ASTEROIDEA 43 



spines or papillae, generally somewhat larger than the other spines 

 on the underside ; they are of very great systematic importance. 

 Along the edge of the disk there is in some forms a double — more 

 rarely [Luidia) a single — series of large, distinct plates, the mar- 

 cjinal i)lates ; they are mostty covered with fine granules or spines 

 of varjdng size. The space between the arms on the under side of 

 the disk forms, where marginal plates are present, or where there 

 is a distinct limit between the upper and under side, a well-defined 

 area, the oral interraclial area ; it has, as a rule, a characteristic 

 covering of spines, different from that of the upper (dorsal) side. 

 The skin of the dorsal side is supported by a skeleton of calcareous 

 plates, with a reticular arrangement. On these plates spines are 

 articulated, scattered or in groups, or arranged in regular bundles, 

 attached to shaftlike processes from the plates. Such groups of 

 spines are designated paxillce'^ and bear some resemblance to 

 a brush. Where paxillse are found, the spiny covering of the 

 dorsal side often forms a regular, beautiful mosaic. In some 

 forms {Porania) the skin of the dorsal side is perfectly naked. 

 In the skin, between the spines, pores occur, through which 

 delicate tubes, ^ja^^^/Zce, may be extended (Fig. 2 G) ; they have 

 the function of gills, and in some forms {e.g. Asterias) may be so 

 numerous that, when well extended, they make the animal look 

 c^uite shaggy. On the dorsal side, about half-way between the 

 edge and the centre of the disk, lies the madreporite, which is 

 generally rather large and conspicuous ; it is finely striate. Ex- 

 ceptionally, there may be 2-3 madrej)orites ; a tropical, many- 

 rayed sea-star (Acanthaster) has numerous madreporites. 



In most sea-stars are found peculiar small organs, small 

 pincers, the pedicellarice (Fig. 26). In their simplest shape they 

 are only common spines, placed 2-4 together so that they can 

 move against each other {Astropecten, Pontaster), or two series 

 of spines opposed to one another (pectinate pedicellarise ; Pseud- 

 archaster) ; in others they have the shape of two small, or two 

 very broad, low valves (valvate pedicellariae, Ceramaster, Hip- 

 pasteria), or narrow valves attached in deep grooves (alveolate 

 pedicellariae). In more developed form they consist of tAvo 

 peculiar-shaped valves, which move on a basal joint ; of these 

 there are usually two forms, the crossed and the straight 

 pedicellariae (or in older terminology, forficiform and forcipiform 

 pedicellariae) {Asterias, Brisinga). They are attached to the skin 



^ The finer distinction between the various forms of spine -groupings : 

 paxillae, pseudopaxillse, parapaxillse, etc., it seems unnecessary to em- 

 phasise in the present work. 



