'66 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



ened into two cerebral ganglia (c.g, A and B), while its sides, the 

 connectives (cc), run downwards and backwards and unite together 

 to form the ventral cord. This dilates slightly into a ganglion (g) 

 in each segment, the ganglia being best marked in the posterior 

 part of the body. The double nature of the cord is indicated by 

 a longitudinal furrow running along its upper surface. 



A nerve passes from each cerebral ganglion to the prostomium 

 and peristomium, numerous nerves run out from the first ventral 

 ganglion, while behind this there are typically three pairs of nerves 

 in each segment, two arising from its ganglion (ft), and one (septal 

 nerves) from the cord in front of this (n r ). 



A number of delicate nerves pass from the inner sides of the 

 connectives to the pharynx, in the walls of which they form a 

 delicate ganglionated network. This arrangement is called the 

 sympathetic nervous system (sy). 



The nervous system is essentially made up of nerve-cells and 

 nerve-fibres. In the nerve-ring and cord there is a very delicate 

 connective-tissue framework (neuroglia) in which these elements 

 are imbedded. The nerve-cells occupy the anterior part of the 

 cerebral ganglia and the under side of the ventral cord both in and 

 between the ganglia. The ventral cord is surrounded by a firm 

 muscular sheath in which the neural vessels run, and imbedded 

 in the dorsal part of the sheath there are three giant-fibres, which 

 perhaps have a supporting function, but in any case are modified 

 nerve-fibres. 



The central organs certainly constitute a correlating apparatus, 

 but scarcely anything is known of their mode of action. 



9. Special Sense Organs appear to be represented only by 

 "goblet-bodies," consisting of aggregates of slender epidermal 

 cells connected with nerve-fibres, and occurring on the prosto- 

 mium, peristomium, and, less abundantly, on the anterior segments. 

 The Earthworm is, however, by no means devoid of special senses. 

 The entire surface of its body is endowed with a delicate sense of 

 touch, while certain kinds of food (e.g., bits of onion) are specially 

 preferred and readily discovered, which seems to prove possession 

 of both smell and taste. The structure and position of the pro- 

 stomial and peristomial goblet-bodies indicate that these are the 

 special organs of taste and smell. Although earthworms possess 

 nothing that can be described as a definite sense of sight, they 

 are sensitive to intense light, which they shun, and thus escape 



