310 CHARACTERS OF INVERTEBRATE ANIMALS 



back a horny ribbon, the radula, beset with transverse rows of 

 flinty teeth. This ribbon, often called the "tongue" or "palate", 

 has often been compared to a finger-nail, and as worn away it 

 constantly grows forwards from a projection (radula sac) at the 

 back of the pharynx, just as the finger-nail does from its root. 

 A fuller account of this organ will be given in another place. 



The kidneys of the Ormer are two irregular brown bodies, 

 opening as described. The left one is very small, and would 

 seem not to act as a kidney at all. 



There are certain characteristic features of the nervous system 

 which require notice. It consists of a nerve-ring surrounding the 

 beginning of the digestive tube, and of other connected parts. 

 The upper part of the ring consists of a transverse band con- 

 necting two swellings, the brain (or cerebral) ganglia, and from 

 each of these two cords run downwards to constitute one side 

 of the ring. The outer cords end below in a pair of lateral 

 (or pleural) ganglia, and the inner cords in a pair of foot (or 

 pedal) ganglia, which are united together in the middle line so 

 as to complete the ring below, while each lateral ganglion is 

 also connected with the adjoining foot ganglion. From the 

 brain ganglia, nerves run off to the sensitive parts of the head 

 including the tentacles, and strong nerves to the foot run back- 

 wards through the substance of that organ from the foot ganglia. 

 There still remains to be described a nerve-loop, which connects 

 the two lateral ganglia and gives off nerves to some of the 

 internal organs. It is a nerve -cord which, starting from one 

 lateral ganglion, runs obliquely backwards, and, turning round in 

 a curve at the level of the hinder end of the mantle - cavity, 

 sweeps forwards again and takes an oblique course to the other 

 lateral ganglion, taking altogether a course which may be compared 

 to the figure 8. Three ganglia are seen as swellings upon this 

 loop, one close to each gill, and the third at the back end of 

 the 8. The curious course of the loop is one result of the 

 twisting of the body which has affected both the visceral hump 

 and shell. 



The most important organs of sense are the tentacles, which 

 have to do with touch, so-called organs of hearing, consisting 

 of a pair of little rounded sacs attached to the foot ganglia, 

 and two small cup-shaped eyes, one at the base of each tentacle. 

 There is also a special sense-organ connected with each gill, 



