230 



ANNELIDA AND ARTHROPODA: 



coordination centers in the anterior end — or, more 

 simply, head formation. In their evolution from an 

 annelid-like ancestor, the arthropods have developed 

 cephalization to a marked degree. That is, they have 

 good heads with all the appurtenances thereof: brain, 

 eyes, chemical sense organs, and tactile organs. They 

 have gone even farther in their differentiation by 

 attaining a separation of their bodies into regions and 

 their appendages into complex structures. Thus, not 

 only are the head's segments and its appendages 

 modified for feeding and sensory function, but the rest 

 of the body is usually laid off in definite regions with 

 definite functions. The evolutionary modification of 

 appendages is in large measure tied in with this 

 regional modification. Segmentation of this type, 

 with widely varying body segments, is unlike that of 

 the annelids whose body segments are essentially 

 alike. 



Head formation and specialized segmentation were 

 not the only changes on the way from ancestor to 

 arthropod. The coelom, or true body cavity, of an- 

 cestral arthropods exists today in early development 

 but is later lost except for small cavities in the ex- 

 cretory and reproductive organs. The major body 

 cavity in the modern adult is a spacious hemocoel. 

 This cavity is created by the breaking down and 

 migration of cells in the endoderm, resulting in the 

 cavity's being lined by derivatives of all three germ 

 layers. The blood (hemolymph) of this cavity is 

 circulated by a dorsal pumping organ which may be a 

 long tube or a compact heart. In living forms the 

 entire body is encased in a protective external skele- 

 ton (exoskeleton) composed of nonliving material, 

 chitin, secreted by the epidermis. The abdominal 

 exoskeleton is usually segmented in agreement with 

 the body wall segmentation. In the thoracic region 

 segmentation is not defined so clearly. In the head 

 segmentation can be traced only through study ol 

 embryonic development. Also, it is assumed that 

 segmentation of the exoskeleton of appendages has 

 evolved with the limbs; otherwise, arthropods would 

 be encased in rigid boxes, incapable of complex func- 

 tion and limited in movement. Despite the great dif- 

 ferences among specialized limbs, they may be re- 

 duced to a basic ancestral type. The ancestral arthro- 

 pod limb is assumed to have been made up of three 

 parts: at the base, a pmlofioihle, from this, an inner 

 endopodile and an outer exopndik. Modifications of the 

 separate parts, along with their segmentation, lead to 

 the diversity of appendages seen in the arthropods. 



Subphylum ONYCHOPHORA (Joint-legged 

 Worms) 



Diagnosis: found in West Indies, Mexico, Central 

 America, and Southern Hemisphere; about 70 spe- 

 cies; of interest because of their "missing-link" 

 features between Annelida and Arthropoda; vaguely 

 caterpillar-like, a head with a pair of eyes, two 

 ''feelers" (antennae), and mouth bounded on each 

 side by a blunt projection (oral papilla), without a 

 shell; resemble annelids in having nephrida for ex- 

 cretion; body externally unsegmented, with poorly 

 developed, barely jointed legs; respiration by air 

 tubes (tracheae) as in many land arthropods; sexes 

 are separate, the young being born alive (Figure 

 14.4). 



Figure 14.4 Subphylum Onychophora, Peripaius, terrestrial, about 

 notural size. 



Subphylum TRILOBIT A (Trilobites) 



Diagnosis: all extinct, Cambrian to Permian; 

 hardly like any living creatures, perhaps vaguely 

 sowbug-like; body covered by a shell and divided by 

 two lengthwise dorsal furrows into three lobes; head, 

 thorax, and abdomen distinct; body could be rolled 

 up as in certain pillbugs; head with one pair of 

 antennae and two eyes; appendages simple (Figure 

 14.5). 



Figure 14.5 Subphylum Trilobita, /sofe/us, an Ordovicion trilobite. 



Subphylum CHELICERATA (Chelicerates) 



Diagnosis: body usually of two parts, cephalo- 

 thorax (head + thorax) and abdomen, in Acarina 



