8o SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



subject to adaptive changes, which may be very extensive, as in 

 certain holometabolous insect larvae. 



Sclerotization of the integument has affected not only the wall of 

 the body, but also the walls of the tubular segmental appendages, and 

 the latter are jointed by definite rings of flexible membrane interposed 

 between the resulting limb segments, or podomeres. Hence, the 

 arthropod limb itself has possibilities of much variety and specificity 

 of action. As a consequence, while probably the appendages in the 

 first place were all simple locomotor organs, many of them have been 

 converted into instruments adapted to various purposes, and those 

 that still subserve the locomotor function are capable of all the kinds 

 of mechanical progression except flying known among animals. 



Concomitant with the evolution of the skeletomuscular mechanisms, 

 the nervous system and the sense organs have necessarily acquired a 

 high state of development, and the elaboration of most intricate in- 

 stincts has been possible because of the facility with which tools may 

 be produced and adapted to their ends. 



The primitive arthropods, being closely related to the primitive 

 onychophorons, and together with the latter derived from generalized 

 annelids, must have been slender, many-segmented, polypodous crea- 

 tures resembling modern centipedes. They differed from their con- 

 temporaneous onychophoran relatives in having dorsal and ventral 

 segmental plates and specifically jointed appendages. The Protarthrop- 

 oda were early differentiated into primitive trilobites and primitive 

 mandibulate forms. From the primitive trilobites were evolved the 

 later Trilobita, Xiphosurida, Eurypterida, and Arachnida, while the 

 Protomandibulata gave rise to the Crustacea, the Diplopoda, the 

 Chilopoda, and the Hexapoda. 



EARLY EMBRYONIC DEVELOPMENT 



The processes of cleavage and germ-layer formation are so variable 

 among the arthropods that they can have little value in a phylogenetic 

 study of arthropod relationships. Cleavage, whether total or partial, 

 results usually in the formation of a superficial blastoderm, and the 

 embryo appears as a germ band on the ventral side of the egg. 

 Gastrulation in some of the Crustacea takes place by invagination, 

 but more commonly both the endoderm and the mesoderm are formed 

 by delamination or by proliferation from the blastoderm or the germ 

 band. Manton (1928) gives a precise account of the proliferation of 

 the germ layers and the primary germ cells from the blastoporic 

 region in the crustacean Ilemimysis, the cells of the several groups 



