Adaptations shown by Animal Parasites.

The Tapeworm and Flea

Both ectoparasites and endoparasites show adaptations which suit them to their mode of life, but the entry of one animal into the body of another causes a greater modification of structure than the mere existence on the surface of the host. Such adaptive features are well shown in the tapeworm, Taenia. This parasite lives in the intestine of the dog, cat and many other mammals, and two species, Taenia solium, the pork tapeworm, and Taenia saginata, the beef tapeworm, are sometimes found in man. Its position inside the body of its host gives the tapeworm protection from its enemies, and frees it from the necessity of finding and catching food. Consequently the power of movement is not required, and the parasite has no organs of locomotion, although the segments are able to perform wriggling movements. Since endo-parasites receive but few and simple sense impressions from the outside, the nervous system has become simplified and there are no sense organs. The nervous system consists of two parallel nerve cords which run through each segment and join a ring of nervous tissue in the head.

The animal possesses no mouth or alimentary canal, but is surrounded by the semi-digested food of its host, which is absorbed over the whole surface of the body. The great length and flatness of the tapeworm provide a sufficient absorptive area. It might be expected that the parasite would be digested by the gastric juices of the host, but this is prevented owing to the secretion by the parasite of an anti-ferment which neutralizes the effect of the digestive juices of the host. Since the food is continually being moved by the constriction of the walls of the alimentary canal, the tapeworm must have some means of preventing itself from being swept along with the food. On the head are four suckers which anchor the animal to the wall of the gut of the host. Taenia solium possesses in addition a ring of hooks for the same purpose.

The Reproduction of the Tapeworm

The main difficulty that a parasite has to face is that of introducing its young into the right host, for the eggs and lame of the parasite are defenceless, and on passing out of the alimentary canal of the host are destroyed in great numbers by animals and weather conditions. In order to compensate for this loss, most parasites produce an enormous number of eggs, Taenia solium itself producing as many as 80 million per year. Although all other organs may be reduced in parasites, the reproductive organs are always present and are often complicated and of large size.

Since Taenia has lost all locomotor organs, fertilization of the eggs is made more difficult, but the possession of male and female organs in each segment lessens this difficulty. The end segments are simply bags of eggs, which break off and pass out of the body of the host with the faxes. No further development of the tapeworm can take place unless the eggs are swallowed by a pig. The digestive juices of the pig ’s stomach dissolve the wall of the egg and set free the six-hooked embryo which has developed in the egg. Inside the pig ’s body this embryo works its way through the alimentary canal and into the blood vessels by means of the six hooks. It is carried to the muscles in the blood stream, where it loses the hooks and develops into a bladder worm or cysticercus. This consists of a bladder in which a tapeworm head is tucked inside. The cysticercus stage cannot develop further unless the flesh of the pig is eaten by man. If this happens, the head of the cysticercus becomes pushed out, attaches itself to the wall of the alimentary canal and commences to bud off proglottides.

Thus in the tapeworm the life cycle takes place in two different hosts, in one of which the sexual stage develops and in the other the immature stages occur. This sucoession of different stages is part of the general adaptation to ensure passage from host to host.

Adaptations shown by the Flea

In an ectoparasite such as the Ilea, specialization has not taken place to the same extent as in the tapeworm, but nevertheless there are certain features which adapt it to the parasitic mode of life. The flea feeds on the blood of its host and possesses mouth-parts modified for biting and sucking, and, unlike Taenia, it has a well-developed alimentary canal. One of the most important adaptations shown by the flea is the shape of the body, which is flattened from side to side. This is the most suitable shape for a parasite which moves between the hairs of its host. The legs of the flea have been modified for jumping, but it cannot fly, for at no time during its life history does it possess wings.

One species of flea is found on man, and the cat and the dog both have their particular kind, but specialization has not proceeded so far that one kind of flea is invariably found on its usual host, since cat and dog fleas, for example, are able to live on man.

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