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, anatomicalljr, the Multivalves differ 
greatly from the two former, and are placed amongst the articulated classes in scientific arrange¬ 
ments. 
