230 THE INVERTEBRATA 



regularly placed openings of the uterine birth pores. The majority 

 of these are parasitic as adults in freshwater fishes, but Dibothrio- 

 cephalus latus occurs in man and Bothriotaenia in birds. Archigetes 

 is parasitic as an adult in body of tubilex, an oligochaete worm living 

 in fresh water. The larva is a plerocercoid which in some forms, 

 Caryophyllaceus and Archigetes develops gonads paedogenetically so 

 that there is no adult with proglottides. These paedogenetic forms 

 closely resemble the Cestoda Monozoa in appearance. 



(v) Cyclophyllidea. The scolex bears four cup-shaped suckers and 

 has a rostellum with a crown of hooks. 



The Cyclophyllidea 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. The number of proglottides varies considerably, the smallest 

 number (3) is found in Taenia echinococcus ^ while many forms have 

 hundreds of proglottides and are several yards in length. The pro- 

 glottides never drop off before they are mature, as they may do in 

 the other groups and develop generative organs later, consequently 

 the separated proglottides always contain fully developed oncho- 

 spheres. Two interesting forms may be mentioned. Dipylidium cani- 

 num is a tapeworm infesting the alimentary canal of dogs and cats. 

 Each proglottis has a double set of generative organs with two 

 separate generative openings, a feature which gives the animal its 

 name, but which may occur in other forms. 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. 



The homologies of the various ducts of the genitalia of the Platy- 

 helminthes (Figs. 165, i66) present great difficulties. While one or two, 

 the oviduct and the vas deferens for example, are quite clearly homo- 

 logous throughout, the homologies of others, particularly the accessory 

 organs such as uterus, bursa copulatrix, vagina, are very doubtful. 

 The ** uterus" of the Trematoda is clearly the ductus communis of 

 the Turbellaria greatly elongated and used for egg storage, while the 



