ASTEROIDEA. 



393 



— which are movably articulated to one another at their inner ends. 

 Running in the roof of the groove, below the line of union of the 

 two rows of ambulacral ossicles, is one of the radiating ambulacral 

 vessels, from which are given off the rows of suctorial " tube-feet." 



The integumentary skeleton of the Asteroids is less developed 

 than in the Echinoids, and has the form of innumerable small 

 calcareous pieces, or " ossicles," united together so as to form a 

 species of chain-armour. The os- 

 sicles are generally united with one 

 another in a reticulated manner, 

 and the interspaces between them 

 are filled by the coriaceous integu- 

 ment; but they may be directly 

 united by their edges. In many 

 genera a specially developed series 

 of ossicles forms a row of plates, 

 known as the " adambulacral 

 plates" (fig. 271, b\ on each side 

 of the ambulacral furrows. In 

 many genera, also, there is a single 

 or double row of large plates, known 

 as the "marginal plates" (fig. 271, 

 ?n), round the margins of the disc 

 and arms, along the line separating 

 the dorsal and ventral surfaces. 



Spines are commonly developed, especially along the margins of 

 the ambulacral grooves, but these structures are in no case movably 

 articulated. The so-called " pedicellariae " of the Asteroids, as in 

 the Echinoids, may be regarded as peculiarly modified pincer-like 

 spines ; but the size of these is too small to render it likely that 

 they can be commonly preserved in the fossil condition. In some 

 genera (as in So/aster, Luidia, Ctenodiscus, &c), there are large 

 spines the summits of which carry bunches or tufts of minute cal- 

 careous processes, and which are known as "paxillse." In other 

 cases, as in Pentaceros, there are very much thickened spines, which 

 may assume considerable dimensions. It is not improbable that the 

 Ordovician fossils upon which the genus Bolboporites has been 

 founded, are really of the nature of dermal spines belonging to 

 some Asteroid like Pe?ttaceros. 



The ambulacral system of the Asteroids is essentially similar in 

 its arrangement to that of the Echinoids. The external opening of 

 the water-vessels is always placed interradially, between the two 

 posterior rays (when five rays are present), and is provided with a 

 porous " madreporite," two, three, or more of these being occasion- 

 ally present. The madreporite admits the water to the short 



Fig. 271. — Diagram of a Star-fish {Goni- 

 aster), showing the under surface, with the 

 mouth and ambulacral grooves, a, Ambula- 

 cral ossicles, with the ambulacral pores be- 

 tween them ; b, Adambulacral plates, bound- 

 ing the ambulacral grooves ; m, Marginal 

 plates (wanting in many species) ; o, Oral 

 plates, placed at the angles of the mouth. 



