378 



PRIMITIVE TYPE OF NERVOUS SYSTEM. 



developed on the assumption of a bilaterally symmetrical form, 

 and the consequent necessity arising for the sense organs to 

 be situated at the anterior end of the body. If this view is 

 correct, the question presents itself as to how far the posterior 

 part of the nervous system of the Bilateralia can be regarded as 

 derived from the primitive radiate ring. 



FIG. 231. THREE DIAGRAMS REPRESENTING THE IDEAL EVOLUTION OK VARIOUS 



LARVAL FORMS. 



A. Ideal ancestral larval form. 



B. Larval form from which the Trochosphere larva may have been derived. 



C. Larval form from which the typical Echinoderm larva may have been 

 derived. 



m. mouth ; an. anus ; st. stomach ; s.g. supra-resophageal ganglion. 

 The black lines represent the ciliated bands. 



A circumoral nerve-ring, if longitudinally extended, might 

 give rise to a pair of nerve-cords united in front and behind 

 exactly such a nervous system, in fact, as is present in many 

 Nemertines 1 (the Enopla and Pelagonemertes), in Peripatus 2 , 

 and in primitive molluscan types (Chiton, Fissurella, etc.). 

 From the lateral parts of this ring it would be easy to derive the 

 ventral cord of the Chaetopoda and Arthropoda. It is especially 

 deserving of notice in connection with the nervous system of the 



1 Vide Hubrecht, "Zur Anat. und Phys. d. Nerven-System. d. Nemertinen," Kon. 

 Akad. Wiss., Amsterdam; and " Researches on the Nervous System of Nemertines," 

 Quart. Jotirn. of Micr. Science, 1880. 



8 Vide F. M. Balfour, " On some points in the Anat. of Peripatus capensis," Quart. 

 Journ. of Micr. Science, Vol. xix. 1879. 



