STRUCTURE AND FUNCTION OF THE SPHA@RIDIA. 41 
distance from the peristome; in Strongylocentrotus and 
Echinus, for example, they extend to the summit of the curve 
of the test as the Urchin lies abactinal surface downwards. 
Among these forms they are situated near the edge of the plate 
nearest the middle of the ambulacral zone, and are seldom more 
than one to the plate. Among other forms (Spatangus, Cly- 
peaster) they vary greatly in position, and often occur two and 
three tothe plate. In Spatangus and Echinocardium they may 
be placed in a cup-like depression near the dendritic organs of 
the peristomial area. Among other genera of the Spatangoid 
group they are to be found most numerous on the plates of the 
bivium in a succession of pockets, while on the plates of the 
trivium they are confined to groups of two or three near the 
base of the tentacles. The depressions in the calcareous plates 
mentioned above are changed into closed cavities in some 
species. 
The process consists mainly in the deepening of the cup, but 
is also accompanied by an overgrowth of the edges of the cup. 
Such closed cavities are found among the Clypeastroids, Cassi- 
dularians, and Spatangoids (Lovénia). In by far the larger 
number of the species the spheridia are on the free surface of 
the test (Regularia and most Spatangoids). 
These organs are easily recognisable with the unaided eye, 
and are about half a milimetre in length, and from one fifth to 
a quarter of a milimetre thick. They increase in number (but 
not materially in size) with age, and are renewed like the spines. 
In the living state the organs are usually colourless, and reflect 
light with the brilliancy of a diamond, but if exposed to the 
air for a time they lose their reflecting powers, and acquire 
a leaden hue, due to the drying of the epithelial layer. 
The calcareous globule remains unchanged by drying. The 
movements of the sphzeridia consist of rotating and jerking 
stomial plates very close to the peristome; among other Regularia they are 
frequently numerous. I have counted fifty on the test of anadult Strongylo- 
centrotus droebachiensis, and Echinus melo possesses even more. 
None have as yet been found in Cidaris; and the Clypeastroids and Spatan- 
goids are not abundantly supplied with these sense organs. 
