NORTHUMBERLAND AND DURHAM. 149 



III. will confirm this statement ; for whereas those from mature 

 animals are flat plates with many perforations, those from the 

 young present a cup-like form having only three to five perfora- 

 tions and a number of rounded projections : intermediate stages 

 show that further growth proceeds in a lateral direction, the 

 "knobs" assuming the character of the little warts depicted in 

 fig. 11, the concavity (of the cup) being obliterated by the fur- 

 ther accretion of calcareous material. Of course it will be un- 

 derstood that all the spicules found in the "foot" of a mature 

 P. phantapus have not had this origin : many are there in their 

 earliest stages of growth and never present any approach to a 

 cup shape, but will all along present the character of flat plates. 

 Examples of these are shown by figs. 1-5, the corresponding 

 growth, so to speak, in the young of P. phantapus being repre- 

 sented by figs. 13-16. 



It would be interesting to trace and figure all the progressive 

 stages, but this is rather beyond the scope of a catalogue ; suffi- 

 cient examples have been given, it is hoped, to enable the spi- 

 cules to be readily identified. Mature spicules from the foot 

 of this species measure about nsW of an inch across, the largest 

 perforations being about toVW in diameter. The spicules from 

 the very young only measure t-oVo in diameter. The plate at 

 the tip of a sucker is often *V of an inch across, or even larger. 



EXPLANATION OF PLATES. 



PLATE I. 



Figs. 1-14. Spicules from body of Cucumaria elougata, x 60. 



Figs. 15-23. „ „ C. lactea, x 200. 



Figs. 24-33. ,, ,, ThyonkUum hgalinum, X 200. 



PLATE II. 



Figs. 1-5. Spicules from body of Thyonidium commue, x 200. 

 Figs. 6-17. ,, ,, tentacles of the same, x 500. 



Fig. 18. Part of plate from sucker tip of the same, x 200. 



