Page:Picturesque Dunedin.djvu/110

98 the tissues of their bodies the partly digested food of their hosts.

There is in the museum a good spirit specimen of the-bladder-worm of the rabbit, which, like many parasites requires two hosts in which to complete its life-history.

It occurs as a bladder about the size of an egg, and although perhaps never directly producing death, it certainly causes the rabbit great inconvenience. Attached to the inside walls of the specimen forming the exhibit are small white structures which, viewed under the microscope, are seen to be provided with suckers and a circle of hooks at their free extremity. When a rabbit having this disease is eaten by a dog, these little structures fasten on to the walls of the intestine, and develop into a tapeworm, which in time deposits its eggs. The rabbits running about eating the grass here and there, get these eggs into their system, where they develop into bladder-worms. Were it not for the dog no tape-worms would be produced, and the bladder-worms could not possibly get into the system of any rabbit not infected with them.

After worms come Crustacea, and to trace the connection between these two groups is not so difficult as it would appear. The segmentation of the body, which first occurs in the higher worms, is the most obvious point of resemblance between them, and this is the first step towards the formation of such appendages as legs found in marine-worms. Crustacea have these appendages developed into fully formed limbs.

Crustacea are very well represented by specimens of all kinds, many of the crabs so simulating the surroundings in which they live as to be easily mistaken for stones or pieces of sea-weed. The king-crab, however, is one of the strangest forms. A strong, hemispherical dorsal shield covers the head, and to it is articulated a smaller abdominal plate, with a long-caudal spine. The king-crab is an old water-breathing type, which, from the presence of a compound eye, and numerous-smaller ones, together with various other characteristics, such as breathing by gills which resemble a series of parallel plates arranged like the leaves of a book, is probably more nearly allied to scorpions and spiders than to crabs and Crustacea in general. Close to the crabs is the peculiar, caterpillar-like Peripatus, exhibiting scarcely any external signs of segmentation,