Chap. 28 ANNELIDS PIONEERS IN SEGMENTATION 571 



the anticoagulant, hirudin, and the muscular pumplike pharynx draws out the 

 blood (Fig. 28.16). The pharynx opens into an enormous crop extended by 

 pairs of sacs, the last of which reaches nearly to the end of the body. This can 

 hold enough blood for several months' food supply. Soon after a blood meal 



Pharynx 



Crop with pouches 



Anterior sucker 



Posterior sucker 



Stomach 

 Capacity to toke in much food at one time 



Fig. 28.16. Leech. A general diagram of the digestive cavity. A leech sucks in 

 enough blood at one time to increase its weight five times. When well inflated it 

 loosens its hold voluntarily but the wound continues to bleed because of the anti- 

 coagulant injected with its saliva. 



much of the water is excreted and the concentrated blood is slowly digested 

 in the small stomach into which the crop opens. 



Leeches are hermaphrodites and, as in earthworms, there is a mutual trans- 

 fer of sperm cells during mating. After mating, usually in summer, eggs 

 and sperm pass into cocoons produced by a clitellum. The cocoons remain in 

 water or earth except in one fresh-water family (Glossiphonidae) in which the 

 cocoon and afterward the young leeches are attached to the underside of the 

 parent (Fig. 28.15). 



Leeches in Medical History. Leeching is an old medical treatment, so com- 

 mon that leech became the name for the physician as well as the treatment. The 

 leeches, placed on the skin, carried on the real treatment that consisted of their 

 sucking out a considerable amount of the "bad blood." During the early nine- 

 teenth century there was an enormous demand for "medical leeches." They 

 were reared in ponds in many parts of Europe. Broussais (1772-1838), a 

 French physician, was a leading advocate of "blood letting." During the year 

 1833 over 41 million leeches were imported into France for medical use and 

 a good number into the United States. 



Class Archiannelida 



This is a small group of inconspicuous worms (e.g., Polygordius) of the sea- 

 shore, that in the adult stage resemble the late larval stages of polychaetes. 

 Internally the adults are segmented, but externally the segments are indis- 

 tinct or missing. The larva is a typical trochophore. The class is merely men- 

 tioned here for completeness. 



