688 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxv. 



the transverse one, in fact is little more than a notch in the upper lip, 

 though its edges are still serrated like those oi folia cem. In many of 

 the other species {7viger, megalojm^ etc.) the longitudinal slit disappears 

 entirely, leaving a curved mouth opening transverse to the long diam- 

 eter of the proboscis tube and often slightly enlarged at either end. 

 In these forms also there is no serration of the edges of the mouth 

 opening. Just behind the labial palps on either side the chitin frame- 

 work is thickened somewhat where the longitudinal ribs join the 

 transverse rods, and to these thickened joints are attached the mandi- 

 bles and maxilla?. 



The mandibles {ind.) are elongated, somewhat triangular or })roadly 

 sickle-shaped chitin plates, which curve inward and upward toward 

 each other so that their pointed and toothed tips almost meet in the 

 mid line of the gullet above the mouth opening. 



The}' are always l^ordered by sharp-cutting teeth on their inner 

 margins, and sometimes {amerlcaiius^ etc.) along the outer margins 

 also, and are evidentl}^ designed to cut and tear the flesh of the para- 

 site's host and so stimulate the flow of blood. The two maxillte (//;,?.), 

 for there is but a single pair, are also thin chitinous plates of various 

 shapes, which are attached at the same place as the mandibles, but 

 which extend backward and outward behind the mandibles nearly 

 parallel with each other. They are sometimes toothed on their inner 

 margins (Thorell), but are usual!}" smooth and terminate in a long 

 sharp spine, which also is evidently used to stimulate the flow of blood 

 from the host. 



The proximal end of the proboscis passes directly into the short 

 oesophagus, which in turn opens into the stomach. 



These portions of the digestive system are the same in the adult as 

 in the larva and do not need to be described again here. 



The only thing to be added is that the side pouches of the stomach 

 become more and more branched with age until Anally they are entirely 

 broken up into ramitications, as is well shown in the photograph of 

 americduus (fig. 84). 



The circulatory system. — There is a well-detined heart and a short 

 aorta which opens directly into the body cavity, but there are no other 

 blood vessels of any sort. Instead, the blood circulates about freely 

 through the lacunar and sinuses, which form a network all over the 

 body, as alread}- described (p. 669). 



The blood itself is a colorless liquid in which float numerous cor- 

 puscles. The latter are smooth, spindle, or pear-shaped, and occasion- 

 ally somewhat fibrous at the ends. 



As soon as the circulation stops they sink to the bottom, but they 

 do not become spherical in any American species so long as the animal 

 remains alive. They contain a well-defined nucleus, which can be 

 brought out clearly by the addition of a little acetic acid (Leydig). 



