io 4 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



body are provided with lateral plates, a pair to each segment. 

 These plates are divided into lobes, and aid the animal in 

 crawling or swimming, and as some of them are well pro- 

 vided with blood-vessels they also serve as respiratory organs. 

 Many of the marine worms swim in the sea, others are 

 more or less closely confined to their burrows, while some 

 form tubes of sand or gravel or secrete tubes of lime which 

 furnish excellent protection. Such tube-like houses are often 

 found on rocks and shells. They may usually be found on 

 the oyster shells in almost any market. The part of the 

 body that is protected has no further use 

 for the lateral appendages and so they have 

 almost or quite disappeared. On the head, 

 however, there have been developed great 

 plume-like appendages which are well sup- 

 plied with blood-vessels and act as gills. 

 These organs are often beautifully colored, 

 and when fully expanded look like gorgeous 

 flowers. Indeed, these worms and the sea- 

 anemones make veritable flower gardens in 

 many a tide pool along rocky shores. 



The Leeches. In their general appear- 

 ance the leeches look much more like the flat- 

 worms (Platyhelminthes), than like the other 

 Annelida. The body is flattened, and com- 

 40 The posed of many segments. Most of the seg- 

 ments are marked by transverse lines, making 

 the animal appear to have many more seg- 

 ments than it really has. The ventral side is 

 provided with two sucking disks. The mouth, 

 which lies in the anterior disk, is provided 

 with sharp jaws which enable the leech to puncture the skin 

 of animals in order that it may suck their blood. Leeches 

 live mostly in the water, and are found most commonly on 

 such animals as fish, frogs and others that are aquatic or 

 semi-aquatic. They will readily attack man when the oppor- 

 tunity offers, and in olden days they were much used by physi- 

 cians to "bleed" their patients. A leech was allowed to at- 



FIG. 



medicinal leech, 

 Ilirudo medicina- 

 lis. (Grows to be 

 six or eight inches 

 long.) 



