Fascioles (arrowed) are dense bands of specialised spines that form distinctive features of the tests of heart urchins. These spines are very small and closely-packed and end in a fleshy tip covered in mucus glands. They are also strongly ciliated along their shaft. In the living animal (above) the bands show up because of the fleshy tips of the spines, but the positions of fascioles are also easily picked out from the skeleton as continuous bands of very small and densely packed granules.
Fascioles are developed primarily in burrowing echinoids and serve two important functions. They are responsible for producing the mucous coat which infaunal heart urchins living in fine sediments require. This mucous coat emanates from the fascioles and is spread over the aboral surface above the spine canopy. It acts as a barrier preventing fine particles from falling in between the spines and thus clogging the burrow. The fasciole spines are also responsible for drawing water into the burrow for respiratory purposes. Currents created by densely ciliated fasciole spines pull water from the surface into the burrow and also pump water out to the rear of the animal into the sediment. The presence of fascioles in fossils is a clear indication that the animal was at least partially infaunal.
Fasciole arrangement is very important taxonomically. A few of the more important fasciole types are shown.