The family of Echinidæ, commonly known as Sea-Eggs or Sea-Urchins, have a beautiful but complicated structure. The calcareous shell of an Echinus is a hollow spheroid with large circular openings at each pole. In the larger of the two, called the corona, the mouth of the animal is situated; in the lesser circle the vent is placed. The spheroid itself is formed of ten bands extending in a meridional direction from the corona to the lower ring; that is, from one polar circle to the other. Each band consists of a double row of pentagonal plates increasing in size from the poles to the equator, nicely dovetailed into one another, and the bands are neatly joined by a zigzag seam. Every alternate band is perforated by a double series of minute double holes for the passage of the tubular feet of the animal. The five perforated or ambulacral bands have rows of tubercules parallel to the series of feet holes, supporting spines movable in every direction. The five imperforated bands are characterized by a greater number of spines, but there are none within the polar circles. The spines may be long rods, or merely prickles, or stout, club-shaped bodies, according to the genera. [Illustration: Fig. 138. Section of Shell of Echinus. _a_, portions of a deeper layer.] The microscopic structure of the shell of the Echinus is everywhere the same; it is composed of a network of carbonate of lime, with a very small quantity of animal matter as a basis. In general, the network extends in layers united by perpendicular pillars, but so arranged that the open spaces, or meshes, in one layer correspond to the solid structure in the next. The spheroid of the Echinus is covered with spines, and both outside and inside by a contractile and extensile transparent membrane, which supports the shelly plates at the poles, and dips between the bands but does not penetrate them. Its extensile nature admits of the addition of calcareous matter to the edges of the plates when the animal is increasing in size. The membrane lining the interior of the shelly globe is tough; it encloses the digestive organs, and forms a muscular lip to the mouth, which is armed with five triangular, sharp-pointed, white teeth, and surrounded by five pairs of pinnate tubular tentacles. The outer margin of the lip is fringed with a circle of snake-headed pedicellariæ visible to the naked eye.