Sea Urchin 
Sea urchins are members of the phylum Echinodermata, which also includes sea stars, sea cucumbers, brittle stars, and crinoids. Like other echinoderms, they have five-fold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive "tube feet". The symmetry is not obvious in the living animal, but is easily visible in the dried test. Echinodermate means "spiny skin" in Greek.
Specifically, the term "sea urchin" refers to the "regular echinoids", which are symmetrical and globular, and includes several different taxonomic groups, including two subclasses : Euechinoidea ("modern" sea urchins, including irregular ones) and Cidaroidea or "slate-pencil urchins", which have very thick, blunt spines, with algae and sponges growing on it.
In most cases, the female's eggs float freely in the sea, but some species hold onto them with their spines, affording them a greater degree of protection. The fertilized egg, once met with the free-floating sperm released by males, develops into a free-swimming blastula embryo in as few as 12 hours. Initially a simple ball of cells, the blastula soon transforms into a cone-shaped echinopluteus larva. In most species, this larva has 12 elongated arms lined with bands of cilia that capture food particles and transport them to the mouth. In a few species, the blastula contains supplies of nutrient yolk and lacks arms, since it has no need to feed.
During early development, the sea urchin embryo undergoes 10 cycles of cell division, resulting in a single epithelial layer enveloping a blastocoel. The embryo must then begin gastrulation, a multipart process which involves the dramatic rearrangement and invagination of cells to produce the three germ layers.
The first step of gastrulation is the epithelial-to-mesenchymal transition and ingression of primary mesenchyme cells into the blastocoel.[13] Primary mesenchyme cells, or PMCs, are located in the vegetal plate specified to become mesoderm.[14] Prior to
Specifically, the term "sea urchin" refers to the "regular echinoids", which are symmetrical and globular, and includes several different taxonomic groups, including two subclasses : Euechinoidea ("modern" sea urchins, including irregular ones) and Cidaroidea or "slate-pencil urchins", which have very thick, blunt spines, with algae and sponges growing on it.
In most cases, the female's eggs float freely in the sea, but some species hold onto them with their spines, affording them a greater degree of protection. The fertilized egg, once met with the free-floating sperm released by males, develops into a free-swimming blastula embryo in as few as 12 hours. Initially a simple ball of cells, the blastula soon transforms into a cone-shaped echinopluteus larva. In most species, this larva has 12 elongated arms lined with bands of cilia that capture food particles and transport them to the mouth. In a few species, the blastula contains supplies of nutrient yolk and lacks arms, since it has no need to feed.
During early development, the sea urchin embryo undergoes 10 cycles of cell division, resulting in a single epithelial layer enveloping a blastocoel. The embryo must then begin gastrulation, a multipart process which involves the dramatic rearrangement and invagination of cells to produce the three germ layers.
The first step of gastrulation is the epithelial-to-mesenchymal transition and ingression of primary mesenchyme cells into the blastocoel.[13] Primary mesenchyme cells, or PMCs, are located in the vegetal plate specified to become mesoderm.[14] Prior to