130 



FOUNDATIONS OF BIOLOGY 



HEAD; segments 6 to 13, the THORAX; and segments 14 to 19, 

 the ABDOMEN. In other words, by the coalescence or com- 

 plete fusion of certain segments, the body has become divided 

 into more or less distinct regions. (Fig. 71.) Also, the primi- 

 tive locomotor appendages of the respective segments have 

 become modified into organs for the performance of widely 

 different functions: those of the head, as sensory organs, 

 jaws, etc.; those of the thorax, as organs for grasping, offense 

 and defense, and walking; and those of the abdomen for 

 swimming, etc. Thus change in structure has gone on hand 



FIG. 70. Diagrammatic representation of the structure of a primitive Arthropod 

 in which very little specialization of the segments has occurred. A, eye; D, digestive 

 tract; F, antenna; G, jointed appendages; H, dorsal blood vessel; M, mouth append- 

 ages; N, ventral nerve cord with ganglia; S, mouth; Sk, ohitinous exoskeleton; OS, 

 cerebral ganglion; Us, suboesophageal ganglion. (After Schmeil.) 



in hand with change in function, so that although there is no 

 superficial resemblance between the jaws of the Crayfish and 

 the legs employed for swimming, nevertheless a study of their 

 development shows beyond doubt that they owe their origin 

 to modifications of one primary type. Accordingly the vari- 

 ous appendages are said to be HOMOLOGOUS, signifying a 

 fundamental similarity of structure based on descent from 

 a common antecedent form. (Fig. 72.) 



On the other hand, organs of dissimilar fundamental 

 structure, which nevertheless perform the same function, are 

 called ANALOGOUS. In the group of the Arthropods known 

 as the Insects, the series of head appendages and the legs are 

 homologous with those of the primitive Arthropod type, 



