TBANSMIGBATIONS AND METAMORPHOSES. 



203 



order to arrive again at the nasal fossae which are the 

 first cradle of the family. 



We find occasionally between the feathers of some 

 birds tubercles of the size of ' a pea, and when we open 

 them we see in each two similar worms, placed so that 

 the stomach of one is applied to that of the other ; this 

 is the monostome of which we have spoken above. These 

 worms are from three to four millimetres in length 

 (about "13 in.), and are found in the titmouse, the sis- 

 kin, the sparrow, the canary, and some other birds. 



A worm very common in the intestines of the green 

 frog is known by the name of Amphistomum sub-clavatum. 

 Its cercariae are usually found in an acephalous mollusc, 

 known by the name of Cyclas cornea. That which 



Fig. 45.— Cercaria of Amphis- 

 tomum sub-clavatum. 



Kg. 46. — Sporocyst of Amphis- 

 tomum sub-clavatum from the 

 Cyclas cornea. 



distinguishes the scolices of this species is the great 

 contractibility of the external membranes of the young 

 individuals ; they lengthen, they shorten, they swing to 

 the right and the left,, describing a semicircle on the 

 anterior half of the body (Fig. 46). We represent side 



