U PHYLUM ECHINODERMATA 417 



its extremity a sucking -disc, by means of which it can be attached ; 

 in a few, however, this sucking-disc is absent. 



The epidermis is ciliated in all but Holothuroidea. In the 

 subjacent dermal layers there are always present calcareous bodies 

 or ossicles, varying very greatly in form and arrangement in the 

 different groups. Movable or immovable calcareous spines or 

 tubercles projecting on the surface are very general. Peculiarly 

 1 in >(li tied spines, termed pedicellariae, are commonly, though not 

 universally, present in certain parts in the Echinoidea and 

 Asteroidea. A pedicellaria consists in essence of two or three 

 calcareous jaw-like pieces or valves, movably articulated together, 

 and capable of being separated or approximated by the con- 

 traction of bundles of muscular fibres ; sometimes there is a long 

 stalk ; sometimes (as in the case of Anthenea, p. 387) a stalk is 

 absent ; during life the jaws or valves keep opening and closing. 

 That such specialised structures have some important function to 

 perforin there can be no doubt, but there is some uncertainty as 

 to what their special purpose is. According to some observers, 

 the pedicellariae of the Sea-urchin have been seen passing from 

 one to another the particles of fsecal matter discharged from the 

 anus, and their function would thus appear to be a cleansing one. 

 On the other hand, it is stated that when a Sea-urchin is attacked, 

 the spines may be bent aside from the assailed portion of the 

 surface so as to allow of the pedicellariae being brought to bear as 

 defensive weapons on the assailant, and from these and other 

 observations that have been recorded, both on Asteroids and on 

 Echinoids, it is concluded that the main function of these appen- 

 dages is to act as defensive organs. Pedicellariae are absent in the 

 Ophiuroids, but in the Euryalida there are peculiar hook-like 

 organs of adhesion, most abundant on the oral surface and 

 towards the extremities of the arms. The sphceridia, which have 

 already been referred to as occurring in the Sea-urchin, are only 

 doubtfully to be regarded as modified spines ; they are confined 

 to the Echinoidea. Also confined to that class are the clavulce — 

 slender spines covered with strong cilia, which occur in bands on 

 the surface of the Spatangoids. Larger spines, resembling the 

 clavulse in being covered with strong cilia, occur also in the 

 Clypeastroids and some Asteroids. The currents produced by the 

 action of their cilia serve to keep constantly renewed the water 

 in the neighbourhood of the anus and of the branchiae. 



There are two principal systems of plates to be recognised, 

 an oral and an apical ; the former corresponding with the oral or 

 actinal, and the latter with the aboral or abactinal surface. The 

 former vary considerably in the different classes : the constant 

 elements are five orals, which may or may not be recognisable in 

 the adult. The apical system consists (1) of a central plate ; (2) 

 of five basals which are inter-radial in position ; (3) of five radials 



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