228 THE INVERTEBRATA 



is called the uterus and is used for the storage of eggs, but it is 

 doubtful whether it is homologous with the uterus of the 

 Trematoda. 



The life history of a cestode is a complicated combination of sexual 

 and asexual reproduction. One, two or three hosts may be necessary. 

 The egg passes to the exterior with the faeces. It contains inside it 

 an embryo armed with six hooks called an "onchosphere". The egg 

 case takes diiferent shapes ; in Dibothriocephalus latuSy which is a more 

 primitive type of cestode, the covering of the embryo is ciliated. 

 In the Cyclophyllid tapeworms, which constitute the most advanced 

 group of the Cestoda the ciliary covering is lost. In Dipylidium 

 caninum, the adult of which occurs in the alimentary canal of the 

 cat or dog, it is replaced by an albuminous coat with a chitinous 

 Uning inside, while in most of the other forms only the chitinous 

 covering persists. The egg hatches as an onchosphere after being 

 swallowed by the first host. The onchosphere then penetrates the 

 wall of the alimentary canal using its hooks for this purpose and 

 lodges somewhere in the peritoneal cavity of the host. Here it 

 develops suckers and a scolex. In primitive forms such as Dibothrio- 

 cephalus, the larval cestode rests inside the first host, a Cyclops, at 

 a stage of its development known as the plerocercoid stage. This 

 stage is ovate in shape and the generative organs are undeveloped 

 and there are no signs of proglottides. The Cyclops is then eaten by 

 a freshwater fish, after which the larva, or plerocercus, bores through 

 the wall of the alimentary canal and rests in the body cavity where 

 it grows still further, reaching the metacestode stage. Proglottides 

 can be distinguished in the metacestode stage but the generative 

 organs are not fully mature. 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 Schisto- 

 cephalus gasterostei are commonly found. The adult in this case 

 reaches maturity when eaten by a bird. Man acquires Dibothriocephalus 

 latus, a nearly related form, by eating pike infected with the meta- 

 cestode. In the Cyclophyliidea 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 pisi- 

 formis was 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 



