CESTODA. 



251 



left in the shell at hatching, and the outer layer either becomes thick 

 and ciliated, as in some Bothriocephalidae (Fig. 203, c), or thin and 

 not ciliated as in other Bothriocephalidae and Taenias which inhabit 

 aquatic animals, or finally it becomes cuticularised as a thick, radially 

 striated layer (Fig. 203, a), as in the Taenias which infest land 

 animals (protection against desiccation). Meanwhile, either while 

 the embryo is free (Bofhrioceplialidae), or while it is within the 

 uterus of the mother, the inner mass develops six hooks, and 

 becomes the six-hooked embryo or Onchosphere (Fig. 205, 6). In 

 the Bothriocephalidae the further history of the embryo is unknown; 

 it loses its ciliated coat and probably soon dies, unless it migrates into 

 its next host, which is unknown. In 

 other cases, in which the life-history has 

 been followed, the development of the 

 embryo into the asexual scolex rarely 

 takes place in the intestine of the original 

 host. It is said to do so in Taenia 

 (Hymenolepis) nana (see p. 261), and it 

 has been suggested that it might occur 

 in the stomach of the same animal, if by 

 reversed peristalsis a ripe proglottis was 

 passed back into the stomach, and there 

 digested (T. saginata). 



As a rule the Scolex, i.e. the head and 

 neck of the tape-worm, is developed from 

 the six-hooked embryo in another host, 

 and in some cases (Coenurus, Echino- 

 coccus) more than one scolex arises from 

 a single embryo (Fig. 206). 



The eggs usually leave the intestine of 



the host in the proglottis, either by active migration or in the faeces. 

 The proglottides are deposited on the ground or in water. Here they 

 crawl about and deposit their eggs as described on page 244, but they 

 soon die, especially if the temperature is unfavourable and the air dry. 

 The embryo, which in the case of the land forms is protected against 

 desiccation by the thick cuticle described above, retains its vitality 

 for a time, which depends on the external conditions. Eventually 

 it dies, unless it passes into the stomach of a suitable host. As a 

 rule this host is an herbivorous or omnivorous animal, but it may 

 be a carnivorous animal. The embryos are usually taken up in the 

 food, or in drinking water, but occasionally they enter accidentally 



FIG. 204. Embryo of Bothrio- 

 cephalus latus pressed out of 

 the egg. EG outer layer (so- 

 called ectoderm) ; Hm the shell 

 (enveloping) membrane. 



