106 



ECHINODERMA 



similar to that of the water-vascular system. Repro- 

 duction is mainly sexual ; as a rule the sexes are separate, 

 but do not differ externally. 



In nearly all echinoderms there is a dermal skeleton. 

 This is calcareous and consists sometimes of isolated pieces, 

 but more usually of rods or plates united by fibres of con- 

 nective tissue and forming a complete shell or test, which 

 may be either flexible or rigid ; spines and other processes 

 are often attached to the plates. When examined micro- 

 scopically each part of the skeleton is found to be formed 

 of a network of calcareous rods (fig. 41). The details of 

 the structure vary in different forms, depending on the 



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Fig. 41. A. Portion of transverse section of a spine of a sea-urchin, 

 Echinometra, Eecent. Magnified. B. Section of interambulacral 

 plate of recent Cidaris cut parallel to the surface. Magnified. 



size and shape of the spaces between the rods. In the 

 spines of sea-urchins the network of rods has usually a 

 radial arrangement, with polygonal or rectangular spaces 

 (fig. 41 A), except at the centre, where the structure is 

 more irregular. Another characteristic feature of the 

 skeleton is that each component part shows the optical 

 characters of a crystal of calcite, and differs only from an 

 ordinary crystal in not having crystal contours and in the 



