PHYLOGENY 



ment of a thick, chitinous cuticle. This made necessary the joints— thin 

 places in the cuticle— from which the phylum takes its name. In order to 

 move the hard pieces of the cuticle, the continuous muscular wall, in- 

 herited from polychaete ancestors, became broken up into specialized 

 muscles, attached to ingrowths of the cuticle. The limbs, being jointed, 

 are far more adaptable than the parapodia of the polychaetes, and they 

 have become specialized for many functions, including sensation, feeding, 

 and locomotion. This thick cuticle undoubtedly minimizes water loss by 

 evaporation, and it may be that this is a major factor in their successful 

 invasion of the land, for no other invertebrate phylum has so large a por- 

 tion of terrestrial species. The concentration of the nervous system and 

 the sensory organs in the head region ( cephalization ) , begun in the poly- 

 chaetes, is carried much further in the arthropods. The coelom is much 

 reduced, and has become largely replaced by a hemocoele. Segmentation 

 is prominent externally, but less so internally. 



As might be expected with so large a phylum as the Arthropoda, there 

 is no general agreement on the taxonomic rank to be accorded to the 

 major divisions of the phylum. For present purposes, five major subphyla 

 will be recognized, and no attempt will be made to treat their component 

 classes because of the vastness of the groups. These are the Trilobita, the 

 Crustacea, the Myriapoda, the Insecta, and the Arachnida. These well- 

 defined groups, with the exception of the Trilobita, are universally famil- 

 iar, and so need not be defined here. 



The trilobites are of especial interest because they are the most ancient 

 arthropods known, and because their structure was very generalized and 

 could conceivably have given rise to the other major groups of arthropods. 

 The trilobites were dominant from the Cambrian through the Silurian. 

 They then declined until the Permian, from which period only a single 

 species is known. With this, the group became extinct. The trilobite body 

 was enclosed in a chitinous skeleton, and was divided into three longi- 

 tudinal lobes by two longitudinal furrows, hence the name of the group. 

 The body consisted of a head and a segmented trunk. The segments were 

 generally movable, but a variable number of posterior segments were 

 joined together to form a rigid unit, the pygidium. There was a single pair 

 of antennae. The remaining appendages were all simple, undifferentiated, 

 biramous appendages. None were specialized as mouth parts, but all had 

 gnathobases, basal processes which could be used for biting. All species 

 were marine. A typical trilobite is illustrated in Figure 48. Nothing is 

 known of the internal structures. 



Fossil remains of transitional organisms from trilobites to the other 

 major arthropod types are entirely lacking and it may be that the several 

 groups arose independently. Indeed, this would avoid some perplexing 

 inconsistencies. Comparative morphology indicates that the crustaceans, 

 myriapods, and insects form one line of descent, while the arachnids must 

 have diverged very early. In the Crustacea, this transformation has in- 

 volved the division of the body into a highly fused cephalothorax and an 

 abdomen in which the original segmentation is retained. The appendages 

 have become greatly diversified, while still retaining the biramous plan 



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