138 Introduction to Animal Morphology. 



connective tissue, whose fibres cross and unite at 

 angles of about i2O-6o-9O. Usually the interstices 

 are filled with protoplasm, in which resides the power 

 of repair after injury. In dried perisomes the organic 

 matter is about 9 per cent. The plates may be flexible 

 (Astropyga), or rigid, and unite either by sutures filled 

 by an organic cement soluble in alkaline carbonates,, 

 and ankylosing as age advances ; or they may be 

 separated from each other by a soft area (Astheno- 

 soma). Before uniting, the plates grow as the body 

 enlarges, by additions to their edges. The rows of 

 plates are in pairs, five of which are pierced with 

 holes for the ambulacral feet, and five are inter-ambu- 

 lacral, each of two rows of imperforate plates. Both 

 series of plates are beset with tubercles (spine-warts) 

 for the articulations of the spine, in transverse rows, 

 and varying in size; for as there are large, medium,, 

 and small spines, so there are primary, secondary, and 

 miliary warts. The first set have often an apical 

 depression for the round ligament of the spine, and a 

 granular "spine-ring" around the base, girdled by 

 smaller warts. 



These plates, though apparently simple in the adult, are 

 compound in most of the regular Echinoidea, and each con- 

 sists of an ad- and an ab-oral entire plate (stretching from 

 interambulacrum to the median suture of the ambulacrum), 

 and one, t\vo, or three intermediate half plates. At first they 

 are all entire, but the rapid growth of the ab- and ad-oral 

 plates causes the others to become triangular with the apex 

 inwards. At first the pore-pairs are in a slightly convex arc 

 near the margin ; but as the test grows, the ad-oral pair are 

 moved inwards, and downwards, and the ab-oral similarly, 

 but more slightly, affected. Thus the arcs of pores in the 

 adult, if counted from below upwards, commence with the 

 second pair of a plate, and end with the first pair of the 



