466 



ZOOLOGY 



SECT. 



ganglion is in close union with the epidermis ; in these also the 

 ventral cord is not segmented into ganglia. Running longitudinally 

 through the ventral cord in many forms are certain giant fibres of 

 very large size ; though these may have rather a skeletal than a 

 nervous function, they are simply greatly enlarged and modified 

 nerve-fibres. Nerve-cells may be confined to the ganglia, or may 

 be distributed over the entire surface of the ventral cord. Giant 

 nerve-cells occur in some forms in certain regions. Small ganglia 

 are found frequently in various peripheral parts, more especially 

 at the bases of cirri or of sensory papillae. 



The organs of special sense are eyes, tentacles and cirri, 

 nuchal organs, and otocysts. Eyes, absent in the Oligochaeta with 



dors, vess 



Fia. 381. Saccocirrus, transverse section, to show the position of the nerve-cords, dors. vets. 

 dorsal vessel ; int. intestine ; ne. co. nerve-cord ; set. setae. (After Fraipont.) 



a few exceptions and in some of the tube-forming Polychasta as well 

 as in a few free forms of that sub-class, are very general in their 

 occurrence. Their structure is, as a rule, very simple, but in some 

 forms reaches quite a high grade of development. Usually they are 

 confined to the prostomium, but Polyophthalmus, in addition to the 

 prostomial eyes, has pairs of eye-like organs probably light- 

 producing (phosphorescent) organs on many of the segments 

 of the body. Leptochone has a pair on each segment, and in Fabricia 

 there is a pair on the anal segment ; while in many species of 

 Sabella and all the species of Dasychone there are eyes on the branchial 

 filaments. 

 Most usually the eye is (as in Nereis, p. 437, Fig. 359) a 



