256 



NATURAL HISfORY. 



grow at the rate of seventy-two millimetres a day, and that thirteen metameres may be produced in 

 the same time. 



The common Pork Tape Worm,* common in the intestines of man, looks like a long, soft, white, 

 jointed thing, which, when alive, elongates and contracts readily. The scolex, or head, is armed with 

 .hooks and suckers, and the metameres present water channels, one on each side, and joined above and 

 -below, besides egg glands and a uterus centrally placed and branching, and also the male elements. The 



embryo scolex forms the pork measle, and, being eaten by man, 

 turns into the common Tape Worm of the intestines. 



A Cysticercus lives in. the mouse, and it produces Tcenia 

 crassicolla in the cat. The cause of the death of many sheep is a 

 hydatid in the brain, called Ccenurus cerebralis, and this, when 

 eaten, by the dog, produces Tcenia ccenurus in its intestines. 



The genus Bothriocephalus contains foreign and Irish Tape 

 Worms. Its segments or metameres do not separate individually 

 so as to become independent organisms. It is a broad worm, at- 

 taining twenty-five to seventy feet in length and aai inch in breadth, 

 and there may be 4,000 joints. In the genus Tetrarhynchus four 

 proboscis-like tentacles exist, thickly set with booklets, retracted 

 near the suckers. The shorter Tape Worms (genus 

 Caryophyllus) do not have the metameres separable, 

 and the head, or scolex, produces one only, which carries 

 the reproductive organs. 



ORDER TREMATODA. 



These worms, many of which are called Flukes, 

 are flat, rarely cylindrical, often bladder-like, broad, 

 elongated creatures ; they are not jointed, and are fre- 

 quently leaf-shaped, and they have no vent. 



The Trematoda are parasitic within or outside 

 animals, and whilst some grow from large eggs, laid 

 ;about the localities frequented by the parent, into the shape of the adult, others present the phe- 

 nomena of alternation of generation, complicated by curious metamorphoses. These last kinds come 

 from very small eggs which have got into water or damp places, and are at first very minute, con- 

 tractile embryos, sometimes ciliated, and which endeavour to settle on some animal or other, ordi- 

 narily some of the Mollusca. This stage is that of the ciliated embryo. The ciliated embryo's office 

 is to get on to a host ; it then loses its cilia and becomes stationary on its host, and then gives 

 exit to a cylindrical sac-like object, which has two lateral prolongations close to a tapering tail. At 

 this stage of growth the parasite is called the Redia, and it has a mouth and a simple intestine, but 

 no other organs. Within this bag-like Redia a process of budding goes on, each bud becoming a 

 creature like the parent of the ciliated embryo in shape; but it is destitute of reproductive 

 organs, and is furnished with a long flat tail like a Tadpole, by which it is propelled after the escape 

 from the Redia. At this stage they are called Cercarise. They burst forth, and, after a free-swim- 

 ming existence, penetrate the body of some animal. They drop their tails and become encysted in 

 the tissues. Finally, they assume the adult form and develop reproductive organs within, out of 

 which pass the eggs. The Redia acts as the " nurse." and the Trematode may pass through life by 

 inhabiting two very different animals, after coming forth from that inhabited by the parent. The 

 stages vary in different genera, and, as a rule, the first are passed in invertebrate and the last in 

 vertebrate animals. 



The first sub-order of the Trematoda is that of the Distoma, with not more than twe suckers 

 -without hooks, and their Redise and Cercarise live principally in Mollusca. 



Distoma hepaticum and D. lanceolatum are species which have been found in the human liver. The 

 nrst-named species also bears the generic title Fasciola, and is very common in the Ruminantia, and it 



* Tcenia solium. 



TAPE WORM. 



Piece of Tape Worm with small head (1 and metameres 

 gradually increasing in size; B. a, he*d with suckers and 

 hooks; c, metameres with ovarian apparatus. 



