140 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



appendages, and the special development of an exite lobe of the basi- 

 podites has given rise to a characteristic biramous structure of the 

 limbs. The primitive protocephalon is retained as the definitive head 

 in the Anostraca and in most of the Malacostraca, but in the majority 

 of the Entomostraca and in the Leptostraca, Amphipoda, and Isopoda 

 from three to five gnathal somites have been united with the proto- 

 cephalon to form a more extensive cephalic structure. A carapace is 

 variously developed in many groups, either from the cephalognathal 

 region, or from the gnathothoracic region, but there is no true cephalo- 

 thorax formed by an intimate union of cephalic and thoracic somites 

 as in the Chelicerata. The mandibles have no movable lobes such as 

 those of the myriapods ; in most forms the jaws preserve the primitive 

 monocondylic articulation with the head, but in the higher Mala- 

 costraca they are secondarily dicondylic. The genital openings are 

 variable in position in the Entomostraca, but are fixed with respect 

 to a specific segment in the Malacostraca. The hatching of the young 

 at an early embryonic stage has resulted in the development of 

 specialized swimming larval forms representing more primitive an- 

 cestral stages in their general structure than the immediate protar- 

 thropod ancestors of the crustaceans. The great antiquity of the 

 Crustacea is shown by the occurrence of highly evolved forms in the 

 Cambrian period contemporaneous with the oldest known trilobites. 



I'j. — The Protomyriapoda, being the direct descendants of the 

 protarthropods, perpetuated the generalized arthropod form after the 

 trilobites, the chelicerates, and the crustaceans had branched off as 

 side issues and taken on variously specialized forms. During their 

 evolution the protomyriapods acquired the structures characteristic 

 of their descendants, which include the modern Symphyla, Diplopoda, 

 Hexapoda, and Chilopoda. The three gnathal somites became inti- 

 mately united with one another and with the protocephalon, forming 

 the standardized head of the above-mentioned groups, composed of 

 the acron and four postoral segments. The compound eyes and the 

 first antennae of the Protomandibulata were retained, but the second 

 antennae became reduced and eventually were lost, though their 

 ganglia were preserved as tritocerebral lobes of the brain. The 

 mandibles lost the telopodites, but each had a strong gnathal lobe 

 (lacinia) movable by a muscle arising within the coxopodite and by 

 another arising on the cranial wall. The two postmandibular maxillary 

 appendages were modified by a reduction of the telopodites and by 

 other adaptations to serve as accessory feeding organs. Since the 

 Symphyla and some of the more generalized Hexapoda have lateral 

 hypopharyngeal lobes (superlinguae) resembling the paragnatha of 



