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A plankton-eating larva, living and feeding in the water column, is considered to be the ancestral larval type for echinoderms, but in extant echinoderms, some 68% of species develop using a yolk-feeding larva. [11]
While both feeding (planktotrophic) and non-feeding (lecithotrophic) larvae exist among the four other extant echinoderm classes, all present day crinoids appear to be descendants from a surviving clade that went through a bottleneck after the Permian extinction, at that time losing the feeding larval stage. [21]
It represents the type of basis of all larval forms of, at least, the eleutherozoans (all echinoderms except crinoids), where the starfish, sea urchins, sea cucumbers and brittle stars belong. The dipleurula is a bilaterally symmetrical, ciliated echinoderm larva (cilia devoted to movement, feeding and perception).
The larvae of echinoderms have bilateral symmetry, but during metamorphosis this is replaced with radial symmetry, typically pentameric. [12] Adult echinoderms are characterized by having a water vascular system with external tube feet and a calcareous endoskeleton consisting of ossicles connected by a mesh of collagen fibres. [95]
Sea urchins or urchins (/ ˈ ɜːr tʃ ɪ n z /) are typically spiny, globular animals, echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal to 5,000 metres (16,000 ft; 2,700 fathoms). [1]
This form of reproduction allows certain species to enhance their survival. And lastly, larval development, following fertilization, brittle star larvae undergo various developmental stages before transitioning into adulthood. In some species, larvae may remain attached to the parent until they can sustain themselves.
Holothurians appear to be the echinoderms best adapted to extreme depths, and are still very diversified beyond 5,000 metres (16,000 ft) deep: several species from the family Elpidiidae ("sea pigs") can be found deeper than 9,500 metres (31,200 ft), and the record seems to be some species of the genus Myriotrochus (in particular Myriotrochus ...
Many species brood developing larvae in the bursae, effectively giving birth to live young. A few, such as Amphipholus squamata, are truly viviparous, with the embryo receiving nourishment from the mother through the wall of the bursa. However, some species do not brood their young, and instead have a free-swimming larval stage.