120 ECHINODERMA. ECHINOIDEA 



new plates are being added next the apical disc ; the fact 

 that some of the fused primary plates are smaller than 

 others, and also the presence of demi-plates, is attributed 

 to the reduction in size of the original plates by the 

 absorption of material under pressure. Lambert, however, 

 considers that the difference in size of plates is due mainly 

 to the more rapid superficial growth of the plates on 

 which large tubercles are developed. The pores in the 

 ambulacra of some echinoids are placed one immediately 

 above the other, so that one vertical row of pore-pairs is 

 seen — such pores are termed unigeminal (fig. 46) ; in other 

 cases the pore-pairs are alternately near to, and more distant 

 from, the margin of the ambulacral plate, and consequently 

 two vertical rows are formed, and the pores are said to be 

 bigeminal ; in a similar way three vertical rows of pore- 

 pairs may be produced, when the pores are known as 

 trigeminal. Sometimes the pores in each pair are united 

 by a groove on the surface of the plate, and are then 

 termed conjugate. In some sea-urchins the two rows of 

 pores in each ambulacral area diverge rapidly after leaving 

 the apical disc, and then come together again before 

 reaching the equator, so that this part of the ambulacrum 

 is leaf-like, and the five ambulacra together form a 

 rosette on the upper surface of the corona, the pores being 

 but irregularly developed on the lower surface; the 

 ambulacral areas in such cases are termed petaloid (e.g. 

 Scutella), but when the rows of pores diverge but do not 

 come in contact at their lower ends they are sub-petaloid 

 (e.g. Nucleolites). When, as in Cidaris, the distance be- 

 tween the two rows of pores increases uniformly and 

 slowly in passing from the apical disc to the equator, and 

 the pores are equally well-developed on the under surface 

 of the test, the ambulacra are said to be simple (fig. 46). 



