ALIMENTARY CANAL. 679 



spicules proper are more common, and are especially numerous in the 

 mesenteries and the enclosing wall of the pyramid of the jaws. 



Hoffman has been the first to call attention to a peculiar calcareous sup- 

 port found in Spatangoids, situated near the actinostome, on the inner surface. 

 It consists of an irregularly shaped triangular or rectangular projection of 

 the test, placed in the left posterior interambulacral space (PI. XXXIV. f. 

 2, 4), from which the main mesenteries of the alimentary canal take their 

 origin. This support seems peculiar to the Spatangina, and is not to be 

 homologized with the prominences of the interambulacral spaces next to the 

 actinostome in Nucleolidae (see PI. XV. f. 1 ; PI. XXXIII. f. 1); these ap- 

 pear, judging from the mode of formation of the auricles of some Clypeas- 

 troids, to be rudimentary auricles. The auricles of those Scutellidae in 

 which the reticulation of the inner actinal floor is very marked (PI XIII c .f. 

 .;). shows that they are mere processes of the interambulacral plates (see 

 also PI. XII". f. 4; PI XIII". f. 5; PL XIIF.f. &). 



