138 



THE CESTOIDEA 



or even the brain and eye ; arrived here it may even burrow 

 through the tissues on its own account. In any case it causes 

 inflammation, and the tissues of the host give rise to a " cyst " 

 around it. In this position it will undergo further development, 

 resulting in a " metacestode " of one kind or another. This, when 



FIG. XXIX. Development of Dipylidium caninum (altered from Grassi and Rovelli). 



1. Proscolex, onchosphere or six-hooked embryo, at present solid. 



2. The embryo has elongated, and the " primitive lacuna " has arisen (a). 



3. The hinder part, bearing the hooks, narrows and elongates, so that a "head" and 

 "tail" are distinguishable; at the anterior end the foundations of the four suckers and the 

 rostellum are indicated in reality at this stage they are mere solid heaps of cells. 



4. The organism is longer ; the body and tail are more distinct ; the rostellum, now armed 

 with spines, is cupped, as also are the suckers. The excretory system is developed (6, c). 



5. The fore body is being invaginated into the hind body ; the excretory bladder is provided 

 with a pore at the base of the tail, the surface of which is shown with the scattered hooklets. 



6. The tail has dropped off, a spherical " cysticercoid " remains, in which the " scolex" is 

 commencing to grow upwards into the sac formed by the fbre body, the outer wall consisting 

 of the hind body. 



swallowed by the final host, will develop into a tapeworm in the 

 intestine of its new host. 



One of the simplest metacestode conditions occurs in the case of 

 D. caninum (T. elliptica), whose history has been worked out by 

 Grassi and Eovelli (13). The eggs discharged with the faeces 



