CONCLUSION. 



271 



Conclusion. A general survey of the appendages throughout the arthropod- 

 vertebrate stock reveals a steady and logical progress in their specialization. Sen- 

 sation, feeding, locomotion, respiration, and reproduction are alike essential 

 functions, but they do not make their local appearance at the same time phylo- 

 genetically or ontogenetically; or make the same demands for space or for special 

 locations. Sensory and feeding appendages, for example, must be located near 

 the mouth, which is the oldest organ of the body, and the first one to be formed. 

 Locomotor appendages must be located where they can raise and move the primi- 

 tive head, or in the later phases of evolution, lift the whole body and support 



be. 



md. P. 



.o.c. 



*^^^^^ 



.c. 



cm. 



FIG. 178. 



FIGS. 176 TO 178. A, Hypothetical form in median section, indicating the probable arrangement of organs in 

 an intermediate condition between that in an arachnid and an ostracoderm; B, median section of an ostracoderm 

 (Bothriolepis) showing the arrangement of the internal organs; in part, hypothetical; C, Amphioxus, in median 

 section. Diagrammatic. 



it in a properly balanced position. Respiration, circulation, excretion, and repro- 

 duction make no such imperative demands for special locations or for early 

 development. 



Thus there is established in the appendages, at a very early period, a definite 

 linear sequence of functions that coincides with the sequence in the historical 

 evolution of the functional demands made upon them. 



In other w r ords, the oldest organs phylogenetically, and those first in demand 

 ontogenetically, are laid down first in time, and consequently at the head end of the 

 series, because growth in segmental animals always takes place by a process of 

 addition at the tail end. Those organs that make no special demand for location 

 or for prior use, even though they may have an equally long pedigree with the rest, 

 are gradually relegated to the more recently added territory in the posterior part 

 of the body. 



