ASTEROIDEA. 



171 



FIG. 121. Diagram of a transverse section through 

 the arm of Astropecten, the hepatic diverticula being 

 omitted. 7 ambulacral, 2 adambulacral ossicle ; 

 3 lower, 4 upper marginal plate ; 5 superambulaeral 

 ossicle ; 6 integument of abactinal surface ; 7 paxil- 

 li ; S adambulacral spines ; 9 spines of the lower side 

 of the ini'ramarginal plate ; 10 lower, 11 upper marginal 

 spine ; a radial water-vascular vessel ; 6 so-called 

 radial blood-vessel ; c radial nerve ; d ampulla of tube- 

 foot ; e tube-foot ; / perivisceral cavity of arm. 



developed, we get the ambulacral peristom (Fig. 120). Whatever may be 

 the condition in the adult, all young starfishes begin by possessing an 

 ambulacral peristom. The adambulacral pieces of the peristom carry 

 opines. 



The terminal plates may be mentioned here. They belong to the 

 actinal surface, and one of them is found at the end of each arm on the 

 abactinal side of the last ambulacral ossicles (Fig. 119 T). They are 

 especially conspicuous 

 and important when the 

 skeletal plates are first 

 making their appearance ; 

 and it can then be seen 

 that they are laid down 

 round the left coelomic 

 ;sac, i.e. on the actinal 

 surface of the future star- 

 fish (p. 147). 



(b) The ambital skeletal. 

 The rest of the skeletal 

 plates are classified as 

 ambital. They consist of 

 the interambulacral plates 

 and the antambulacral. 



The interambulacral 

 plates are of three kinds : 

 ( 1 ) the inner intermediate 



pieces (o, Fig. 119), of which there is one in each interradius ; they lie on 

 the abactinal side of the two adambulacral pieces which form the buccal 

 angles ; (2) the ventrolateral plates, which lie between the adambulacral 

 and the inferior marginals (V, Fig. 119); the ventro-lateral plates are 

 often only found on the interradial portions of the actinal surface of the 

 disc ; (3) the inferior marginals, which constitute a row of plates placed 

 at the edge of the arm, just external to the ventro-laterals (Fig. 119, MV). 



The antambulacral plates (Fig. 122) constitute the bulk of the ambital 

 skeleton. They consist of (1) the superior marginals '(M), which may 

 either be in contact with the inferior marginals or separated by intercalated 

 plates; (2) the eleven primary plates of the apical system, viz. five radial 

 (R), five interradial (JR), and one central (C) ; (3) the secondary radial 

 plates of the arms and disc (Rb and Rd) (carinalia of Perrier, medioradials 

 of Sladen) ; these are often not distinguishable in adults from the other 

 antambulacral plates, e.g. many Archasteridae, Porcellanasteridae, 

 Asteriidae, Solasteridae, Echinasteridae ; (4) the dorsolateral plates 

 (dl), corresponding to the ventro-laterals of the interambulacral skeleton ; 

 (5) the supplementary plates of the arms and disc (s) which may effect the 

 connexion of the dorsolaterals into a meshwork (hence reticularia of 

 Perrier) ; (6) the madreporic plate (Md) placed in the left anterior in- 

 terradius (the mouth being downward, and the anal interradius forward) ; 

 the madreporic plate may be either the interradial plate of the apical 

 system (JR, MD), or it may be outside this plate (Md), or it may be fused 

 with it (Md 1 ). 



The primary plates of the apical system are quite distinct in the young 

 starfish, but in later growth are only rarely distinguishable by size or form 

 .(some Asterinidae and Pentacerotidae, many Pentagonasteridae) from 



