ZOOLOG Y. 37 
transparent case or hardened skin, which is slightly seg- 
mented, 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 con- 
cealed. They resemble slightly the Sea-cucumbers, ani- 
mals 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, jor Phractelminthes, and are considered by 
Haeckel as furnishing the link between the Worms and 
the Star-fishes. We will refer to them again. The An- 
nelida, 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 appear- 
ance,—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 admira- 
tion 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 
