SECT. VI. 



ECHIXODERMA TA. 



'77 



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 

 ambulacra! bands have rows of tubercules parallel to 

 the series of feet holes, supporting spines movable in 

 every direction. The five imperforated bands are charac- 

 terized 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, accord- 

 ing to the genera. 



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 ani- 

 mal matter as a basis. In 

 general, the network ex- 

 tends 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 exten- 

 sile 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 



VOL. II. N 



Fig. 138. Section of Shell of Echinus. ' 

 a, portions of a deeper layer. 



