| Zoology 250 (2002) PORIFERA II: Cell types, feeding & development (Porifera study images; click on tree to see full cladogram) |
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- 1) Specialized cell types -- the "lego blocks" -- of sponges include:
- pinacocytes- form the pinacoderm= outer covering [not a true epithelium because cells lack: a) intercellular junctions (cells can pull apart!), b) a basal lamina, and c) apical-basal polarity]
- porocytes- tubular cells that form incurrent pores (ostia) & canals
- mesenchyme cells- amoeboid cells that make spicules, spongin; can transform into other cell types; may transport food & make eggs
- contractile cells- encircle oscula, can open & close oscula in response to environmental stimuli
- choanocytes- pumping, feeding
- 2) Choanocytes feed like choanoflagellates:
- they phagocytize very tiny particles: bacteria, spores, gametes, fine organic debris, etc.; most not visible in a light microscope!
- particles are trapped on the 'collar' of microvilli (remember: sponges have no mouth and no gut)
- pinacocytes & amoebocytes may also phagocytize food particles
- 3) Reproduction
- sponges are hermaphroditic (sperm from choanocytes, eggs from choanocytes or mesenchyme cells)
- fertilization is internal! (collar cell captures sperm, carries to egg)
- early cleavage is radial; in most a blastula larva develops inside the adult; the larva swims only briefly before settlement
- 4) Phylogenetic relations
- Sponges are clearly the sister group to the Metazoa
- Hexactinellid sponges may be the most primitive living metazoan, they have many bizarre features, even for a sponge
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Copyright © 2002 by A. Richard Palmer. All rights reserved.
(revised Feb. 3, 2002)