15 September--Multicellularity

What are the limitations on cells size for unicellular organisms?

Partially related to cell processes such as

• Diffusion--movement of a substance from area of higher concentration to lower
  Osmosis-movement of water through a  selectively semi-permeable membrane from area of higher concentration to lower concentration
• Partially related to consumption and being consumed

Multicellularity as a compromise between large size and the problems associated with diffusion and osmosis.With multicellularity can come a division of labor

How do cells stick together

• physical connection--
  
• Intercellular cement
  

Tissue-a collection of cells that perform a similar function
specialization, differentiation, cellular morphogenesis

Sponges--colony, do have simple tissues, but no "organs or organ systems" probably evolved from a group of sarcoflagellates.

ciliates--mitotic division, syncitial
longitudinal versus transverse binary fission
some ciliates remain together after division.

Flatworms

acoels--small number of cells, the outer layer is cilliated, the inner layer is syncitium, and the mouth is simply a pore through the body wall. outer layer of cells and inner undifferentiated mass of cells.

Germ layers

• ectoderm
• endoderm