Wednesday, August 16, 2006

Porifera

PORIFERA
from UCMP website

"An early branching event in the history of animals separated the sponges from other metazoans. As one would expect based on their phylogenetic position, fossil sponges are among the oldest known animal fossils, dating from the Late Precambrian.
Poriferans don't have mouths; instead, they have tiny pores in their outer walls through which water is drawn. Cells in the sponge walls filter goodies from the water as the water is pumped through the body and out other larger openings. The flow of water through the sponge is unidirectional, driven by the beating of flagella which line the surface of chambers connected by a series of canals"

Fossils: "the first probable sponge, Paleophragmodictya, from the Ediacara of Australia"

Sponges reproduce by both asexual and sexual means. Most poriferans that reproduce by sexual means are hermaphroditic and produce eggs and sperm at different times.

"Sponges have a cellular grade of organization. They do not possess any structures that can be considered organs. For instance, sponges do not have stomachs or kidneys. Instead, sponge cells of various types are responsible for bodily functions, the day-to-day activities that sustain life. Many of the most common types of cells are illustrated below in a cartoon view of the wall of a poriferan. "

http://www.ucmp.berkeley.edu/porifera/pororg.html here

Choanocytes (one of the 7 types of cells found in sponges)
"These distinctive cells line the interior body walls of sponges. These cells have a central flagellum that is surrounded by a collar of microvilli. It is their striking resemblance to the single-celled protists called choanoflagellates that make many scientists believe that choanoflagellates are the sister group to the animals. Choanocytes are versatile cells. Their flagella beat to create the active pumping of water through the sponge, while the collars of the choanocytes are the primary areas that nutrients are absorbed into the sponge. Furthermore, in some sponges the choanoflagellates develop into gametes. "


sponges: no nervous system; cells not organized into tissues; toti-potent cells, any cell can grow a new sponge. no distinction between germ & soma, as in eumetazoa [the rest of the metazoa].

sponge & placyzoa most primitive living group of multi-cellular animals; sometimes classed as parazoa for this reason
Dawkins, AT, p.497-501

porifera (fm choanomonads),ancestor to coelenterates, Mayr, WEI, p52-3

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