218 LECTURE X. 



of a circular disc, like a clock-face, on one side of the granular mass : 

 five double lines, like pointers, radiate from the centre of this disc, 

 and their extremities expand into circles with a double outline. 

 These circles form the bases of as many hollow tentacles, like those 

 that traverse the ambulacral pores of the adult Echinus, but which 

 are here single, not in pairs. Opposite this disc there appear upon the 

 dome, on each side, two triradiate pedicellaria, which exhibit spon- 

 taneous motion, the arms of the pincers opening and shutting. The 

 disc progressively expands, extending over the cellular mass to which 

 it owes its origin, and developes tubercles, which push through the 

 transparent dome of the Pluteiis, and are transformed into spines and 

 tentacles, both of which, together with the pedicellariae, manifest 

 their characteristic motions and combine these seemingly voluntary 

 actions with the automatic continued vibrations of the ciliated 

 epaulets of the larva ; so that it can now both swim and creep : the 

 mouth of the larva still remains in its first position and, like the oeso- 

 phagus, is in full action. By the time the disc has grown over half 

 the granular sphere little of the larva remains, except some of the 

 slender calcareous rods, and the mouth is obliterated : the perforated 

 oviducal plates are formed round the pentagonal space, as yet imper- 

 forate, in the centre of the original disc, determining the anal pole of 

 the developing spiny globe, close to which the cicatrix of the ruptured 

 pharynx of thePluteus is completed by the so-called "madreporiform 

 plate. The mouth of the Echinus is quite distinct and remote from 

 that of the Pluteus : it is first indicated in the centre of the naked 

 or unspined moiety of the granular sphere by the formation of the 

 hard summits of the five characteristic dental masses of the Echinus, 

 The spines, which soon acquire a considerable length, contain a cal- 

 careous framework. When the latter is perfectly developed, it forms 

 a hexagonal prism placed within tlie cylindrical skin of the spine, 

 which consists of a regular calcareous lattice-work, terminating at 

 the extremity in minute teeth. The horizontal arrangement of the 

 axial network of the spine is radiate. Before the framev/ork of the 

 spine is thus far developed, when it first appears, it has exactly the 

 form of a candelabrum. The basis of the framework of the spine is 

 thus a star with six rays, from the centre of which there arises a 

 simple axis, which immediately subdivides into other branches which 

 subsequently re-unite. The segments of the delicate nascent shell of 

 the Echinus growing into tentacles and spines cannot be considered 

 as the subsequent plates of the shell, but as the foundations of the ten- 

 tacles and spines. The outer skeleton is at length completed by the 

 progressive extension of the " ambulacral " and " interambulacral," 

 or spine-bearing plates, and by the less regular " oral " plates. 



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