Tapeworms. Cestoids. 241 



5. Larva, Hydatid Bladder-worm, Scolex. Becoming 

 embedded in a tissue or organ of its new host, the embryo grows 

 into a cystic organism, developing one or more heads attached to 

 its sac of clear or milky fluid, and encreases at the expense of 

 the surrounding tissue, but never acquires reproductive organs, 

 nor any feature of the mature intestinal worm except the head 

 which exactly represents that of the fully developed tapeworm. 

 It is only when the animal, which acts as host to the larva, is 

 devoured by another animal adapted to entertain this particular 

 parasite in its mature form, that it is set free by the digestion of 

 the tissues around it and grows into a taenia or strobila. The 

 caudal sac disappears, the neck becomes gradually elongated and 

 segmentation commences, being first indicated by the formation 

 of transverse striae behind the head which become more and 

 more separated until distinct, sexually mature segments are 

 formed. It is now a tape- worm or strobila and grows to a vary- 

 ing length determined by its species and drops off at intervals 

 its ripe ovigerous segnients from its caudal extremity. 



Divisions of Taenia. The taenia are divided into armed and 

 unarmed according as they are or are not provided with a double 

 circle of booklets on the proboscis. The armed are sub-divided 

 into : ( I ) Those which in their larval state have the caudal 

 vesicle proceeding from the head or proscolex by a simple en- 

 crease and modification of structure without any new independ- 

 ent part. These are known as the cystotseniae ; (2)' Those in 

 which the caudal portion is enlarged by budding and the forma- 

 tion of an additional part or blastogene, which retains its embry- 

 onic structure. The caudal vesicle is only slightly developed so 

 that the larva retains more of the appearance of a taenia. When 

 it passes into the condition of the taenia the blastogene'xs sloughed 

 off. These are known as cystoidotaeniae. This larva Hves in 

 invertebrate animals. 



The cystotaeniae are divided into the cysticerci, ccenuri, 

 and echinococci, differentiated by their respective mode of de- 

 velopment in the larval or cystic stage. 



The cysticercus has a well developed but simple caudal sac 



bearing only a single head which may become retracted and in- 



vaginated within the sac leaving only a small opening at the point 



of involution, or it may be projected outward into the surround- 



16 



