78 STAR-FISHES, SEA-URCHINS, ETC. 
obstructions from the surface of the test, where this 
is necessary, can be easily seen. 
SEA-CUCUMBERS. 
The sea-cucumbers, or holothurians, constitute an 
interesting group of animals whose members live 
both upon the rocks and buried in the sand or mud. 
Although so different in gen- 
eral appearance, they repre- 
sent only extreme modifica- 
tions of the structure seen in 
the star-fish or the sea-urchin. 
Take for example that singu- 
larly attractive creature the 
Synapta, whose elongated 
leech-like body can be se- 
cured from the mud-flats by 
the aid of a garden-trowel, and 
examine it. The transparent ~ 
cylindrical form, permitting 
the yellow intestinal canal to 
be clearly visible in the in- 
terior, shows at first little to 
connect it with either star-fish or sea-urchin, but 
soon you will perceive five well-defined bands 
traversing the length of the body from one ex- 
tremity to the other. These are indeed the am- 
bulacra, although in this instance the tubes are 
closed and, so far as locomotion is concerned, func- 
tionless. In its fundamental structure, therefore, 
the sea-cucumber is only a greatly elongated sea- 
urchin, being pushed out axially, as it were, to its 
SYNAPTA. 
