6. _Cirrhopoda, or Curled-footed Mollusca._ These are generally attached, and enclosed in a shell of several pieces; they are furnished with a mouth, armed with jaws, and with several pairs of jointed and fringed organs, called cirri, by the protrusion and retraction of which they capture their prey. Examples of this class are the Barnacles and Acorn shells. These animals have long ceased to be regarded as Mollusca, the investigations of modern naturalists having proved them to be true articulated animals most nearly related to the Crustacea. THE ARTICULATED ANIMALS Have no back-bone. The covering of the body is sometimes hard and sometimes soft, but it is always divided into segments by a number of transverse incisions. The limbs, when the body is provided with any, are jointed; and they can be separated from the body without any serious injury being sustained by the animal, new limbs being shortly after formed to replace them. The senses of tasting and seeing are more perfect than those of the Mollusca, though that of feeling seems much less acute. In other respects the four classes differ considerably from each other. [_The Entozoa, or Intestinal Worms_, placed by Cuvier and others among the Radiata, are now arranged amongst the lowest forms of articulated animals, as are also those animalcules known as _Rotifera_.] I. _The Annelida, or Red-blooded Worms_, have no heart, properly so called, but have sometimes one or more fleshy ventricles. They breathe through branchiæ. Their bodies are soft, and more or less elongated, being divided into numerous rings or segments. The head, which is at one extremity of the body, can scarcely be distinguished from the tail, except by having a mouth. These animals have no feet, properly so called, but they are furnished with little fleshy projections, bearing tufts of hairs or bristles, which enable them to move. They are generally of carnivorous habits. They lay eggs, but the young are frequently hatched before exclusion, and hence these creatures are said to be ovoviviparous. Their study is called Helminthology. As examples of the three orders of this class may be mentioned the serpulæ or worm-like animals, often found on shells, the common earthworm, and the leech family.