438 



ZOOLOGY 



SECT. 



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 sur- 

 face of the ventral cord. Giant nerve-cells occur in some forms in 

 certain regions. Small ganglia occur frequently in various peri- 



FIG. 344. Saccocirrus, transverse section, to show the position of the nerve cords, dors. vess. 

 dorsal vessel ; int. intestine ; ne. co. nerve cord ; set. set?e. (After Fraipout.) 



pheral parts, at the bases of cirri or of sensory papillae more 

 especially. 



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

 otocysts. Eyes, absent in all the Oligocha^ta with a few exceptions, 

 and in some of the tube-forming Polychseta, 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 reaches quite 

 a high grade of development. Usually they are confined to the pro- 

 stomium, but Polyophthalmus, in addition to the prostomial eyes, has 

 pairs of eye-like 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 Dasi/chone there are eyes or eye-like organs 

 on the branchial filaments. 



