• 1) Specialized cell types 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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