The Sommeling forme pige resents the bever soron of

be van muut time of me metatarsal bones, correaponding to the fire ling se, rembined: their els where the tea jeh n upearing separate, while the metatural bone of the hom the in the side remains separate from the other

At the arRejoint, or trets, dere are no separate tarsal ivoner to the seen, nor wind their existence be known, except theoretkang, without an examination of the embryo.

The folowing figure represents the appearance of the leg

No ** Two or An Pussro BIRD GREATLY ENLARGED-Only the lower portiou of the tibia and fibula is drawn.

of an embryo bird greatly enlarged, showing the bones in place.

At this early stage, not only the metatarsal bones are seen separate, but the bones of the tarsus, four in number, are well marked and distinct.'

The many strange modifications in the shape and proportions of the legs of vertebrate animals are accompanied by similar changes in the arrangements, number, and proportion of the bones of these parts.

Thus, in the short leg of the salamander (see Fig. 152) the bones of the tarsus occupy a space about as long as that of the metatarsus. In birds, on the contrary, the tarsus is very short, while the metatarsus is very long.

159. If the pupils will observe the hind-leg of the toad or frog, they will see that not only is it much longer than the fore-leg, but that an extra joint seems to be added in the foot. An examination of the bones shows that the first two tarsal bones are very long, while the other tarsal bones are very short; so that in this case the tarsus combined is much longer than the metatarsus. The following figure represents the right hind-leg of a young toad while still in the tadpole state, though the adult shows the same features.

The above brief lessons on the vertebrates do not even embrace an outline of the structure and habits of any of the classes, and they are given only as suggestions toward a method of study, and to point out the essential paths to fol

1 The author has seen the tarsal bone, marked i, in the heron, tern, and seapigeon.

low, in order to gain an insight into the affinities existing between the various groups composing this great branch of

animals.

fibula.

tibia.

tarsus.

metatarsus.

phalanges.

FIG. 158.-BONES OF THE RIGHT LEG OF A YOUNG TOAD GREATLY ENLARGED.-The femur is not shown in this drawing; the tibia and fibula are combined together.

There are still other large classes of animals forming branches or sub-kingdoms quite distinct from those already studied, and represented by animals which live in the sea, such as the star-fish, sea-urchin, jelly-fish, and sea-anemone, of which no mention will be made in this book. There are also many classes, belonging to branches already touched upon, which have not been alluded to. These will be fully dealt with in the second book, now in preparation.

CHAPTER XXIV.

CLASSES AND SUB-KINGDOMS.

THE following figures illustrate the classes and sub-kingdoms which have been dealt with in this book, with their technical names :

SUB-KINGDOM MOLLUSCA (Clams, Snails, Squids, etc.).

Class Gasteropoda (snails, periwinkles, limpets, etc.).— Animals whose bodies are generally inclosed in a mantle or sac, which usually secretes a shell composed of one piece, and this often assuming a spiral shape. The body rests upon a broad, creeping disk called the foot, and this part represents the ventral portion or belly of the animal. The name Gasteropoda is derived from two Greek words, gaster, the belly, and pous, foot. The following figures represent a few animals of this class:

GASTEROPODS.

Class Acephala (clams, oysters, mussels, etc.).—Animals whose bodies are protected by a mantle which secretes a bivalve shell, or a shell composed of two pieces. They have no well-defined head, and hence the name Acephala, derived from two Greek words, a, without, and cephale, head. These animals are also called Lamellibranchiates, because the gills form leaf-like membranes or plates on the sides of the body; the word being derived from a Latin and a Greek word, lamella, a plate, and branchia, gill. The following figures represent a few animals of this class: