transparent case or hardened skin, which is slightly segmented, and through which the jointed intestine may be seen. The Sipunculus (Fig. 33) resembles somewhat our earth-worm, and is found at low-water mark buried in the sand. When the tide comes in, the Sipunculus, rising to the surface, exhibits a circle of tentacles surrounding its mouth: this can be drawn in by the animal and quite concealed. They resemble slightly the Sea-cucumbers, animals included in the Echinodermata. Within a few years a remarkable group of Gephyrea have been found well preserved in a fossil condition. They have been called Mailed Worms, or Phractelminthes, and are considered by Haeckel as furnishing the link between the Worms and the Star-fishes. We will refer to them again. The Annelida, or second division of Articulated Worms, are among the most beautiful of living creatures, of every size and color, sometimes seen as pretty little white or red worms swimming about in our fresh-water streams and ponds, or living, as sedentary organisms, in tubes constructed out of the sand and other materials found near the sea-shore, or swimming along by a kind of undulatory movement. The Nereids (Fig. 34) are composed in some examples of many hundred joints or segments; each segment is furnished with a little paddle attached to the side. The blood, rushing into the little tufts of hair, which are seen on the upper surface of each segment, gives the animal a brilliant appearance,—the little hairs refracting the light make so many rainbows. The whole effect of the Nereid gliding through the sea is so beautiful that it has called forth the admiration of the poets. The Annelida increase their length by adding segments to those already formed. In this respect they resemble the Centipedes, etc., which belong to the Myriapoda, of which we will speak again. The Rotatoria (Fig. 35), or third division of the Articulated Worms, are microscopical. They live in fresh or salt water; they are
