552 INVaBTEBRATE MOBFHOLOGT. 



II. Class Asteeoidea. 



The Asteroidea, or Starfishes, are all flattened forms, at 

 no period of their lives attached by a stalk, but creeping 

 about freely upon the oral surface. In some forms the body 

 is a flattened disk pentagonal in outline (Asterina), but more 

 usually (Fig. 246) the five radii are prolonged out into five 

 stout unbranched arms, and in some forms, such as Brisinga, 

 the arms may be long and slender and more than five in 

 number. The mouth is situated in the centre of the oral 

 surface, and the anus slightly excentrically upon the aboral 

 surface, while the hydrocoel system of tubes is confined, as 

 in the Crinoids, to the oral surface of the body, except that 

 the madreporiform tubercle by which the system communi- 

 cates with the exterior is upon the aboral surface in the in- 

 terradius CD. 



The ectoderm is throughout ciliated, and contains usually 

 numerous mucous glands, while in its lower layers ganglion- 

 cells and nerve-fibrils form a plexus extending over the entire 

 surface of the body. 



Calcareous matter is deposited in the connective tissue, 

 but in the majority of forms the primitive apical plates are 

 not recognizable in the adult ; more usually the aboral sur- 

 face is covered by a large number of small plates arranged 

 without anj' regularity, or else the calcareous matter forms 

 a reticulum composed of numerous fused bars, short spines 

 rising frequently from the points of union. In embryos, 

 however, and in some adult forms, such as Zoroaster (Fig. 

 247), the apical system can readily be made out, and con- 

 sists of a centrodorsal plate (Fig. 247, CD), sometimes 

 grooved upon the edge for the anus {an), surrounded by five 

 under-basals (2) usually small, alternating with which are 

 five basals (3). At the base of each arm is a radial (_4), and 

 in embryos beyond this there is in each radius another plate, 

 which as growth takes place is carried further and further 

 from the radial and finally forms the terminal plate {T) oi the 

 arm, by which name it is known. 



Of the oral system the orals are possibly represented by 



