TYPES OF CONSTRUCTION IN THE ANIMAL KINGDOM. 23 f 



lation. A third stage is represented in the elongation of the 

 body in such a manner that it may move in part without any 

 actual locomotion, the one end becoming sessile, or attached, 

 as in the case of the Coelenterata. In this case no specialized 

 organs of locomotion are developed, but the mouth-parts 

 are moved in relation to the body, and are moved also 

 in relation to the source of food. In Vermes there is loco- 

 motion, but no special articulated parts are developed in 

 this lower type. In the Arthropoda articulated organs 

 subserving the function of locomotion are developed ; associ- 

 ated with the specialization of motion, as local motion, we 

 find a specialization of the poles of the body into an anterior 

 and posterior end, relative to the direction of the motion. 

 The first mode of differentiation spoken of has not to do with 

 locomotion, but rather with relation to reception of food. 

 An oral end of the alimentary canal was established through 

 which food reached the interior of the organism. The polar- 

 ity was a polarity between the oral end of the alimentary 

 canal and the excretory end, or rather between the approach 

 of food and the discharge of effete results of digestion. 



In the Cffilenterata the oral orifice serves also for the dis- 

 charge, and therefore the oral and aboral poles are brought 

 together, typically, at the same point. In the Echinoder- 

 mata, in some cases, the aboral corresponds to the oral pole. 

 In other Echinoderms there is a distinct separation of the two 

 ends of the alimentary canal. With the setting up of the 

 antero-posterior polarity of the chief axis of the body, and 

 of these specializations of locomotion, there was expressed a 

 decided advance by the appearance of sense-organs at the 

 anterior pole. 



Differentiation of Nervous System a Concomitant of Locomotion. 

 — Motion, bringing about a change of place, implies the selec- 

 tion of better conditions of environment, and the guidance of 

 the locomotion toward such favorable conditions. Thus the 

 differentiation of the nej'voiis system follows, or is intimately 

 associated with, the specialization of motion into locojnotioii. 

 Again, we notice that the head, being thus specialized, is only 

 one of the kinds of differentiation. Thus the metameric 

 mode of development first makes possible heteronomy of 



