NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 65 



species. In some forms the last four thoracic segments are not cov- 

 ered, as in the adult of Squilla (G), in others such as Squilla latreillei 

 (F) only the eighth segment is exposed in the larva, while in species 

 of Lysiosquilla the carapace of the last larva may cover the entire 

 thorax and the first two abdominal segments. Probably these varia- 

 tions in the length of the larval carapace are only differences in the 

 extent to which the free posterior margin is produced beyond the 

 attachment of the plate on the third or fourth segment of the adult 

 thorax. Otherwise the changes during the growth of the larva are 

 merely developmental stages of growth and have no metamorphic 

 value. It is difficult even to see any functional reason for the differ- 

 ences between the two larval forms on hatching. 



IV. STRUCTURE AND EVOLUTION OF 

 ARTHROPOD APPENDAGES 



Inasmuch as changes in the form and function of the appendages 

 are important features in the metamorphoses of Crustacea, and various 

 conflicting views have been held concerning the primitive nature and 

 the evolution of arthropod limbs, we must give some attention to this 

 controversial subject. 



Most studies on the comparative structure of the arthropod ap- 

 pendages, and deductions as to the origin and primitive form of the 

 limbs give the impression that conclusions have resulted too much 

 from an attempt to fit the facts into a preconceived theory. Widely 

 accepted has been the idea that the primitive appendage was a 

 biramous limb ; and many carcinologists would derive all kinds of 

 arthropod appendages from an original phyllopodial type of limb, 

 such as that of the branchiopod crustaceans. 



The trilobites are among the oldest known arthropods, and, with 

 respect to their appendages, they are the most generalized, since all the 

 postoral limbs are fully segmented legs. The base of each leg bears a 

 branched lateral process (fig. 25 A, Eppd), which, arising on the coxa, 

 is clearly an epipodite and hence cannot be an equivalent of the 

 crustacean exopodite, which by definition is an exite of the basipodite. 

 The trilobite limb, therefore, is not "biramous" in the manner of a 

 crustacean limb, and hence does not relate the trilobites to the 

 Crustacea. Raymond (1920), however, explicitly states the opposite 

 view. "The trilobites," he says, "are themselves crustaceans, as is 

 amply proven by their biramous appendages." More recently, Hee- 

 gaard (1947) has argued that the trilobite limb is truly biramous, in 

 spite of the evident coxal position of the "exopodite," and he further 

 attempts to show that remnants of a primitive biramous structure are 



