462 



The Animal Kingdom 



the bottom of polluted waters, the red-colored Tuhijex occurs in large 

 numbers, and is used by many animals as a food. One family of this 

 class, the Branchiobdellidae (Fig. \S\,A), live as commensals on the 

 gills and exoskeleton of crayfish. These have suckers and have lost their 

 setae ; they serve as intermediate hosts for the important kidney worm 

 {Dioctophyme renale), a nematode parasite of carnivores. Throughout 

 the soil, the members of another family, the Enchytraeidae, occur in vast 

 numbers. These are small threadlike worms. 



THE CLASS HIRUDINEA 



The fourth major class of the annelids is that containing the leeches 

 (Fig. 151,5), the class Hirudinea. These highly modified annelids 

 typically live in fresh water in the temperate and tropical regions of the 

 world. Only a few are known from the ocean, and a few forms have 

 become terrestrial in moist, tropical areas. 



Fig. 151. — Other eucoelomates. A, Camb arm cola, a branchiobdellid; B, Glos- 

 siphonia, a leech; C, Dendrostonia, a sipunculid; D, Echiurus, an echiuroid. 



With but a few important modifications, the body plan of the leeches 

 is essentially that of the other annelids. Chief among these modifica- 

 tions is the presence of two suckers, a small oral one around the mouth 

 and a large posterior one. The somewhat flattened body contains thirty- 

 three segments which are externally divided many times by additional 

 rings. Setae are lacking, and the entire coelom is filled with par- 

 enchymatous tissue. The crop is the largest part of the digestive tract 



