670 PEDICELLARLE. 



/. 8 ; PL XTII f . f. 7), and of the Spatangoids (PL XV". f. / ; PL XIX 1 f. 

 9; PI XXI'. f. 5). 



In Ophiurans we find all the intermediate stages between plates, claws, 

 and slender spines; in starfishes, between the simplest granules, the most 

 complicated serrated spines and pedicellarise ; and in Holothurians, between 

 mere spicules, anchors, and the pavement-like covering of such genera as 

 Cuvieria and Psolus. All this shows plainly enough that the spines and 

 pedicellariae are strictly homologous, whatever modifications they may as- 

 sume in the different orders of Echinodernis, whether they serve as prehen- 

 sile scavengers, or simply protect the test against the violence of the waves on 

 the rocks or the attacks of their enemies. Sea-urchins are the favorite food 

 of many species of fish ; the}- would find it rather dangerous to attack the 

 bristling Diadema, and require pretty strong jaws to get the better of the 

 armored lleterocentrot us. The spines are not simply organs of defence; 

 they also act as means of locomotion, and in such genera as Arbacia the am- 

 bulacra! suckers perform only a secondary part in the displacement of the 

 sea-urchin, the spines of the lower side serving as stilts by which the sea- 

 urchin raises itself and move- along by a kind of halting gait. In Ophiurans 

 and Holothurians. the pedicellariae hooks and anchors perform the part of 

 organs of prehension and locomotion at the same time. 



There is nothing in the history of the development and in the homologies 

 of these organs to show that they have been suddenly brought into exist- 

 ence ; on the contrary, the modifications of the spines and pedicellarise show 

 the most complete homology between appendages which have lately been con- 

 sidered as strong proofs of the possibility of the sudden appearance of organs 

 for which no utilitarian motive could be given. 1 trust 1 have made it suffi- 

 ciently plain that in the most complicated pedicellariae known, with a freely 

 movable stalk and with snapping jaws, we have only a very gradual modifi- 

 cation of the simplest sort of limestone network found in all Echinodernis, in 

 the earliest stages of the embryonic development, while still in the Pluteus 

 stage, and that we have an unbroken sequence from this primitive network 

 to form, on the one side, the most diversified spines, and on the other equally 

 variable pedicellariae, and that we must consider the latter in their most 

 complicated forms as nothing but highly specialized spines. 



