III. ANNELIDA: CfLTOPODA 



271 



to form the ventral chain, which consists of as many pairs of ganglia, 

 united by longitudinal commissures, as there are somites present. These 

 ganglia of the ventral chain are closely similar, since the segmentation 

 of the body is homonomous. There is but the slightest division of labor 



FIG. 253. Anterior end of Nais elinguis. h, cerebrum, connected by commissure 

 with ventral chain, ; dg, dorsal, vg, ventral blood-vessel; ;;/, muscular layer of skin; 

 df, if, dorsal and ventral duetse; d, septa; k, prostomium; o, mouth. 



among the somites, and hence they differ but slightly among themselves. 

 The prostomium always bears tactile organs and frequently eyes, which 

 in many marine forms are highly developed, \vii.h lens, vitreous body, 



rm vt , bin tm 



FIG. 254. Schematic cross-section of an annelid (after Lang), ac, aciculum; 

 b, choetae; bm, ventral nerve cord; dc, dorsal cirrus; dp, notopodium; k, gill; Im, longi- 

 tudinal muscles; md, digestive tract; />, nephridium; 07', ovary; rm, circular muscles; 

 tm, transverse muscles; tr, nephrostome; vc, ventral cirrus; vd, vv, dorsal and ventral 

 blood-vessels; vp, neuropodium. 



and retina (fig. 84, 1, II) . Statocysts are rare, but occur in diverse groups. 

 Ciliated pits (olfactory ?) occur on the head, goblet organs (taste) on head 

 and trunk, and lastly, lateral organs, sensory structures of unknown 

 function, may be metamerically arranged. 



