MOLLUSKS 



267 



plants on the surface of the aquarium and the action of the hngual ribbon 

 noted. 



Breathing. — Most gastropods breathe; by means of gills which are lo- 

 cated, as in the clam, in the mantle cavity. But in some snails, the 

 pulmonates, the mantle cavity forms a sac which opens to the outside of the 

 body by a tiny slit. In this cavity air is taken. Its walls are filled with 

 blood vessels, and oxygen is then taken up by the blood as carbon dioxide 

 is released. The pulmonates are true air breathers, and may frequently be 

 seen in an aquarium taking down a bubble of air for use under water. 



Senses. — Snails and slugs can distinguish light from darkness, as may be 

 easily proved by experiment. The tentacles are the most sensitive parts of 

 the body. Certain parts of the animal seem to be used for testing the water, 

 and in the land snails these organs seem to be used to smell with. The 

 nervous system as in the clam consists of three collections of nerve cells, or 

 ganglia, nerves connecting these centers, and numerous cells in the outer 

 part of the body called sensory cells. These cells are 

 sensitive to any stimulus received from outside the 

 body. 



Development. — Eggs of the pond snails are laid in 

 little masses, sometimes in strings, and are often found 

 fastened in a jelly to the side of the aquarium. The 

 snails hatch and at first swim about, later settling 

 down as the shell is formed. Thus they undergo a 

 complete metamorphosis, as do their relatives the clams 

 and oysters. 



Cephalopods. — Squid, Cuttlefish, and Octopus. The 

 name cephalopod means head-footed. As the figure 

 shows, the mouth is surrounded with a circle of ten- 

 tacles. The shell is internal or lacking, the so-called 

 pen of the cuttlefish being all that remains of the shell. 

 The squid, or cuttlefish, is strangely modified for the 

 hfe it leads. It moves through the water more swdftly 

 than a fish by squirting water from the siphon. It can 

 seize its prey with the suckers on the long tentacles 

 and tear it in pieces by means of its horny parrotlike 

 beak. It is protected from its enemies and is enabled 

 to catch its prey because of its ability to change color ^^"JJJ 

 quickly. This change of color is caused by the move- 

 ment of certain color-bearing cells under the skin. In 

 this way the animal simulates its surroundings. The cuttlefish has an mk 

 bag near the siphon which contains the black sepia. A few drops of this ink 

 squirted into the water effectually hide the animal from its enemy. 



To this group of animals belongs also the octopus, or devil fish, a ceph- 

 alopod known to have tentacles over thirty feet in length. The paper 



id. One fourth 

 ral size. Da- 

 vison, Zoology. 



