88 WORMS, ROTIFERS, LEECHES, POLYZOA 



very regularly into six teeth, which curve in one direction, and 

 one other curved so as to face these. The combs stand side by 

 side parallel to each other, along the whole length of the ribbon, 

 and there are muscular fibres seen affixed to the smaller end of 

 every plate, which doubtless give it independent motion. I counted 

 136 plates on one ribbon ; there are two ribbons on each tho- 

 racic segment, and there are seven such segments hence we 

 may compute the total number of prehensile comb-like plates to 

 be about 1,900, each of which is wielded by muscles at the will of 

 the animal ; while, as each plate carries seven teeth, there are 

 between 13,000 and 14,000 teeth hooked into the lining membrane 

 of the cell when the animal chooses to descend. No wonder, 

 with so many muscles wielding so many grappling hooks, that the 

 retreat is so rapidly effected ! " 



The Leeches (Hirudinea), or Suctorial Annelida, are worm-like 

 creatures, frequenting both fresh and sea water, with segmented 

 bodies bearing a flat disk at each end. The Medicinal Leech was 

 formerly a familiar object to be seen in small aquaria in chemists' 

 shops, and was used very largely by doctors for drawing blood from 

 patients. Now that blood-letting is no longer a regular practice, 

 the leech is rarely seen. The colour of the Medicinal Leech is 

 olive or dark green, with six ruddy yellow bands along the back, 

 while the abdomen is yellowish-green with black spots. On taking 

 a leech out of the water it will contract its body and diminish 

 in size. If placed on the hand, it will soon begin to fix its front 

 sucker to the skin, a series of sharp pricks being felt as the 

 sucker becomes firmly fixed. The leech now elongates and 

 moves its body with a slightly undulatory motion. Gradually it 

 becomes more and more distended, until it suddenly lets go its 

 hold and falls off, gorged with blood. The banquet is followed 

 by a prolonged fast, and the length of time that the animal will 

 live without food is remarkable from six months to two years. 

 According to Cobbold, three species of leeches were formerly 

 used in medicine, namely the Grey Leech (Hirudo medicinalis), 

 the Green Leech (Hirudo offlcinalis), and the Dragon Leech (Hintdo 

 interrupta). 



All the leeches lay eggs, and they may be deposited singly or 

 in numbers, and in the latter case they may be covered with a 

 viscous web or with a spongy envelope called the cocoon. When 



