246 ZOOLOGY 
and contains small scattered calcareous ossicles. The mouth 
is surrounded by a circlet of retractile tentacles, into which the 
water-vascular ring sends extensions. The madreporie plate 
usually opens into the body-cavity. The anus is usually 
terminal. 
The body of the Holothurians is elongated along an oral- 
apical axis. The ambulacra are five in number; they may be 
equally developed, or three of them, 
the trivium, may be flattened and 
form a creeping sole upon which the 
animal rests; the bivium is then con- 
vex. This occurs in Psolus and in all 
the Elasipoda. When this specialisa- 
tion of radii takes place, the tube-feet 
are modified on the trivium. In other 
cases the tube-feet are scattered all 
over the body, and in others—the 
Synaptae—they are entirely wanting. 
The skin is covered by an ectoderm 
with an external cuticle; within this 
is a layer of connective tissue, in which 
cells laden with pigment and cal- 
eareous ossicles are scattered. This 
layer also includes a nervous plexus. 
The connective tissue sheath surrounds 
a muscular layer whose fibres run in 
a circular direction, and more inter- 
nally are five radial bands of longi- 
tudinal muscles, one running along each 
ambulacrum, and lying beneath the water-vascular vessel and 
nerve ; anteriorly these bands are attached to the pharyngeal 
ossicles, which are radial and interradial in position. The 
ossicles in the integument are always small in size; they may 
be simple spicules, or may assume a number of very elegant 
forms in the different genera. In the Elasipoda they exist 
in the mesenteries and in the walls of the alimentary canal, 
as well as in the integument. 
The coelom is large, and is lined with ciliated cells; a 
special section of it surrounds the pharynx, and in the outer 
Fic. 148.—Holothuria 
papulosa. 
