4 io THE BUBBLE-SHELL. 



shallow boat-like 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 PLANORBIS. 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 them- 

 selves 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 FRESH-WATER or RIVER 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, and 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, extend- 

 ing 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-branchiae, or Rearward-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 pos- 

 sessing 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, the shell is very thin, globular, and cylindrical, and the aperture is 

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



