///. ANNELIDA: CH^TOPODA. 



307 



pouches of the right side from those of the left (figs. 275 and 

 272). The alimentary canal also shows distinctions; for while it 

 differs greatly in the various species, it has constantly a terminal 

 anus, while the mouth is ventral and is overhung by the preoral 

 segment, the prostomium. 



Xervous system, blood-vessels, and excretory organs are influ- 

 enced by the segmentation. The nervous system is built on the 

 ladder plan. It begins with a supraoesophageal ganglion ('brain') 

 lying in the prostomium, from which the c8soj)hageal commissures 

 pass around the oesophagus to form the ventral chain, which con- 

 sists of as many pairs of ganglia, united by longitudinal commis- 



FiG. 275.— Anterior end of Nais eUnguis. 7i, cerebrum, connected by commissure with 

 ventral chain, u; d(j, contractile dorsal, vg, ventral blood-vessel; m, muscular 

 layer of skin; d/>, vh, dorsal and ventral chsetse; d, septa; k, prostomium; o, 

 mouth. 



sures, as there are somites present. These ganglia of the ventral 

 chain ar? closely similar, since the segmentation of the body is 

 homonymous. There is but the slightest division of labor 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, with lens, 

 vitreous body, and retina. Otocysts are rare, but occur in diverse 

 species. Ciliated pits (olfactory) occur on the head, goblet organs 

 (taste) on head and trunk, and, lastly, lateral organs, sensory struc- 

 tures of unknown function, may be metamerically arranged. 



The blood-vessels are most frequently represented by two main 

 trunks which frequently (as in earthworms) contain blood colored 

 red by ha-moglobin. One trunk runs dorsal, the other ventral, 

 to the intestine, the two being connected by vessels (figs. 272, 276) 

 in each segment. The blood passes forward in the dorsal vessel, 

 backwards in the ventral. It is propelled by contractile portions 

 of the vessels; usually the dorsal vessel pulsates, but, as in the earth- 

 worms, certain of the circular vessels in the anterior part of the 



