THE ANATOMY OF THE MADREPORARIA. 419 
Fic. 4.—Maprerora.—Typical section across a segment of a decalcified 
stem, to show the relations between the branching tubes, the ccelentera of the 
polyps, and the external body wall. The oral disc and tentacles of the 
polyp have been cut off, showing the stomodeum opening below into the 
ceelenteron and slung on the twelve mesenteries. Between the directive 
mesenteries are the grooves left by the removal of axial and abaxial septa. 
While the tubes lying directly under the body wall are hemicylindrical, the 
more central tubes are generally completely cylindrical or oval ; for an account 
of their formation, cf. Fig. 11, and p. 415. <A solid section in a plane at 
right angles to this is represented in the woodcut on p. 410. 
Fic. 5. HeTEROPSAMMIA MULTILOBATA.— View of the corallum from below, 
showing the pear-shaped, slightly flattened base, with the pore for the pro- 
boscis of the Sipunculid and the tiny pores round the bases of two of the 
calicles. 
Fic. 6. HetTEROPSAMMIA MULTILOBATA.—Transverse section through the 
body of the corallum, passing (at a lower plane than the polyp-cavities) 
through the spiral chamber tenanted by the Sipunculid. The arrow is supposed 
to have entered through the terminal pore. 
Fic. 7. HeTEROPSAMMIA MULTILOBATA.—Transverse section through the 
centre of a colony, showing parts of two adjacent polyps at a point where 
fission and consequent new growth of coral has occurred. The corallum is 
dotted, the mesogloea black (the endoderm has been omitted, except for a 
diagrammatic representation of the mesenterial filament). The four spaces 
marked with across are now empty, but have obviously at one time been 
mesenterial chambers. 
Fre. 8. HetTEROPSAMMIA MULTILOBATA.—Part of the transverse section 
represented in Fig. 6, passing from the external surface to the spiral chamber. 
The external body wall resting on the echinulations dips inwards to line one 
of the tiny pores discussed on pp. 418, 416. The soft tissues do not, however, 
line the spiral chamber itself. Ectoderm ‘blocked ;” mesogloea, a black 
line; endoderm, light grey ; corallum, dotted. 
Fig. 9, HeteROPSAMMIA MULTILOBATA.—The external layer of soft tissues 
stripped off, and viewed from the exterior (7. e. the tissues represented dia- 
grammatically in Fig. 11, a), showing the nearly vertical tubes under the 
body wall, with transverse connecting tubes between the echinulations. 
Fic. 10. BarHyacTIs sYMMETRICA.—Diagrammatic vertical section, to 
show the general anatomical relations. The basal body wall is supported on 
the peripheral lamelle of the mesenteries, as in the corals with a vertical 
theca, and is similarly composed. 
Fig. 11.—Diagram showing the formation of the cylindrical internal tubes 
of a Perforate coral by growth and fusion of adjacent echinulations across the 
hemicylindrical tubes underlying the body wall. (Cf. p. 415.) 
