THE CESTOIDEA 



137 



proglottids are in different stages of development, so that by 

 examining the proglottids from behind forwards, earlier and earlier 

 stages are met with independent of the time of the year, whereas 

 in the Dibothridiata all the eggs in a strobila, at the same time of the 

 year, are in the same stage of development. The general later history 

 of the onchosphere, or proscolex, is as follows : Ripe proglottids drop 

 off from the strobila, and pass out of the host with its faeces ; the 

 proglottid either decays and sets free the eggs, which are then 

 swallowed by browsing animals, or the proglottids themselves are 

 devoured. But in order to attain maturity the egg of a given 

 tapeworm must be devoured by a definite intermediate host (only in 



FIG. XXVIII. -Eggs of Taeniidae (modified from Monies). 



1. Dipylidium caninum, L. 2. Drepanidotaenia anatina, Kr., from duck. 3. Pyriform 

 apparatus of Andrya wimerosa, Moniez, out of rabbit. 4. Moniezia expanse, Rud., out of 

 ruminants, a, egg shell (vitelline membrane of Moniez) ; b, albuminous coat, fonned probably 

 in all cases from some of the earliest formed blastomeres ; two nuclei are represented in 4 ; the 

 cells undergo histolysis, and become filled with refringent globules ; c, internal shell, fonned, 

 according to Moniez, from modification of an outer layer of blastomeres, but possibly secreted 

 by this layer ( = striated coat of Fig. XXVII.) ; c, is the pyriform apparatus in 8, 4 ; d, the hexa- 

 canth embryo ; c', a " delaminated " layer of cells formed before the internal shell (c), according 

 to Moniez ; /, peculiar modification of this mass. 



a few cases do several different animals serve as hosts for one and 

 the same worm) which is, as a rule, an herbivorous vertebrate, and 

 in the majority a mammal, where the final host is a mammal ; but 

 in the case of bird and fish and amphibian tapeworms, the inter- 

 mediate host may be some invertebrate. 



Arrived in the intestine of the intermediate host, the egg- 

 envelopes are softened and dissolved ; the proscolex is thus set free, 

 and by means of peristaltic movements, and by the movements of the 

 six hooks it bores its way into the intestinal wall, and arrives in a 

 blood-vessel (some constituent of the portal vein), in which it is 

 carried along till it reaches a capillary of less diameter than itself. 

 In this way it may be conveyed to the liver, peritoneum, mesentery, 



