44 ANIMAL LIFE PAST AND PRESENT. 



represented by the common Acorn-Barnacle covering 

 the rocks on our coasts, and the Stalked- Barnacle which 

 is more commonly found on the bottoms of ships. In 

 their young state these curious creatures are free- 

 swimming Crustaceans, but after a time, becoming 

 tired of a roving life, fix themselves on their backs by 

 the front of their heads to some solid object, and then 

 develop their well-known shells; the feather-like fan 

 which protrudes from the aperture of these shells being 

 the greatly modified legs, now acting as feelers for the 

 purpose of capturing food. What induced the strange 

 belief that the Stalked-Barnacles underwent a further 

 metamorphosis to appear as Barnacle- Geese, passes 

 ordinary comprehension. 



Passing on to the Arachnids (Spiders and Scorpions) 

 and Insects, we find that these creatures, whether 

 aquatic or terrestrial, breathe atmospheric air by means 

 of a system of tubes known as tracheae, and we are 

 accordingly led to conclude that such of them as are 

 adapted to an aquatic life have acquired this habit. 

 This is especially well shown by the instance of the 

 Water-Spider, which, while agreeing in structure with 

 other spiders, has the limbs fringed and somewhat 

 flattened for swimming, and is in the habit, when 

 diving in the water, of carrying down with it a bubble 

 of air clinging to the hairs of the abdomen. 



Among the Insects, the larve of many groups in 

 which the perfect animals inhabit the air, such as the 

 Dragon-Flies, May-Flies, and Gnats, are aquatic. 

 Whereas, however, the larvre of the two former groups 



