50 HOWARD AYERS. 
EXPLANATION OF PLATE V, 
Tllustrating Mr. Howard Ayers’s Paper, “On the Structure 
and Function of the Spheridia of the Echinoidea.” 
Fic. 1.—A surface view of a living spheridium of Echinus melo, show- 
ing the outer openings of the canal system. The microscope was focussed on 
the surface of the calcareous body just below the epithelium, which is seen in 
optical section at the sides of the body. 
Fic. 2.—A spheridium of Strongylocentrotus droebachiensis de- 
prived of its soft tissues by treatment with caustic soda, The concentric 
lamelle of the calcareous body appear as layers of unequal thickness in 
different parts of the globule. The canal system is not figured. 
Fic. 3.—The same as Fig. 2, but of another globule. The calcareous body 
is seen to be made up of three very thick concentric plates, enclosing a 
central spherical body. 
Fie. 4.—A spheridium of Strongylocentrotus droebachiensis, 
showing the manner in which the superficial layers of the body grow into 
projecting tubercles. Caustic soda preparation. 
Fies. 5, 6, 7.—Three spheridia from the same species, treated with caustic 
soda to remove the soft parts, washed in water and 95 % alcohol, then heated 
to expel all liquid. The globules were then mounted in hard Canada balsam, 
warmed sufficiently to allow the bodies to sink into the balsam, but not 
enough to cause the balsam to enter the canals. The canals are by this 
means left filled with air, which owing to its different refracting power from 
that of the calcareous matter and of the balsam, causes the canals to appear 
as black lines and rods. In Figs. 5 and 6 the balsam has entered the outer 
branches of the canal system, and in consequence the canals here can be 
distinguished merely by the line of contact between the balsam and calcareous 
substance. 
Fie. 8.—A part of the canal system of a spheridium of Strongylo- 
centrotus droebachiensis, projected on the plane of the paper. Caustic 
soda preparation. 
Fic. 9.—A spheridium of the same species, treated with diluted HNO®, 
The calcareous matter has been partly dissolved, thus exposing the internal 
canals as grooves. At the edges of the body they appear as notches in the 
body. 
Fie. 10.—A spheridium of Strongylocentrotus droebachiensis, 
which has been treated with caustic soda, washed in water and 95 % alcohol, 
placed in an aqueous solution of safranine, passed through absolute alcohol 
and clove oil and mounted in balsam. The canals were filled with the stain- 
