242 



THE POPULAR EDUCATOB. 



sides of the intestines. It has a proboscis, and around this are 

 two rows of hooks, which point backward, so that when the 

 animal plunges its proboscis into the soft mucous coat it cannot 

 be pulled away, and hence holds its position, notwithstanding 

 the continued transmission of food and the constant motion of 

 the alimentary canal by which it passes forward its contents. 

 Besides these, four suckers are situated below the booklets on 

 the rounded head. An attenuated neck gradually enlarges as it 

 proceeds downwards, and, at a little distance from the head, 

 closely set and fine constrictions are observed, which become 

 larger and more especially longer as we trace them downwards 

 towards the tail-end. These constrictions become also more 

 and more definite and deeper, dividing the animal into segments, 

 which, being longer than wide and very flat, look like a series of 

 oblong cards. 



In each of these segments a complete set of reproductive 

 organs, both male and female, are found, and each in course of 

 time produces an immense brood of eggs. When this condition 

 has been arrived at, the segment drops off and crawls about, 

 making its escape from its host, and finally bursting from the 

 increased growth of the contained eggs, it scatters an immense 

 multitude of germs. At a first consideration, one would have 

 thought that these germs, cast forth into the world, and dependent 

 for development upon their admission to other hosts, would have 

 but little chance of complete life ; and this is no doubt true with 

 regard to each single germ. In this case, however, Nature makes 

 up by multitude for efficiency. If we consider that each tape- 

 worm has many hundred segments or joints when found in the 

 interior of man, and that these joints are continually renewed 

 from above as they fall away, and that each segment contains 

 thousands of ova which are cast into various situations into 

 garbage, water, etc. it is not wonderful that some are taken 

 into the interior of some suitable animal. 



These animals, being constantly provided with digested food 

 which has been elaborated by their hosts, do not need any food 

 canal of their own to digest aliment, but absorb it when already 

 dissolved through the walls of the body. In fact, their stomachs, 

 so to speak, are external, and correspond to the skin of other 

 animals. Running along each side of the animal is a narrow 

 duct. Cross-branches unite the two ducts, one to each segment, 

 and run across at the part farthest from the head. These two 

 lateral canals were long considered as the alimentary system of 

 the animal ; but it was found that it had no opening forward 

 that is, it was without a mouth and although there is an open- 

 ing at the other end of the body, yet it is now considered to be 

 the atrial system corresponding to the water-vascular or ambula- 

 oral system which we have described in the Echinodermata. 



The egg with its contained embryo being swallowed by some 

 animal, the latter does not remain in the food canal and become 

 developed into a tapeworm, as might have been supposed, but 

 immediately that the coatings of the egg are dissolved away, the 

 embryo, which is armed with six boring-hooks, makes its way 

 through the walls of the alimentary canal, and traverses the 

 body in any direction until it reaches some structure suited to it, 

 and there it rests and becomes more fully developed. The deve- 

 lopment is commenced by the formation of a bladder which is 

 proper to the animal, while the soft organ in which the parasite 

 is lodged forms a self-defensive cyst around this of common 

 (areolar) tissue. Thus the creature is snugly ensconced in a 

 cavity, through the walls of which the liquids penetrate, and are 

 absorbed by the bladder-like animal. By the aid of this 

 nutriment fresh changes occur with the growth of the larva. 

 Thus on one side of the interior of the bladder a round body 

 grows and so projects into the cavity, and in this the head and 

 neck of the future perfect worm are formed. On this head 

 the circles of hooks and the suckers are developed, so that the 

 examination of the larval form when at an advanced stage will 

 enable the examiner to determine to which species the creature 

 belongs. When this process is completed, the larva has reached 

 a stage beyond which it cannot become more developed unless it 

 changes its position, and this change of position is not an active 

 but a passive one. Hence multitudes of these creatures probably 

 die and become disintegrated without ever attaining the perfect 

 form. Those, however, whose life-circuit becomes complete, are 

 transferred to the stomach of a carnivorous animal by the flesh 

 in which they are lodged being devoured. Thus the animal has 

 two different hosts, one of which entertains it in the immature 

 condition, and the other when it becomes perfect and sexually 



capable of reproducing its species. Most of these cystoid 

 animals, when in the cystoid or bladder-like state, inhabit the 

 soft structure of herbivorous or grain-feeding animals, while 

 when they arrive at the cestoid or tape- worm condition they are 

 found in the carnivorous animals which feed upon their former 

 hosts. It has been shown that the Cysticercus fasciolaris of the 

 liver of a mouse becomes the Tcenia crassicollis (the thick-necked 

 tapeworm) of the intestines of the cat, and the Cysticercus pisi' 

 formis (the pea-shaped bladder-tail) of the rabbit becomes the 

 Tcenia serrata (notched tapeworm) of the dog. In the case of the 

 species we have been describing, the host of the larva is usually 

 the pig, and the host of the adult worm is man. As might be 

 expected, it is found that the Tcenia solium infects those most 

 who are especially fond of ill-cooked sausages. In Germany 

 this unfortunate taste for nearly raw pork has produced the 

 most harmful results, not only by introducing this worm, but 

 also another called Trichina spiralis, a worm of much higher 

 organism, and belonging to an order to be referred to hereafter. 

 When the flesh containing the encysted entozoa is being digested 

 by the animal who has been unfortunate enough to swallow it, 

 the digesting operation goes on not only so far as to liberate the 

 creature, but also to dissolve away the bladder which encloses 

 the head. Then the creature, like the liberated genius in the 

 "Arabian Nights," begins to take revenge on its liberator for its 

 long imprisonment. It fixes itself by its hooks and its suckers 

 to the walls of the intestines, and its tail grows and becomes 

 segmented as before described. As compared to the immense 

 length and size of the chain of segments, the head is ridiculously 

 small; and thus the simile of the genius, who, when liberated 

 from his bottle, assumed such vast and formidable dimensions, 

 is not inappropriate to the rapid development which follows the 

 liberation of this worm from its cyst. 



The effect upon the human system occasioned by a tapeworm 

 is extremely distressing. The patient suffers not only from loss 

 of appetite, emaciation, and lassitude, but the sympathetic 

 nervous system is affected so as to produce convulsions and 

 epilepsy. Distressing, however, as these effects are, they are 

 not so fatal as are those produced by the presence of the im- 

 mature form, because the adult worm is confined to the intestines, 

 and is thus, so to speak, in a situation external to the body, while 

 the larvae, as we have seen, penetrate into all parts of the body, 

 and their presence is more or less injurious as they take up their 

 abode in the more or less vital organs. If they find their way to 

 a position under the skin or in the muscles, they are compara- 

 tively harmless ; but if they penetrate the eye or the brain, 

 they occasion pain and sometimes death. 



In tracing the circle of life of the Tcenia, we find it runs 

 through all the forms named, in the following order : 



1. The egg. 



2. The embryo, actively travelling by a six-hooked boring 

 apparatus. 



3. The resting larva, consisting of a head enveloped in a 

 terminal bladder. 



4. Immature tapeworm liberated from its bladder. 



5. Segmented and sexually mature tapeworm. 



6. Free segment, called a proglottis, from its likeness to the 

 tip of the tongue. 



This creature belongs to the sub-class Anenterelmintha, which 

 is distinguished from the sub-classes to which the other animals 

 of the class belong, by having no alimentary canal of any kind. 

 This animal, in common with all belonging to this sub-class, 

 is entirely nourished by absorption, and for this reason we have 

 taken the tapeworm as the type of an entozoon. 



The animals of the sub-class Sterelmintha differ from these in 

 having an alimentary canal channelled out in the substance of 

 an otherwise solid body. Our best English writer on the Entozoa, 

 Cobbold (whose books should be read by those who wish for a 

 more intimate acquaintance with the class), takes the Distoma con- 

 junctum, which he found in the intestines of the American red 

 fox, as a type of the sub-class. The animal belonging to this 

 sub-class, with which we are unfortunately best acquainted, is 

 the liver-fluke, which occasions the disease called the rot in 

 sheep. This creature is found abundantly in the liver of sheep so 

 affected. Sometimes as many as a thousand have been found in 

 a single liver. The animal is of considerable size, measuring 

 from I to 1 inch in length, and about | inch in breadth. It is flat, 

 and shaped like a little sole. Its anterior extremity is extended 

 into a nipple-shaped projection, at the end of which is the 





