ix PHYLUM ECHINODERMATA 383 



is to be specially noticed, extend in the direction of the axis 

 joining mouth and anus, and not at right angles to it as in the 

 Starfishes and Sea-urchins. 



In considering the general external form in the various classes 

 of Echinoderms, we have to take into account the arrangement of 

 the tube-fed the organs of locomotion as these have important 

 relations to the other parts, and to the whole plan of organisation 

 of the animal. These organs, as previously explained, are tubular 

 appendages with highly elastic and contractile muscular walls, 

 capable of being stretched out so as to extend a long way from the 

 surface of the body. In the majority of cases the tube-foot has at 

 its eJ&reinity a sucking-disc, by means of which it can be attached ; 

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



The entire surface is covered with a ciliated epidermis. 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 

 modified spines, termed pedicellarice, 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. 357) a stalk is 

 absent. During life the jaws or valves keep opening and closing. 

 That such specialised structures have some important function to 

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

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

 the pedicellaria? of the Sea-urchin have been seen passing from 

 one to another the particles of fcecal 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 pedicellarise 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 append- 

 ages is to act as defensive organs. Pedicellariaa are absent in the 

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

 organs of adhesion, most abundant on the ventral surface and 

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

 already been referred to as occuring 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 elavuloe 

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

 the surface of the Spatangoids. Larger spines resembling the 



