142 Dynamic Theory. 



It was observed that among the zoophites, or plant animals, as sponges, 

 corals, etc., the animal has but one orifice opening into his bag-like 

 stomach, and this serves both for ingress and egress. In animals of 

 somewhat higher development we find the single external orifice replaced 

 by two ; but they are not far apart. It is as if a partition reaching not 

 quite to the bottom of the original cavity had been inserted so as to form 

 a continuous U shaped canal with an opening at each end, one for 

 ingress and one for egress. As an important condition of animal life is 

 to have a steady income of food, whatever organs were developed for its 

 procurement would obviously be located near the entrance. Accordingly 

 we find the parts around the mouth becoming modified into cilia, flagella, 

 tentacles, and finally jaws, &c. , and this modification is accompanied by 

 a corresponding development of nervous machinery. All this finally 

 promotes the part about the mouth, and including it, to the position of 

 head. 



By reference to table V in Chap. VI it is seen that the Gastrsea, or sac- 

 like animal, is a common parent of two diverging stocks; viz. , radiate ani- 

 mals and bilateral animals. There is a certain degree of bilateralism even 

 in the radiate animals, and whether the rays are of even number, four or 

 six, or of odd number, five, the mechanical requirements of locomotion 

 will invariably tend to make the animal two sided in action, and action, 

 reacting upon the parts engaged, tend to make them over and better 

 adapt them for locomotion. Those animals which remain radiate in 

 structure are usually sedentary in their habits, many of them becoming 

 fixed and stationary for life. True bilateral locomotion begins with the 

 worms. There is almost conclusive evidence that the original worm is 

 simply a co-operating colony of Gastrsead or sac animals strung along 

 in a row one behind the other. These pieces are called somites (bodies), 



FIG. 78. A Chsetopoda worm. (The Grubea fusifera.) 



Segmented worm. 



ph. pharynx, d. alimentary canal. /.tentacle, 

 c. cirrus. ( Glaus after Quatrefages.) 



or sometimes segments, and originally they must have 

 been of equal value from one end to the other. But 

 in the interest of economy differentiations would soon 

 take place. One mouth, one set of tentacles, &c., 

 would answer for all the somites, and so all the others 

 might be dispensed with. In such worms as the 

 Polychaeta, Chaetopoda, &c. , the segments are each 

 provided with a pair of limbs which all work in unison 

 to carry the animal forward. But the two end seg- 

 ments are different from the rest. The one in front 

 ' FIG. 78. containing the principal sense organs, mouth, &c. , is 



called the peristomium, or "part around the mouth." In locomotion 



