202 THE INVERTEBRATA 



tapeworms only the chitinous covering persists. The egg hatches as an 

 onchosphere after being swallowed by the first host. The onchosphere 

 then bores through the wall of the alimentary canal and lodges some- 

 where in the peritoneal cavity of the host. Here it develops suckers 

 and a scolex. In primitive forms such as Bothriocephalus^ the young 

 cestode rests inside the first host, a Cyclops, which is then eaten by 

 a freshwater fish. It again bores through the wall of the alimentary 

 canal and rests in the body cavity where it grows still further, reaching 

 the metacestode stage. Growth now ceases but the metacestode stage is 

 often inconveniently large for the body cavity, causing it to bulge. 

 Sticklebacks thus infected with the metacestode of Schistocephalus 

 gasterostei are commonly found. The adult in this case reaches 

 maturity when eaten by a bird. Man acquires Bothriocephalus latus, 

 a nearly related form, by eating pike infected with the metacestode. 

 In the Tetrabothriata the resting stage in the first host is the " bladder 

 worm " (or cysticercus). The onchosphere on reaching its resting place 

 becomes hollowed out into a ball filled with fluid, A depression then 

 forms in the wall of the sphere and becomes an inverted scolex. In 

 Taenia serrata, the common tapeworm of the dog, the bladder stage in 

 the rabbit (to which the name Cysticercus pisiformis wsis given before the 

 connection with the adult was discovered) has but one head inverted 

 into the cyst. In the bladder-worm stage of Taenia coenurus, which is 

 found in the brain of the sheep and causes the disease known as 

 "gid" or "staggers", many heads are formed and invaginated into 

 the cyst so that multiple infection may occur when the 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 re- 

 markable 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 diflicult 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 cysticercus 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 



