CENTRAL ORGANIZATION OF VISION 



513 



must be employed and central exchanges must be introduced. And so the 

 diffuse network became canalized into trunk-pathways between important 

 parts — a stage reached in Echinoderms — and then telephone exchanges were 

 introduced in the form of ganglia which are characteristic of the nervous 

 system of the worms, Arthropods and Molluscs ; therein not only the relay 

 but the integration of messages became possible (Figs. 668-9). All through 

 this process the head-end of the animal tended most readily to encounter 

 external stimuli; in this region which first made the acquaintance of predators 

 or prey, the sense-organs became concentrated. All through the process 

 the degree of nervous develoj)ment depended on the richness of the stimuli 

 provided by the sensory organs — at first the tactile, chemical and olfactory, 



Figs. 671-2.- — -Contractile Myo-epithelial Cells. 



Fig. 671. — In the sponge, Sycon gelalino- 

 sum,the myo-epithelial cells surround the 

 central ajoerture, which is capable of 

 being contracted or dilated. 



Fig. 672. — An isolated cell from the 

 Coelenterate, Pelagia, showing the flagel- 

 lum and the long striated muscular base 

 (after Krasinska). 



but eventually the visual ; and so at the head-end became concentrated the 

 main exchange -centre which in course of time assumed control of all the 

 others for the common good. This process of centralization resulted in the 

 final development of the brain of Vertebrates (Fig. 670). To achieve this final 

 development the whole of the economy of the body has been subordinated; 

 on the supremacy of the main cerebral centre the eventual predominance of 

 the Vertebrates and of man is built ; and in the end the evolution of vision is 

 determined not by increasing specialization of the eye but on progressively 

 more efficient analysis and integration by the cerebrum. " The law of 

 progress is this — the race is not to the swift, nor to the strong, but to the 

 wise " (Gaskell. 190S). 



In Protozoa extracellular conducting nervous tissue was, in general, 

 not required ; in the sponges (Porifera) no nervous elements exist, for these 

 loose afforegations of cells with little community-life and without observable 

 cohesion can be torn in pieces and reassemble again. These organisms 

 thus exhibit none of the rapid reactions characteristic of the higher forms 



8.0. — VOL. I. 33 



