278 THE WORLD OF THE SEA. 
drawn, the form of these spots is somewhat altered, being more 
like little pins with eyes drilled in large heads. 
If a leech be cleaned, and then powdered with flour or chalk, 
and fixed upon its back so that it cannot move, pairs of spots will 
be seen on the under surface of its body, the flour which covers 
them being moistened with a mucous liquid which they exude. 
These pores in the skin are also found to be upon every fifth ring. 
If we now dissect the animal, we shall be surprised to find a 
remarkable correspondence between the external machinery and 
A SECTION OF A LEECH. 
the internal anatomy. The gazglions—the nerval centres which 
supply the place of brains in the lower animals—are found dis- 
tributed beneath the spots on the skin, that is, at a distance of 
five rings from each other. 
The stomach is composed of eleven pairs of pockets, each one 
having a ganglion; hence, they also ‘are at an interval of five 
rings. Between the pockets of the stomach are small canals which 
terminate in the pores whose existence was indicated by the flour. 
The secretion of these canals lubricates the leech. There are 
seventeen pairs of them, each at a distance of five rings. 
The vessels by which the circulation is completed are arranged 
