x PHYLUM ANNULATA 449 



ment being regulated by a number of valves in the " hearts," the 

 dorsal vessel, and the chief vessels connected with it. 



The nervous system (Fig. 367) consists of a dorsal bilobed 

 brain or cerebral ganglion and a double ventral nerve-cord together 

 with a pair of cesophageal connectives, by which the former is 

 connected with the anterior end of the latter. The brain, which is 

 of small size, is situated in the third segment, above the beginning 

 of the alimentary canal ; it is divided by a median constriction 

 into two lateral parts of pyriform shape with their broad ends in 

 contact. The connectives pass from this round the sides of the 

 alimentary canal to unite in the middle below with the anterior 

 end of the ventral nerve-cord. In this way a complete nerve- 

 ring or nerve-collar surrounds the anterior part of the enteric 

 canal in the third segment. From this the ventral nerve-cord 

 extends backwards to the posterior end of the body, and in each 

 segment it presents a slight enlargement or ganglion, as it is 

 usually termed, most conspicuous in the more posterior segments. 

 The whole cord is double, consisting of two intimately united 

 right and left parts. From the brain, nerves are given off to 

 the prostomium ; and from the ventral cord three pairs of nerves 

 arise in each segment. From the oesophageal connectives a 

 series of stomatogastric nerves pass to the pharynx and neighbouring 

 parts of the alimentary canal. 



The Earthworm is devoid of organs of sight or hearing. It 

 exhibits sensitiveness to bright light, the sensitive elements being 

 large cells of the epidermis devoid of pigment. The sense of 

 hearing appears to be absent ; but a faculty analogous to taste 

 or smell, enabling the animal to distinguish between different 

 kinds of food, is well developed. The goblet-shaped bodies, groups of 

 narrow epidermal cells, most abundant on the prostomium and 

 peristomium, have probably to do with this faculty. 



The organs of excretion the segmental organs or nephridia 

 (Fig. 368) are similar to those of Nereis, but somewhat more 

 complicated. They are slender tubes which occur in pairs in all 

 the segments of the body except the first three and the last. 

 Externally each nephridium opens by one of the small nephridio- 

 pores which have already been mentioned as occurring on the 

 ventral surface ; internally it ends in a funnel-shaped ciliated 

 extremity with a crescentic slit-like aperture, the nephrostome 

 (nst), opening into the cavity of the segment in front of that in 

 which the external aperture occurs. The tube is thrown into 

 three loops attached to the posterior surface of the corresponding 

 septum by a fold of membrane. Two parts are clearly recognis- 

 able an inner narrow and an outer wide part : in the former the 

 narrow central lumen is a perforation through the axis of a string 

 of cells, and is thus intracellular : it is lined in parts with cilia 

 VOL. i. G G 



