TYPES OF COX STRUCT ION IN THE ANIMAL KINGDOM. 229 



food to flow to the mouth of the organism. The tentacle is 

 an enlarged cilium in so far as the function is concerned. 

 The Coelenterata exhibit this mode of elaboration of the motor 

 organs in a typical way. 



Water-vascular System of Echinoderms. — In tiie Echinoder- 

 mata a higher elaboration of this kind of action of muscular 

 tissue is expressed in the watcr-vasciilar system. This is a 

 peculiar adaptation of simple muscular contraction. 



Cilia in Molluscoidea and Mollusca. — The Molluscoidea have a 

 system of ciliary motion drawing the food particles to the 

 mouth-opening by setting up currents in the water. Some 

 of the Mollusca have a similar method of producing currents 

 by means of cilia on the edge of their mantles. In the Gas- 

 tropod and Cephalopod motion is accomplished by speciali- 

 zation of muscular contraction. Various types of modifica- 

 tion of the foot are elaborated in the different classes of 

 these interesting forms. 



Skeletal Parts. — In the Arthropoda and the Vertebrates 

 organs of motion are more highly elaborated by the addition 

 of hard parts acting as levers, and thus giving special direction 

 and change of direction to the simple contraction of the mus- 

 cles passing between two articulated parts. The general dif- 

 ference betw^een the motor systems, or the modes of motion, 

 in these two grand divisions of the Animal Kingdom, is seen 

 in the different relation which the contracting part (the mus- 

 cle) bears to the mechanism, or skeletal part. In the Ar- 

 thropod the muscles are attached on the inside of hollow 

 skeletal elements. In the Vertebrates the muscles are out- 

 side and around the levers which they move, and in these two 

 groups of organisms motion, and both the muscles and the 

 machinery of motion, reach a high degree of elaboration." 



Multiplication of Like Parts Preceding Specialization of their 

 Functions.^ — -The course of differentiation is from simplicity 

 or homogeneity of parts, first, to multiplication of the parts 

 possessing like functions and often uniformity of form, and, 

 second, to the specialization of function of these parts, their 

 division into groups, their consolidation, and, finally, definite 

 Tiess in number and precision of use or function. Hence 

 division of labor follows the multiplication of parts and does 



