ON ECHINODERMATA. 541 
more or less considerable, which are left below or above, by 
the assemblage of the coronal part. 
The coronal pieces are subdivided into ten groups or series, 
which radiate from one orifice to the other, a little like the 
ribs of melons, and which form areas alternately full and per- 
forate, equal or unequal. The name of ambulacraria, is 
given to the series which are perforate, and that of anambu- 
lacraria, to those which are not. 
The anambulacraria are themselves constantly formed of 
two series of pieces, more or less hexagonal, and usually 
transverse, which are united at one extremity in the middle 
of the anambulacrarium, and at the other, but less angularly, 
with the ambulacraria. Each piece is raised at its external 
surface, with a variable number of mammillz, more or less 
projecting, well-rounded, polished at their summit, and wi- 
dened at the base, without any trace of perforation. 
The ambulacraria, sometimes more narrow than the others, 
are, nevertheless, likewise formed of two series of polygonal 
pieces, united angularly together in the middle line of the 
ambulacrarium, and externally with the pieces of the anam- 
bulacraria. They are also raised with mammillz, more or 
less salient; but, besides, they are pierced at their external 
side by pores, variable in number, and in disposition, for each 
species, but which always traverse the testa from one part 
to the other. This is what constitutes the ambulacra, pro- 
perly so called. 
The breadth of the anambulacraria is generally greater at 
the middle than at the two extremities; but this is not the 
case with the ambulacraria. They are always larger towards 
the mouth, and the last presents at the interior a sort of apo- 
physis or lamina, pierced with a hole in its middle, and which 
affords an attachment for the motor muscles of the teeth. 
These are named auricule. 
The skin which surrounds the mouth is scarcely rough ; 
