Figure 12.13: An echinoderm, the giant California sea cucumber. ( 9 )digestive system, often leading directly from mouth to anus. They also possess an open and
reduced circulatory system, but no heart. Their nervous system consists of a modifiednerve
net(interconnected neurons with no central brain).
In most species, eggs and sperm cells are released into open water, where fertilization takes
place. The release of sperm and eggs is coordinated temporally (to occur at the same time) in
somespecies andspatially (tooccur withinthe same location)in others. Internalfertilization
takes place in a few species. Some species even have parental care!
Many echinoderms have amazing powers of regeneration. Some sea stars are capable of
regenerating lost arms, and in some cases, lost arms have been observed to regenerate a
second complete sea star! Sea cucumbers often discharge parts of their internal organs if
they perceive danger. The discharged organs and tissues are then quickly regenerated.
Feeding strategies vary greatly among the different groups of echinoderms. Some are passive
filter-feeders, absorbing suspended particles from passing water; others are grazers; others
are deposit feeders, which feed on particles of organic matter, usually in the top layer of soil,
and still others are active hunters.
Types of Echinoderms
Theechinodermsaresubdividedintotwomajorgroups, theEleutherozoa, whichcontainsthe
more familiar, motile classes, and the Pelmatozoa, which contains the sessile (permanently
attached and not freely moving) crinoids, including the feather stars (Figure12.14), which
have secondarily developed a free-living lifestyle.