64 ECHINODERMATA. 



also in respiration. A nervous system is also found 

 having a distribution similar to that of the water-vascular 

 system. 



In all the echinoderms there is a dermal skeleton. 

 This is calcareous and consists of isolated pieces, or more 

 usually of rods or plates, often forming a complete exo- 

 skeleton, which may be either flexible or rigid; spines 



FIG. 14. A. Portion of transverse section of a spine of a sea-urchin 

 Echinometra, Eecent (magnified). B. Section of interambulacral 

 plate of recent Cidaris parallel to the surface (magnified). 



and other processes are also often present, attached to 

 the plates. When examined microscopically each part 

 of the skeleton is found to be formed of a network of 

 calcareous rods (fig. 14). The details of the structure 

 vary in different forms, depending on the size and shape 

 of the spaces between the rods. In the spines of the sea- 

 urchins the network of rods is frequently arranged so as 

 to form radiating perforated plates (fig. 14 A), which are 

 united by rods placed at right angles to them. The 

 axial portion of the spine often has a coarser structure 

 than the other parts. Another characteristic feature of 

 the skeleton is that each component part shows a uniform 

 optic orientation, being in fact a crystal of calcite, differing 

 only from an ordinary crystal in not having crystal 



