Turtle. But there are Turtles with a soft covering, and there are some Toads with a hard shield over the head and neck at least, and both groups are alike distinguished by the shortness and breadth of the body and by the greater development of the limbs as compared with the lower Reptiles. But here again there is the same essential difference in the mode of development of their young as distinguishes all the rest. The two series may thus be contrasted:-- _Naked Reptiles_. Toads and Frogs, Salamanders, Caecilians. _Scaly Reptiles._ Turtles, Lizards, Serpents. Such corresponding groups or parallel types, united only by external resemblance, and distinguished from each other by essential elements of structure, exist among all animals, though they are less striking among Birds on account of the uniformity of that class. Yet even there we may trace such analogies,--as between the Palmate or Aquatic Birds, for instance, and the Birds of Prey, or between the Frigate Bird and the Kites. Among Fishes such analogies are very common, often suggesting a comparison even with land animals, though on account of the scales and spines of the former the likeness may not be easily traced. But the common names used by the fishermen often indicate these resemblances, --as, for instance, Sea-Vulture, Sea-Eagle, Cat-Fish, Flying-Fish, Sea-Porcupine, Sea-Cow, Sea-Horse, and the like. In the branch of Mollusks, also, the same superficial analogies are found. In the lowest class of this division of the Animal Kingdom there is a group so similar to the Polyps, that, until recently, they have been associated with them,--the Bryozoa. They are very small animals, allied to the Clams by the plan of their structure, but they have a resemblance to the Polyps on account of a radiating wreath of feelers around the upper part of their body: yet, when examined closely, this wreath is found to be incomplete; it does not, form a circle, but leaves an open space between the two ends, where they approach each other, so that it has a horseshoe outline, and partakes of the bilateral symmetry characteristic of its type and on which its own structure is based. These series have not yet been very carefully traced, and young naturalists should turn their attention to them, and be prepared to draw the nicest distinction between analogies and true affinities among animals. VIII. After this digression, let us proceed to a careful examination of the natural groups of animals called Families by naturalists,--a subject already briefly alluded to in a previous chapter. Families are natural assemblages of animals of less extent than Orders, but, like Orders, Classes, and Branches, founded upon certain categories of structure, which are as distinct for this kind of group as for all the other divisions in the classification of the Animal Kingdom.