442 



CHARACTERS OF INVERTEBRATE ANIMALS 



D. 



B. 



shows that at its front end there is a double circlet of horny hooks? 

 behind which four large slickers are arranged at equal distances. 

 By means of these hooks and suckers the head is attached to 

 the mucous membrane lining the intestine of the host. Such 

 organs of attachment are common among parasites, and another 



example is given further 

 on in the case of the 

 Thorn -headed Worm (p. 

 449). No trace of a mouth 

 can be seen, and indeed 

 digestive organs are en- 

 tirely absent, for the para- 

 site lives at the expense of 

 its host, absorbing through 

 the general surface of its 

 body the digested food 

 with which, in the small 

 intestine, it is surrounded. 

 Nor are there any special 

 circulatory or respiratory 

 organs. The excretory 

 organs are well developed 

 and quite unlike the paired 

 nephridia of segmented 



Fig. 272. Common Tape-Worm ( Ttznia solium) 



A, slightly reduced ; B-G, enlarged to various scales. A, Adult, 

 showing head and chain of joints (proglottides), the ripest of which 

 have been broken off. B, Head, c, Six-hooked embryo. D-F, Stages 

 in development of head. G, Bladder- Worm stage (cysticercus), with 

 head protruded. 



worms. 



narrow 



They consist 

 of a 



tube on each side, 

 not far from the edge of 

 the body, and connected 



with elaborate branching canals which ramify in the soft substance 

 filling up the spaces between all the internal organs. The nervous 

 system is feebly developed, and its central parts consist of a longi- 

 tudinal cord on either side, these being connected in the head by 

 a transverse band, which may be regarded as equivalent to a pair 

 of brain-ganglia. 



The joints which make up the trunk are largely filled with 

 egg-producing organs, and, at the hind-end, joints full of ripe eggs 

 are from time to time detached, passing out of the body of the 

 host to the exterior. If any of these joints are devoured by a 

 pig, the eggs hatch out in its stomach and the spherical embryos, 



