Spicules in Amhulacral Tubes of Eehinoidea. By Prof. Bell. 299 



perforated in that portion (Fig. 3). It would seem likely that the 

 rarity of these spicules may be ascribed to the great thickness of 

 the walls of the suckers, the development of muscular and con- 

 nective tissue being so considerable that there is no such necessity 

 for the spicules here as there is in cases where the walls are 

 thinner ; but the spicules themselves are proportionately large. 



The bihamate spicules of the Echinidse, the tri-radiate ones of 

 Diadema, the flattened centrally enlarged form of EcJiinocidaris, 

 present little in common, and, while there would be no difficulty in 

 distinguishing them, it is hkewise impossible at present to make a 

 suggestion as to how they might be derived from one another. 

 When with these we compare the ambulacral spicules of Salenia it 

 is not perphaps too hardy to suggest that in the irregular forms 

 there to be found we may have something hardly more than 

 " amorphous," from which the forms of the later groups have been 

 derived. 



There is no close resemblance between the spicules of Cidaris * 

 and those of Phormosoma and Asthenosoma (Figs. 8 and 9); the 

 reticular character of the spicules of the Echinothuridse is doubtless 

 to be associated with the comparative tenuity of their tests. 



* See Stewart, Quart. Journ. Micr. Sci. xi. (1871) pi. iv. 



