The simplest animals
Animal Evolution
 1st animal fossils found in
Period (650-545 mya)
 by beginning of Cambrian period (545 mya) all modern animal phyla
found in fossil record
 Vendian animals were:

or looked like:

molecular data calculates that animals evolved probably 300-700 million
yrs. earlier
Phylum Placozoa
 simplest animals
 1 known species (
 phylogeny debated – before sponges or after
 made of just a few 1000 cells of:
transparent, 3mm in size, hard to see
 tropical and subtropical marine envs

Phylum Porifera
 sponges

sp.
 marine organisms, some freshwater (150 sp.)
 dead-end in evolution
General characteristics:
1)
2)
3)
4)
5)
6)
Cell types:
 can be found in gelatinous matrix called:
) & possibly more
1)
- flat, thin cells that line outer surface
 some are contractile which changes shape of sponge
2)
3)
- pore cells - regulate water circulation
- collar cells line inner cavities
 involved in water circulation & filtering water for food
4)
- found in the mesohyl
 amoeboid cells that perform many functions:




Energy acquisition

diet is mainly bacteria, phytoplankton, zooplankton & suspended organic
matter

filter food by trapping it on collar. food
particle is then carried down collar & phagocytized

digest larger particles of food
 dissolved nutrients may be absorbed thru active transport

Reproduction
Asexual - freshwater & some marine sp

- mass of amoeboid cells w/in a capsule of spicules
 released when parent dies in winter
 in spring, amoeboid cells leave capsule & develop into sponge
 budding - small bud grows from parent & breaks off to form new
individual
Sexual Reproduction
 sponges usu.
 no self-fertilization because eggs & sperm not produced at same time
 some choanocytes undergo meiosis & become:
 either choanacytes or archaeocytes undergo meiosis & become:
 eggs stay in mesohyl, sperm exit thru osculum to another sponge enter
with incurrent water
sperm trapped by choanocytes & incorporated into vacuole
 choanocytes become amoeboid & transport sperm to egg
 zygote usu. develops in mesohyl to flagellated larval stage
 larvae swim away from parent sponge & settle down after a brief period
 develop into adult body

Mesozoa
 very small animals, ranging from 0.5 - 7 mm in length
 typically just 20-30 cells in all.
 worm-like with a ciliated epidermis.
 parasitic in different marine invertebrates.
Phylum Rhombozoa
 sexual and asexual stages to their life cycle
 parasitizes:
Phylum Orthonectidea
 parasitic on:
 asexual stage called a: