AMONG THE SEA-URCHINS. 17 



is found to be composed of plates varying from 5 to 8 in number, 

 distinctly notched on the margin in a most beautiful and regular 

 design (see Plate I., Fig. 3), full of numerous cavities. These 

 plates are held together in position by a calcareous ring of other 

 oblong plates (Figs. 4, 13. and 15), while the walls of the tube feet 

 are filled with the curious " C "-shaped spicules, shown at Figs. 16 

 and 18, which closely resemble the sponge spicules obtained from 

 Desmacidon incrustans. 



It was when partially reducing the muscular fibre of the tube 

 feet, with a view to obtain a few perfect microscopic mounts, i7i 

 situ, of the calcareous rosette, that we were surprised to meet with 

 these spicules, which vary greatly in size and shape, and which 

 neither Gosse, Forbes, nor any other popular English writer that 

 we have met with appears to have ever noticed. The American 

 Professor Alex. Agassiz, however, fully describes them in his 

 " Revision of the Echini," 1874. Like the sponges, each distinct 

 species has its own peculiar form, some of which are drawn in 

 Figs. 12 to 18; those of Echifiometra, shown at Figs. 17 and 18, 

 being very remarkable. 



We have not yet recounted all the wonders in connection with 

 the Sea-Urchin, but an account of the digestive system (see wood- 

 cut, Fig. 2, A), and a description of the mouth (Fig. 2, C), which is, 

 perhaps, the most complex in nature, as also the history of the 

 development of the urchin, from the larval form (PI. I., Fig. 5), 

 must be left for a future paper. 



EXPLANATION OF PLATE I. 



Fig. 1. — Sea-Urchin, Brissus lyrifer (Forbes), with half body cleared 

 of spines, etc., to show ambulacra and tiddle-shaped dorsal 

 impression. 



,, 2. — Portion of Tube-foot of Echinus sphcera, reduced in liquor 

 potassa3 to show " C "-shaped spicules and calcareous rosette 

 (ambulacral disc). 

 3.— Single plate from ambulacral disc of Echinus sphcera. 



4. — Plate forming a portion of the calcareous ring round centre 

 of rosette. 



5. — Larval form of Sea-Urchin. 



6. — Pedicellaria of Spatangus purpureas, with muscular stalk. 

 7. — The same, with blades partly open. 

 8. — Skeleton of Head of P. tridens from Echinus sphcera. 





J) 



Journal of Microscopy and Natural Science. 

 New Series. Vol. III. 1890. 



