Objects for the Microscope. 151 



(Nepa), and worthy of attention in their metamorphosis, 

 also when mounted thus as objects for the microscope. 



There are several species; some bright scarlet (Hydrachna), 

 others dotted with black, having blue legs (Atax), some 

 parti-coloured black and scarlet (Diplodontus), one, bright 

 green (Arrenus) ; all of them to be found in rivers and ponds 

 merrily swimming about, and laying millions of small red 

 eggs on leaf and stem of water plants. They seize on small 

 crustaceans, such as the Cyclops andDaphnia, and suck them. 



The metamorphosis is as follows : The eggs are laid in 

 great abundance throughout May and June, six weeks after 

 a curious larva comes out, having a long blue snout and 

 two large round eyes. We do not know how long it 

 remains free in the water, but towards the end of the 

 summer we find it change into a scarlet oblong pupa, 

 fixed by a strong hook to the tail of the Water-scorpion, or 

 under the elytra of Dytiscus. These pupa3 were once mis- 

 taken for eggs ; but the French naturalist Duges watched 

 them well, and saw every stage of the metamorphosis. 

 From the pupa emerges a six-legged mite, which moults 

 and becomes perfect, with eight ciliated legs for swimming. 



The claws and palpi should be particularly noticed, and 

 the epidermis of the green mite Arrenus, mounted for its 

 dotted appearance. 



ENTOZOA. 



These are parasites attached to the internal parts of the 

 animal body, and consist of intestinal worms, some 

 extremely minute, burrowing in the skin, others of larger 

 size infesting the viscera. No part of the human body is 

 free from their attacks, the liver, the kidneys, the intestines, 

 and the brain, are their food and abiding-place. There is 

 scarcely one animal, especially of the vertebrate classes, 

 which is not infested by several species. The human body 

 has eighteen internal parasites, and those which inhabit 

 one animal are rarely found in one of another genus. 

 Those who desire further knowledge of these parasites, had 

 better read l Owen's Hunterian Lectures/ vols. iv., v., vi. 



