316 DEVELOPMENT OF TAPE-WORMS. 



which they make their way through the walls of the stomach 

 and the tissues of the body until they reach the organ in which 

 their development is to take place, or, getting into the blood- 

 vessels, allow themselves to be carried along by the stream to 

 some suitable locality. Here they establish themselves, and soon 

 afterwards the little vesicle swells and becomes surrounded by a 

 cyst, formed by exudation from the tissues of its host. A 

 curious process of gemmation then begins in the little bladder, 

 the result of which is the production of a head resembling that 

 of the Tape-worm, but turned into the cavity of the vesicle, 

 which is filled with a fluid resembling the white of egg, and 

 serves both as a reservoir of nourishment and as a protective 

 covering for the budding head. In some cases nothing but the 

 head is produced ; in others a neck of considerable length, with 

 indications of segments, is formed ; in some again the vesicle only 

 produces one head, whilst in others it attains an enormous size 

 and gives origin to a multitude of such productions. These 



bladder-like parasites have long been 

 known under the name of Cystic- 

 worms (Cystici), and regarded as 

 nearly allied to the Tape-worms ; but 

 it is only recently that it has been 

 discovered that they are mere steps 

 in the development of the latter. 

 The worm in this stage is now gener- 



FIG. 636. CYSTICERCUS CEr.LTJLOs.aB 5 11 a j. j 7 T i 



, head enlarged. ^"7 denominated a scolex. In this 



condition it remains until the animal 



in which it harbours is devoured by some particular carnivorous 

 animal, when the head, which up to this period has been turned 

 into the vesicle, is protruded and attaches itself to the intestine 

 of its new host by means of the hooks and suckers with which 

 it is provided (Fig. 636). The vesicle (and neck when present) 

 is then cast off, and the head proceeds to develope a series of 

 segments which in course of time form the true Tape-worm 

 colony as first described. This is called the Strobila. Some 

 differences are exhibited, especially in the second stage, by the 

 different species of Cestoidea, especially those which pass this 



