nothing to do with appendages. Antennae of molluscs, 

 for example, are constructed on a completely different plan, yet there is 

 no reason to suppose that thev function less eflSciently than those of the 

 Crustacea. Nor is there any intrinsic reason why mouth parts, copulatory 

 organs, and gills should be based upon a leg-like structure. Yet it is so in 

 the Crustacea. These facts, so puzzling on any other theory, are easily 

 understandable on the basis of the evolutionary concept. Given a primitive 

 crustacean in which all of the appendages are in a simple condition, 

 somewhat like the swimmerets of the crayfish, natural selection should 

 favor diversification of function. There is a general tendency among the 

 higher phyla for centralization of sensory functions in the head region; 

 hence the sensory functions of the anterior-most appendages become in- 

 tensified at the expense of other functions, and they become antennae and 

 (possibly) eyes. Those appendages nearest the mouth naturally are used 

 for feeding, and so they become specialized for chewing, biting, and food 

 handling. Those appendages nearest the reproductive organs are, in the 

 male, modified for the transfer of sperm to the female. Other appendages 

 continue to serve the original locomotor function in various ways. 



If the Crustacea as a whole be considered, then the range of adaptations 

 of these appendages is still greater. In some of the Crustacea, mouth parts 

 are more numerous than in the cravfish described above. In these, there 

 are fewer legs. Why such a relationship should hold is utterly inexplicable 

 except upon the theory that both the mouth parts and the legs have been 

 derived from primitive appendages by adaptive modification. In the bar- 

 nacles, most of the appendages have been suppressed, and the thoracic 

 appendages have been specialized as plume-like cirri which sweep a cur- 

 rent of food-bearing water toward the mouth. In the lobster, the abdomi- 

 nal appendages are flattened out to form broad, oar-like plates, effective 

 as swimming organs. In the crabs, which normallv hold the abdomen 



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