sand dollars have convex symmetry.
sand dollars have convex symmetry.
what type of symmetry do sand dollars have?
convex
within a group of organisms, taxa showing morphological extremes are favorite objects in constructional morphology, and the west african sand dollar is no exception. the west african sand dollar is a bilaterian (animals with bilateral symmetry) even though adults possess radial symmetry, because the larvae possess bilateral symmetry. since the organism is a bilaterian, it has three tissue layers: endoderm, ectoderm, and mesoderm. the sand dollar is part of the phylum echinodermata, and so the organism has an endoskeleton composed of calcareous ossicles which grow from the mesodermal tissue. this endoskeleton is covered in ciliated tissue. the west african sand dollar is a deuterostome, meaning that the blastopore forms an anus instead of the mouth. the organism has an oral surface with podia (tube feet) and ambulacral grooves and an aboral surface. these tube feet are not used for locomotion in sand dollars, but instead for gas exchange. instead, locomotion is achieved by movable spines. in the center of the test, the west african sand dollar has a feeding device called “aristotle’s lantern.” aristotle’s lantern is a complex system of jaws and muscles which are capable of a variety of feeding types including suspension feeding, herbivory and detritivory feeding, and occasionally predation. adaptations to this lantern have allowed sand dollars to live in habitats which have fine, shifting substrates. just like other sand dollars, the west african sand dollar has a shallow dome with a thin edge that is ideal for burrowing. equally important, as echinoids living on an unstable sediment surface swept by waves and currents, their form minimizes drag and facilitates maintenance of position. however, their low, domed profile acts as a hydrofoil and generates lift, which could send the sand dollar flowing into a harsh current. one adaptation to this generation of lift is the presence of lunules. lunules reduce lift by interfering with attachment of flow or by bleeding off pressure from the under surface. excess pressure on the oral surface of lunulate sand dollars is relieved by flow along pressure drainage channels which lead from the central region of the disc into the lunules and ambital notches. it is also hypothesized that lunules shorten the food path of sand dollars, however this is not fully supported by observation. west african sand dollars also have small spines that help them sieve food particles from sandy or silty sediments. these spines have lost their protection function and are used more for burrowing and feeding. types of sand dollar spines include: shoe spines (broad and tuberculate), ambulatory spines (longer than shoe spines on lower surface), frill spines (flatter than ambulatory and larger), non-ambulatory spines (food collecting spines of ventral surface), and miliary spines (small and short).
heliophora
bullet::::- myxosporea (marine parasites)









































bullet::::- scyphozoa (true jellyfish)









































bullet::::- staurozoa (stalked jellyfish)









































bullet::::- polypodiozoa (marine parasites)









































section::::cycliophora (tiny marine animals).









































bullet::::- eucycliophora









































section::::dicyemida (rhombozoa).









































bullet::::- dicyemida









































section::::echinodermata (starfish, sea urchins, sand dollars, sea lilies, and others).









































section::::echinodermata (starfish, sea urchins, sand dollars, sea lilies, and others).:crinozoa.









































bullet::::- crinoidea (sea lilies and feather stars)









































section::::echinodermata (starfish, sea urchins, sand dollars, sea lilies, and others).:asterozoa.









































bullet::::- asteroidea (star fish)









































bullet::::- ophiuroidea (brittle stars)









































section::::echinodermata (starfish, sea urchins, sand dollars, sea lilies, and others).:echinozoa.









































bullet::::- echinoidea (sea urchins)









































bullet::::- holothuroidea (sea cucumbers)









































section::::entoprocta.









































bullet::::- entoprocta









































section::::gastrotricha (hairybacks).









































bullet::::- gastrotricha









































section::::gnathostomulida (jaw worms).
list of animal classes