410 THE BUBBLE-SHELL. 



shape. It then looses its hold of the supporting plant, and boldly launches itself on 

 the surface. 



Hundreds of these creatures may often be seen floating in successive squadrons along 

 the surface of a stream, knocking against all kinds of obstacles without fear, and 

 travelling farther in one hour than they could otherwise have achieved in a week. If, 

 however, the spectator throw a stone into the water all is changed in a moment, the 

 expanded foot is suddenly contracted as the circling ripples reach the voyagers, and the 

 mimic fleet sinks to the bottom as if by magic. 



The tentacles of this animal are rather short, broad, compressed, and triangular. The 

 eggs of the Pond-snail are laid in ribbons of transparent gelatinous substance, and the 

 process of their gradual development can easily be watched by means of the microscope. 

 Most of the Pond-snails feed on vegetable substances, more especially decaying leaves, but 

 the present species is carnivorous in its habits. 



ON the right-hand of the engraving, and exactly opposite the pond-snail, is an equally 

 common British shell, called from its flattened whorls the PLANOBBIS. In this animal, 

 the foot is short and round ; the tentacles are long, slender, and leave the edges at their 

 inner bases. Both this and the preceding species are in the habit of burying themselves 

 in the mud during a drought, and there passing a semi-torpid existence. 



BELOW the planorbis is a very remarkable species, called, from its peculiar shape, the 

 FEESH-WATEE or EIVEE LIMPET. This appears not to be a British shell, but is found in 

 various parts of America, Madeira, and some portions of Europe. It inhabits swiftly 

 running streams, and is mostly seen attached to stony and aquatic plants. Although the 

 shell is so limpet-like, the animal does not partake of the resemblance, being very like 

 that of the pond-snail, arid having triangular tentacles with the eyes at their bases. The 

 generic term, Ancylus, is of Greek origin, and signifies a small round shield or target. An 

 empty shell is represented lying on the ground near the animal. 



THE little elongated shell shown crawling over the body at the right-hand top corner 

 of the engraving is the POUCH-SHELL, a British species of a rather small genus, 

 extending over the greater part of the globe. In the greater number of species, the 

 mantle is fringed with long filamentary appendages, but in the present example the edges 

 are quite plain. It is, however, always flat and much expanded. The tentacles are long 

 and slender, and the eyes placed at their bases. The shell is thin, spiral, polished, and 

 the aperture is rounded in front. 



IN the accompanying illustration are represented some of the strange and almost 

 grotesque forms which are assumed by many of the molluscs. These belong to a fresh 

 order, in which the shell is sometimes altogether wanting, and even when present is of 

 very small dimensions, and is almost, if not wholly, concealed by the soft parts. In fact, 

 they may be considered as the marine analogues of the common land slugs. The gills of 

 these animals are rather curiously formed, not being placed in a definite cavity as is the 

 case with the previous species, but projecting boldly from the surface of the animal, and 

 set towards the rear of the body. On account of this position of the gills, the animals 

 are termed Opistho-branchias, or Eearward-gilled molluscs. The whole internal structure 

 of these creatures is fully as curious as their external form, and well repays dissection, 

 the organs of digestion especially being rather complicated, and possessing many points 

 of interest. 



IN the centre of the engraving is seen an empty and rounded shell, with the exterior 

 spotted and the interior of the lip shown. This is the BUBBLE-SHELL, one of a moderately 

 large genus of molluscs, that are found in almost all tropical and temperate seas, and may 

 generally be captured where the bed of the ocean is of a sandy nature. Another species 

 is drawn on the upper left-hand corner, for the purpose of showing the manner in which 

 the shell is enveloped in the tissues of the animal. In all the family to which the 

 Bubble-shells belong, tne shell is very thin, globular, and cylindrical, and the aperture is 

 long and rounded. Tho large side lobes are said to be often used as fins for swimming. 



