198 LOWER INVERTEBRATES. 



Considerable difference of opinion exists in regard to the systematic position of the 

 dicyemids, some contending that they form an independent group, intermediate 

 between the Protozoa and the higher animals, others, that they represent a degenerate 

 branch of some division of the worms, perhaps of trematods. 



Sub-Class III. — Cestoda. 



The tape-worms or cestods are among the most dreaded enemies of mankind, and 

 they inflict terrible destruction upon nearly all vertebrates, especially upon carnivorous 

 species. A tape-worm is an elongated animal, usually so broad and thin as to suggest a 

 ribbon or tape ; one end is thickened and represents the head, having an imperfect or 

 rudimentary brain, but lacking entirely organs of special sense, although possessing 

 some specialized organs, hooks and suckers by which the worm anchors itself in the 

 tissues of its unfortunate host. The long body is either a continuous band or else is 

 divided up into parts called proglottids, forming a longer or shorter chain as the case 

 may be. They have no trace of any digestive canal, but on the contrary have lost 

 even that by the degeneration consequent upon their exclusively parasitic life ; appar- 

 ently their nutrition is effected by the absoi'ption of the juices of the host through the 

 skin of the parasites. 



It is not difiicult to trace out a series of genera by which one can pass gradually 

 from the trematode type to the extreme of the cestode type, for there are certain 

 flukes which approximate to the simpler tape-worms. In the genus Caryophyllmus 

 we have in fact a tajie-worni, which might well be described as a fluke that had lost 

 its digestive tract by excessive degeneration. In Ligula the body is partially divided 

 up, while in Tcenia it is completely jointed, each joint or proglottis having its full set 

 of reproductive organs. Tcenia is in fact the extreme product of the changes which 

 have occurred in the parasitic Plathelminths, and as it is also one of the best known 

 as well as most feared of the whole class, we will consider their life history first. At 

 least seven species of Tcenia attack man ; but of these only three are frequent, namely, 

 T. solium, T. mediocannellata, and T. echinococcus. They all inhabit the intestines, 

 where they burrow their heads into the walls. The two species named first grow to a 

 very large size, solium reaching sometimes a length of three yards, mediocannellata a 

 length of four yards, while echinococcus does not exceed four millimeters, scarce one 

 one-hundredth of the length of m.ecUocannellata. 



The Tcenia solium has a small head, about the size of that of a pin, within a long, 

 thin neck, which gradually widens until the body is some six or seven millimeters 

 broad. The whole body behind the head is divided up into proglottids or joints, 

 which are so fine in the region of the neck as to be undistingnishable by the naked 

 eye, but a couple of inches further back the narrow joints are plainly visible ; the 

 further down, the longer and larger the joints become. In each joint a set of eggs is 

 formed, and in the last proglottid the eggs are mature ; the last joint falls off and is 

 discharged with the egesta ; the new last joint, previously the penultimate, ripens and 

 falls off in its turn ; now the new joints are formed in front just behind the head, and 

 each new joint as it shoves the others back acquires its place in the series, to be in its 

 turn shoved back by fresh joints ; thus the process continues, how long is not exactly 

 known, but at least until hundreds of joints, each crowded with eggs, have been separ 

 rated from the parent band. The head is remarkable for its armature ; upon the crown 

 is a circle of some six and twenty hooks, and four suckers arm the sides of the head. 



