212 PARASITES OF THE DOMESTIC ANIMALS 



vesicles. The exogenous vesicles are capable of implantation upon 

 organs somewhat remote from the primary vesicle. This occurs more 

 commonly in the pig and ruminants than in man. 



The daughter vesicles may, in the same manner, give off grand- 

 daughter vesicles which, like the parent vesicles, maj^ be endogenous 

 or exogenous. 



All of these vesicles develop proligerous vesicles and consequently 

 the larval tapeworm heads, or they may remain sterile, in which case 

 they are referred to as acejjhalocysts. 



It will be noted from the foregoing that one onchosphere ma}^ develop 

 hundreds of tapeworm heads. 



The echinococcus is usually considered as one species, though there 

 is a form which has received the name of Echinococcus multilocularis 

 {E. alveolaris), thought to be due to a tapeworm -.hffering slightly from 

 E. granulosus. Its main distinguishing character is the size of the 

 vesicles, which does not exceed that of the pea. They have a honey- 

 comb arrangement, and are filled with a gelatinous material, the majority 

 of the cysts remaining sterile. The mass of vesicles may grow to the 

 size of a child's head, and constitutes a verj^ fatal form of echinococcosis. 

 It has been found in the ox and pig, but more frequently in man. 



Development. — Embryos finding their way to the intestine with 

 food or water that has been contaminated Avith egg-containing excrement 

 of dogs are probably carried to the liver by the portal system. Four 

 weeks after infecting pigs, Leuckart found small white nodules about 

 1 mm. in diameter, each with a capsule derived from the hepatic con- 

 nective tissue, and containing within it the globular echinococcus. At 

 about five months the cysts were the size of a hazelnut, and each con- 

 tained a thick-walled whitish-colored vesicle, — the mother vesicle. 



The development of the echinococcus is slow. It may remain simple 

 and its growth be limited to increase in volume and thickening of the 

 cuticular membrane, in which case it may reach a diameter of 15 cm. 

 (6 inches) . Generally its size does not exceed that of an orange, and its 

 growth is attained by the formation of secondary vesicles. Where these 

 pass to the inside of the mother vesicle this becomes dilated in an irreg- 

 ular manner, influenced somewhat by the compression of the adjacent 

 organs of the host (Fig. 117). 



As regards the formation of the daughter vesicles, the process has 

 usualh^ been described as a normal one following the complete develop- 

 ment of the hydatid. Deve, in a paper upon this subject (1917), states 

 that every multivesicular cyst is one which has had its vitahty menaced, 

 and that the endogenous vesicles are the result of a defense reaction. 

 The simple cyst with its brood capsules, according to this authority, 

 represents the normal Iwdatid. 



