246 THE STELLEROIDEA 



The Variations in Structure from the typical genus are less 

 remarkable among the Asteroids than among the Echinoids and 

 Ophiuroids. 



In shape the extremes are genera such as Brisinga (Fig. V.) or 

 Freyella, and Culcita (Fig. VI.). In the first two the arms are 

 numerous and slender, and sharply marked off from the disc. In 

 the last the body is bun-shaped, and the ambulacra extend for a 

 short distance over the abactinal surface. 



The most conspicuous variation from Asterias in skeletal 

 structure is due to the presence of a series of thick plates round 

 the margin both of arms and disc. These marginal plates are 

 best developed in the order Phanerozonia. There may be two 

 series, one above the other, and known respectively as supra- 

 marginals and infra -marginals (Fig. VII. d and c) ; in some 

 genera intermarginal plates occur between these *two series. 



The spaces (actinal interbrachial 

 areas) between the marginal 

 plates and the ambulacral fur- 

 rows may be occupied by a 

 series of accessory plates, form- 

 ing a pavement like mosaic. 



The accessory plates on the 

 abactinal side may be large or 

 small, equal or unequal. In 

 some species the central plate 



Segment of ann n^ iro^arfa. a, Is J^ 6 ' and S me f the ^ 



supra-ambuiacrai plates ; b, ambulacral plates ; inaining plates are arranged in 



c and rf, inferior and superior lateral plates: e, i i ., mi ij r 



dorsal plates with paxiiiae. circles round it. The plates of 



three of these circles have been 



regarded as the homologues of the calycinal plates of Crinoids, and 

 are accordingly sometimes named the radials, basals, and infra- 

 basals (but see p. 14). In Cnemidiaster wyvillei the central, and 

 the so-called radials and basals are present ; in Zoroaster fulgens 

 there are infra-basals as well. The order of development of these 

 plates is as follows : First five plates appear round the centre 

 of the abactinal surface ; these plates move outwards to the arm- 

 tips, and form the terminals ; a second ring appears, the plates of 

 which are the " basals " ; the central plate next develops in the 

 centre of this ring ; the radial circle follows, and after that come 

 the " infra-basals." Between the plates of the last set and the 

 central some genera have additional plates, which cannot be 

 homologised with any in Crinoids. 



The Oral Skeleton consists of a ring of plates round the mouth. 

 The ring is composed of as many segments as the starfish has rays, 

 and each segment is interradial, and forms an " oral angle " ; it 

 consists of two sets of plates, usually three in number. 



