3 12 
E CHINODERMS. 
between the membrane round the mouth and the plates of the test. In the 
irregular urchins some tube-feet are modified for respiration, becoming broad, 
flat, and somewhat lobed; the hinder end of the intestine seems to be respiratory 
in function. Some sea-urchins possess eyes. In a Diadema there are five 
ovate pigment-masses of a brilliant ultramarine blue, placed at equal distances 
around the vent. There are certain other peculiar bodies supposed to be sense- 
organs of some kind, called sphseridia, which are of microscopic size, and in 
structure not unlike tiny spines. They lie near the mouth and on the lower 
ambulacral plates, are often set in small holes, and are provided with special nerves. 
Perhaps they test the water in which the sea-urchin lives, and thus may be said 
to serve the sense of smell. Sea-urchins are both animal and vegetable feeders, 
and are even cannibals when opportunity offers. 
The Sea-Cucumbers, —Class Holothuroidea. 
Sea-cucumbers are, as we have seen, elongated and worm-like creatures, with 
a mouth at one end and a vent at the other. The skin is leathery, and 
contains a comparatively small amount of calcareous matter. Usually this 
occurs in small spicules, which assume very definite shapes, such as the anchors of 
Synapta, or the wheels of Chiridota ; but in such forms as Psolus the spicules 
increase in size, so as to form a plated integument. 
There may also often be a ring of calcareous plates 
round the gullet, five of which plates have the same 
relation to the radial water-vessels as the auricles around 
the jaws of a sea-urchin, and they likewise serve for the 
attachment of muscles. In such a common form as 
Cucumaria planci there are five rows of tube-feet pass¬ 
ing from mouth to vent. The five-rayed symmetry is 
not obscured, and is traceable in the arrangement of 
nerves and muscles, although it does not affect any 
portion of the digestive or generative systems. Around 
the mouth, in Cucumaria, is a fringe of branched 
tentacles, connected with the water-vascular ring. In 
most other echinoderms, it will be remembered, a canal passes from this ring and 
opens to the exterior by a madreporite; and in a few liolothurians of primitive 
structure this is similarly ( the case. But in Cucumaria, as in most, the connection 
with the exterior is lost, and the canal, with its madreporite, hangs down into the 
body-cavity. In Cucumaria the tentacles are used like a net to intercept floating 
organisms in the surrounding water. Many liolothurians swallow a great deal of 
sand, and the intestines of those that live near coral-reefs generally contain 
fragments of coral. They usually attach themselves by their tube-feet to rocks 
or seaweed, and wave the tentacles around. Holothuria atra, which lives on the 
great Australian barrier-reef, inserts its hinder extremity within a crevice of the 
rock, into which on being disturbed it speedily retreats. 
Some curious modifications of form have taken place among the liolothurians. 
In the plated sea-cucumbers (Psolus), of which a specimen is illustrated on p. 314. 
U-SHAPED SEA-CUCUMBER 
(§ nat. size). 
