408 ZOOLOGY SECT. 



In considering the general external form in the various classes 

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

 the tube-feet 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 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, except in Pelago- 

 thuria and Rhabdomolgus (Holothuroidea), 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 contraction 

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

 sometimes (as in the case of Anthenea, p. 380) 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 pedi- 

 cellariae of the Sea-urchin have been seen passing from one to 

 another the particles of faecal 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 (fascioles) on 

 the surface of the Spatangoids. Larger spines, resembling the 

 clavulse in being covered with strong ciSa, occur also on the aboral 

 surface in the Clypeastroids and some Asteroids. The currents 



