CESTODA MEROZOA 229 



occur when a sheep is devoured and torn to pieces by dogs or wolves. 

 In Taenia echinococcus ^ the adult of which lives in the alimentary 

 canal of the dog and is remarkable for having but three proglottides, 

 the cysticercus stage is found in domestic animals and also in man in 

 countries where men live in close association with dogs. The cyst 

 stage is very large and the bladder may contain a gallon or more of 

 fluid. Such a cyst, known as a " hydatid ", rapidly proves to be fatal. 

 It is particularly dangerous and difficult to eradicate because the 

 walls of the cyst have the power of budding off asexually daughter 

 cysts. A still further development of asexual budding in the cysti- 

 cercus stage occurs in Staphylocystis, where the onchosphere imbeds 

 itself in the liver and then develops a stalk or stolon which buds off 

 cysts which are detached and fall into the body cavity of the host. 



Where the cysticercus is swallowed by the final host the head is 

 everted from the bladder, the bladder is digested and proglottides 

 forthwith make their appearance from the neck region of the scolex. 

 So far as is known the production of proglottides continues for the 

 duration of the life of the host. 



The subdivision of the Cestoda Merozoa depends on the shape of 

 the scolex. There are five divisions, the last of which contains the 

 forms commonly found as adults in the alimentary canal of the 

 Mammalia and is the only group of economic importance. 



(i) Tetraphyllidea. The four suckers are usually stalked out- 

 growths of the scolex. Parasitic in fish, amphibia and reptiles. 

 Onchosphere enters a copepod and develops into a larva known as a 

 plerocercoid, in which condition it remains until the copepod is 

 eaten, when it develops into the adult. Size moderate usually 

 20-30 cm. long but occasionally as small as i cm. or as large as. 

 I metre. 



(ii) Diphyllidea. There are two suckers only and the scolex has a 

 long neck armed with spines. There is only one family and one genus, 

 Echinobothrium, which is found in the spiral intestine of Selachians. 

 The larva, which is of cysticercoid form, is found in the prawn 

 Hippolyte. 



(iii) Tetrarhynchidea. These have four suckers each provided with 

 a long spiniferous retractile process. The adult is parasitic in the 

 alimentary canal of Elasmobranchs and especially Ganoids. The larva 

 which may be of either the procercoid or cysticercoid type occurs in 

 marine invertebrates of many kinds, fish and occasionally reptiles. 



(iv) Pseudophyllidea. The scolex has two suckers which may 

 be absent in some forms, there is no clearly marked neck and 

 hooks are usually absent. Occasionally as in Triaenophorus , a 

 common parasite of freshwater fish, the external divisions between 

 the proglottides are indistinct and these are only indicated by the 



