VII. Mollusca.--Another large group is the Mollusca. This phylum gets its name from the soft, unsegmented body (_mollis_ = soft). Mollusks usually have a shell, which may be of one piece, as a snail, or two pieces or _valves_, as the clam or oyster. [Illustration: The skeleton of a dog; a typical vertebrate.] VIII. The Vertebrates.--All of the animals we have studied thus far agree in having whatever skeleton or hard parts they possess on the outside of the body. Collectively, they are called _Invertebrates_. This exoskeleton differs from the main or axial skeleton of the higher animals, the latter being inside of the body. The exoskeleton is dead, being secreted by the cells lining the body, while the endoskeleton is, in part at least, alive and is capable of growth, _e.g._ a broken arm or leg bone will grow together. But a man has certain parts of the skeleton, as nails or hair, formed by the skin and in addition possesses inside bones to which the muscles are attached. Some of the bones are arranged in a flexible column in the _dorsal_ (the back) side of the body. This _vertebral column_, as it is called, is distinctive of all _vertebrates_. Within its bony protection lies the delicate central nervous system, and to this column are attached the big bones of the legs and arms. The vertebrate animals deserve more of our attention than other forms of life because man himself is a vertebrate. [Illustration: The sand shark, an elasmobranch. Note the slits leading from the gills. (From a photograph loaned by the American Museum of Natural History.)] Five groups or classes of vertebrates exist. _Fishes_, _Amphibians_, _Reptiles_, _Birds_, and _Mammals_. Let us see how to distinguish one class from another. [Illustration: The sturgeon, a ganoid fish.] Fishes.--Fishes are familiar animals to most of us. We know that they live in the water, have a backbone, and that they have fins. They breathe by means of gills, delicate organs fitted for taking oxygen out of the water. The heart has two chambers, an auricle and a ventricle. They have a skin in which are glands secreting mucus, a slimy substance which helps them go through the water easily. They usually lay very many eggs. CLASSIFICATION OF FISHES ORDER I. _The Elasmobranchs._ Fishes which have a soft skeleton made of cartilage and exposed gill slits. Examples: sharks, skates, and rays. ORDER II. _The Ganoids._ Fishes which once were very numerous on the earth, but which are now almost extinct. They are protected by platelike scales. Examples: gars, sturgeon, and bowfin. [Illustration: A bony fish.] ORDER III. _The Teleosts, or Bony Fishes._ They compose 95 per cent of all living fishes. In this group the skeleton is bony, the gills are protected by an operculum, and the eggs are numerous. Most of our common food fishes belong to this class.