158 



mVMRTEBRATE MORPHOLOGY. 



some forms, such as the Tcenia cmumerina of the dog, whose 

 primary host is the Dog-tick, into a Cysticercoid. This resem- 

 bles a scolex, whose head has been withdrawn into and enclosed 

 bythebody,andwhenitis swallowed by the secondary host, the 

 dog in the instance cited, the head is pushed out, fastens itself 

 to the wall of the digestive tract, and begins to grow and form 

 proglottides. In other cases, however, the posterior part of the 

 scolex into which the head is retracted becomes enormously 

 swollen by the accumulation of fluid within it, forming a large 

 vesicle, into the interior of which the head projects, having 

 become completely invaginated. Such a form as this is 



Fig. 83. — A, six-hooked embryo of Tcenia; B, diagram of Cystieercus ; G, 

 diagram of GcBnurus ; D, diagram of Echinocoecus. 



termed a Cystieercus (Fig. 83, B) ; when it is swallowed by the 

 secondary host the head evaginates, and the cyst remains for 

 some time attached to the hinder end of the scolex, but later 

 disappears, and the formation of the proglottides occurs. 

 Further modifications arise by the formation in the wall of the 

 cyst of not only one but several invaginated heads, forming 

 the Coenurus (Fig. 83, C) ; or even secondary cysts may arise 

 from the inner wall of the original vesicle, and each of them 

 may develop several heads, forming what is known as the 

 Ecliinococcus (Fig. 83, D). 



Several of the Cestoda are especially interesting from a medical stand- 

 point, inasmuch as they are parasitic in man either during the adult or the 



