CHAMBERS'S INFORMATION FOR THE PEOPLE. 



that, when the animal wants to separate the 

 valves, it has only to relax the muscles, and the 

 shells fly apart, in virtue of the elasticity of the 

 elastic ligaments and the cartilage. In the Brachi- 

 opoda, the valves are both dosed and opened 

 by means of muscular action. The heart is always 

 well developed, and consists of at least two cham- 

 bers, and the blood is aerated in the plate-like 

 ciliated gills. The nervous system consists of 

 three well-defined ganglia. In many of the Lam- 

 dlibranchiata, there is present a muscular organ 

 called the foot, which is used in progression. 

 This class comprises two sections, namely : 



Section (A). Asiphonida in which no respir- 

 ator)' siphon is present, the pallial line is not 

 indented, and the lobes of the mantle are free, 

 and not united. This section includes the Oyster 

 family (Ostrcadce\ represented by the Ostrea 

 edulis, the Common Edible Oyster, which is 

 deficient in a foot, and is fixed to the sea-bottom 

 by the shell alone. It has its headquarters in 

 Kritnin, and is considered full-grown for the mar- 

 ket in from four to seven years. The Common 

 Pecten, the type of another family, is so called 

 from the resemblance of its shell to a comb, has 

 a well-developed foot, and a number of brightly 

 coloured spots placed along the edge of the 

 mantle, which are supposed to represent eyes. 



The Afytilidce, represented by the Common 

 Mussel (Afytilits edit I is), so abundant on our 

 shores, has a quantity of hair-like filaments 

 (fyssus) developed in connection with the foot, 

 by which it attaches itself to solid objects. Of 

 the family Unionida, the Unio, or Fresh-water 

 Mussel, is noted for producing small pearls. They 

 all inhabit fresh water, and the shell is covered 

 by a very thick epidermis. The Unio pictorum 

 is so called from its shell having been used by 

 painters to hold colours. 



Section (B). The Siphonida possess a siphon, 

 and the mantle lobes are more or less united. In 

 some, the siphons are short, and the pallial line 

 simple, as in the Cockle family (Cardzadtz), in 

 which the foot is well developed, and fitted for 

 burrowing in the sand of the sea-shore. The 

 Common Cockle (Cardini edttle), which is eaten 

 in some localities, can spring to a considerable 

 height by means of its bent foot. In others, the 

 siphons are long, and the pallial line is indented, 

 as in the Myas, which burrow and form a habitat 

 for themselves. The Solen, or Razor-shell, sinks 

 in the sand with great rapidity. The Pholas has 

 a shell composed of arragonite, and can perforate 

 timber and solid rocks, thus producing great 

 destruction by attacking ships and wooden piles 

 in water. Another celebrated species is the 

 Teredo navalis, or ship-worm, a worm-like 

 animal, furnished with two small shells at its 

 anterior extremity, often attaining the length of 

 one or two feet, and doing immense damage by 

 boring into timber. 



CLASS V. GASTEROPODA. 



Animals with the head well developed, and 

 never inclosed in a bivalve shell. Locomotion 

 effected by a horizontally flattened ventral disc 

 ('foot'), or by a vertically flattened ventral fin- 

 like organ. This class includes the univalved 

 shells, such as the Snail and Whelk. In the 

 mouth, there is a peculiar strap-shaped mastica- 



152 



tory apparatus called an odontophore. The mantle 

 is continuous round the body. The heart consists 

 of an auricle and a ventricle. In one group, 

 respiration is effected by an apparatus for breath- 

 ing the air in water (Branchifera) ; in the other, 

 the respiration is aerial (Pulmonifeni). The 

 sexes are mostly distinct. The young always 

 possess an embryonic shell. The shell is com- 

 posed either of a single piece (univalve), or of 

 many pieces (multivalve). This class comprises : 



Section (A). Branchifera in which the respira- 

 tion is aquatic. This section includes three 

 orders, characterised according to the position of 

 the branchiae. 



ORDER i. The Prosobranchiata have the mantle 

 so arranged as to form a vaulted chamber over the 

 back of the head, in which the respiratory organs, 

 or branchise, are usually lodged. These gills are 

 situated in front of (proson) the heart. This order 

 is divided into two sections : 



I. The Siphonostomata, in which the aperture 

 of the shell is notched or produced into a canal. 

 They are all marine, and carnivorous in their 

 habits for example, the Whelk family (Buccinidce), 

 represented by the Common Whelk (Buccinum un- 

 datum), in which the shell is of a spiral form. The 

 VoZuttdtz contain many beautifully marked shells, 

 and are chiefly confined to warm latitudes. The 

 Cowry family (Cyprceidce), so remarkable for their 

 beauty, are largely used as mantel-piece orna- 

 ments. In certain parts of Africa, the shell of 

 Cyprtza moneta, or Money Cowry, is used by the 

 natives as money. The Muricidce (murex, purple) 



Murex tenuispina. 



have a univalve spiral shell, with a small oval 

 aperture ending in a straight ascending canal. 

 The StrombidcB, or Wing-shells, have the aperture 

 of the shell much dilated, and the lips expanding 

 and extended into a groove leaning to the left. 



2. The Holostomata have the aperture of the 

 shell entire. They are chiefly plant-eaters, and 

 are either marine or inhabitants of fresh water. 

 It includes a great many families for example, 

 the Pate Hides, or Limpet family. In the Common 

 Limpet (Patella vulgaris), locomotion is at a low 

 ebb, for they seldom move far from the place 

 where they are produced. The Haliotidce (Ear- 

 shells) are used in making those mother-of-pearl 

 ornaments which constitute so much of the beauty 

 of works in papier-mache'. The Marine Snail 

 family (Turbinidce) have the shell of a regular 

 turbinated form. They are found in great abund- 

 ance in the Indian seas, and are used as food, 

 many being of large size. The Common Peri- 

 winkle (Littorina littored) is nearly allied to this 

 family. In the Dentalidce, the shell is tubular, 

 slightly curved, open at both ends, and shaped 

 like an elephant's tusk, hence their name of 

 Tooth-shells. The Chiton family (Chitonidce) have 

 multivalve shells, the mantle being covered by 



