6) 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 [NOTE- recent evidence suggests this may be present in some sponges], and c) apical-basal polarity]
archaeocytes (=amoebocytes)- amoeboid cells that can transform into all other cell types (totipotent); special amoebocytes make spicules and spongin fibers (collagen); some may transport food and make eggs
contractile cells (=myocytes)- encircle oscula and flagellated chambers, can open and close oscula in response to environmental stimuli
choanocytes- in choanocyte chambers, function in pumping and feeding
porocyte- a tubular cell; forms ostia in some groups
7) 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); actual method still unclear
pinacocytes & amoebocytes may also phagocytize food particles
a few are predatory! spicules on surface act like velcro
8) Reproduction: budding and fragmentation are common
most 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 ciliated blastula develops inside the adult (brooding); the larva swims only briefly before settlement
overwintering cysts (gemmules): all freshwater spp., some marine spp.
9) Three classes of Porifera to note (a fourth class is not covered):
CALCARAE:calcareous spicules (3-rayed), no spongin, small: less than 15cm
DEMOSPONGIAE:silica spicules (many shapes, 4-rayed), spongin in some; most successful group (80-90% of living sp.); can reach >2m!
HEXACTINELLIDA: "glass sponges"; mostly deep sea taxon; can be large (>1m); silica spicules (6-rayed); lack: canal system, pinacoderm, spongin; body formed of syncytial sheets, even choanocytes lack nuclei! many bizarre features, even for a sponge
10) Phylogenetic relations unclear: clearly the sister group to the Metazoa; hexactinellids may even be the most primitive living metazoan