204 THE INVERTEBRATA 



the scolex. There are two divisions: (i) Dibothriata, (ii) Tetra- 

 bothriata. 



The Dibothriata have two grooves, one on either side of the scolex. 

 The uterus opens to the exterior by a birth pore. The embryo oncho- 

 sphere has a ciliary covering. There is no cysticercus,the resting stage 

 being a metacestode. Bothriocephalus latus belongs to this group. 

 Triaenophorus nodulosus, another member of the group parasitic in 

 the gut of freshwater fish, has hooks as well as grooves in the 

 scolex and the proglottides are not marked by divisions. They are 

 only distinguishable by the uterine birth pores. 



The Tetrabothriata comprise the majority of the common tape- 

 worms. Those infesting the gut of mammals all have a scolex closely 

 resembling that of Taenia with four well-defined suckers and a circlet 

 of hooks. Those found in the gut of fish have a more elaborate 

 scolex. Two interesting forms may be mentioned in addition. 

 Dipylidiuni caninum is a tapeworm inifesting the alimentary canal of 

 dogs and cats. The first host is the flea, and puppies and kittens are 

 early infected by catching and eating these insects. The mature 

 proglottis has a double set of male and female generative organs with 

 an opening on either side. Hymenolepis nana is one of the smallest 

 tapeworms. The adult has ten to twenty proglottides and only 

 measures half an inch in length. It occurs in children in certain places, 

 particularly Lisbon and New York, where it is said to be increasing. 

 It is remarkable among tapeworms for being the only one known to go 

 through all its life history in one host. The embryos bore into the 

 intestinal wall where they pass through the cysticercus stage and 

 emerge again into the alimentary canal when adult. 



