52 PLATYHELMINTHES. 



CESTODA. 



The Cestoda are endoparasites which possess very few of 

 those organs that are characteristic of free-living animals. They 

 have no alimentary canal, no organs of special sense, and, 

 except in the head, the nervous system is feebly developed. 

 On the other hand, the organs needed for the reproduction of 

 the species are enormously developed, so that in the more mature 

 portions of the animal, the ovaries, testes, and accessory organs 

 occupy nearly the whole space. Can you explain why this is 

 true? 



CROSSOBOTHRIUM LACINIATUM. 



This form passes its adult life in the intestine (spiral valve) 

 of the sand-shark. Cestode larvae which may be the young of 

 this species are abundant in the cystic duct of the squeteague. 

 How the developing eggs and embryos are conveyed from the 

 shark to the squeteague is not known. The transfer of the 

 larvae from the squeteague to the alimentary canal of the shark 

 can be easily understood. 



Adult Stage. 1. Notice specimens that are attached to the 

 wall of the intestine of the shark. 



'2. Observe movements of specimens in a dish of sea-water. 



Do the swkers have independent movements? 



3. With a low power of the compound microscope, or with 

 a hand-lens, note that the worm is made up of a head portion, 

 the scolex, and of numerous segments, the proglottids. What 

 is the relative size of the proglottids in the different regions of 

 any specimen? Where are new proglottids produced? (See 

 Curtis.) Are the proglottids attached to one another with 

 equal firmness in all parts of the body? Note their peculiar 

 shape, and how they are connected together. In the above ex- 

 amination, if living material is used it will often be desirable to 

 stretch portions of the animal very gently with your forceps. 



4. Note the number and arrangement of the disk-like suck- 

 ers. How are they borne on the scolex ? Do you find each 

 sucker to be entirely simple ? 



Draw the adult worm. 



