406 ON THE STRUCTURE AND HABITS 



hitherto considered, possessing a mouth, stomach, heart, and other organs in a 

 high state of development. We have here two kinds of animals ; the one may be 

 represented by the Pyrosome, which is an aggregated floating animal, whilst the 

 other will be represented by the Cynthia, a single animal, and fixed for life to the 

 same spot. The Pyrosome has a large tubular base, upon which are fixed 

 hundreds of little animals. This tubular base varies in size from three to 

 twelve inches, and it is by the passage of the sea-water through its hollow 

 interior, as it passes to supply the animals with air and food, that it is kept in 

 constant motion. This animal has also the property of phosphorescence, and is 

 one of the most brilliant of the numerous tribes of lower animals that gild the 

 ocean with their sub-marine fires. The single form of these animals is not 

 uncommon on our own coasts, where they are known from the singular fact that 

 when caught by fishermen they squirt from their bodies a large quantity of water 

 in the face and eyes of their luckless captor, so as often to prevent him securing 

 his prey. These being single, and having a thick fibrous covering over them, 

 lead us by an easy transition to the next group of animals, which are the Shell- 

 fish ; animals in general living in shells and called Molluscous. 



These animals are divided into three kinds ; those with a single shell called 

 Univalve, those with two shells called Bivalve, and those with more than two 

 shells Multivalve.* These may be respectively represented by a Snail, an 

 Oyster, and a Barnacle. To begin, then, with the Univalves, of which the Snail 

 is the type. All the classes previously examined are inhabitants of the water, 

 but a small proportion of those we have entered on live upon the land. The 

 most common of these are the various kinds of Snails {Helix), but there are 

 several kinds which assume a variety of interesting forms. 



Many of them inhabit the ditches and fresh- water ponds in the interior of the 

 country ; they are very numerous in many districts, and as their forms are easily 

 studied, and their collection is not difficult, they form a good subject for the 

 initiatory studies of a young naturalist. 



The most attractive and curious part of the Mollusca is certainly their shell, 

 and in this organ we have a beautiful exemplification of the way in which 

 creation gradually passes from a simple to a more complicated form. Now 

 belonging to this class is the Common Slug, which, though often looked upon as 

 an object of disgust, is endowed with a structure, functions, and instincts as 

 elevated as the princely inhabitants of those gorgeous shells that so often orna- 

 ment the halls and rooms of our dwellings. The Common Slug has no shell, but 



* I have used this division as the most convenient, although, anatomically, the Multivalves differ 

 greatly from the two former, and are placed amongst the articulated classes in scientific arrange- 

 ments. 



