PHYLUM ANNELIDA. SEGMENTED WORMS 



191 



Medicinal leech attached to ar 



m 



Partly extended medicinal leech (ventral view) 



Piacobdella attached 

 to neck of turtle 



Piacobdella with 

 eggs (ventral view) 



Figure 105. Leeches are commonly called bloodsuckers. A full-grown medicinal leech is four 

 inches in length. Piacobdella, common on turtles, is about one inch long. 



complex individuality. Some zoologists main- 

 tain that the segmental arrangement of or- 

 gans, such as nephridia, blood vessels, and 

 reproductive organs, has arisen by division 

 of a single ancestral organ, and not by for- 

 mation of new organs as the fission theory 

 demands. 



The coeiom 



The coeiom (Fig. 92) is a body cavity 

 lined with tissue of mesodermal origin; from 

 it the excretory organs open; and from its 

 embryonic walls, the reproductive cells or- 

 iginate. The importance of the coeiom 

 should be clearly understood since it has 

 played a prominent role in the progressive 

 development of complexity of structure. The 



appearance of this cavity between the diges- 

 tive tract and body wall brought about great 

 physiologic changes; it is correlated with the 

 origin of nephridia for transporting waste 

 products out of the body, and of reproduc- 

 tive ducts for the exit of eggs and sperms. 

 The coeiom also affected the distribution of 

 digested food within the body, since it con- 

 tains a fluid which takes up material ab- 

 sorbed by the digestive tract and carries it 

 to the tissues. Excretor)' matter finds its way 

 into the coelomic fluid and thence out of the 

 body through the nephridia. 



So important is the coeiom considered by 

 most zoologists that the Metazoa are fre- 

 quently separated into two groups: (1) the 

 Acoelomata without a coeiom, and (2) the 

 Coelomata with a coeiom. The Porifera, 



