Lecture—17 September 2003

Theme of lecture--2^nd hypothesis related to the origin of multicellular organisms—the syncitial hypothesis

Before examining that hypothesis, let’s look at two groups of invertebrates, the Cnidaria and Platyhelminthes

Cnidaria

Three groups:

Hydrozoa—both polyp and mesdusal forms

Scyphozoa—Dominant medusae, small polyp

Anthozoa—Dominant polyp, medusae lacking

Diploblastic--Two adult layers—ectodermis and endodermis derived respectively from two germ layers ectoderm and the endoderm. Muscle fibers are associated with the basal portions of both the adult layers.

The layer between the two is called a mesoglea and is non-cellular

Sac-like, either polyp or medusae

The space in the middle of the body is called the Gastrovascular cavity

Incomplete digestive tract

Stinging cells that are called nematocysts

Has a larval stage called the planula that is cilliated

Platyhelminthes—Primarily three groups

Free living worms—planarian worms, acoels

Parasitic worms-flukes

Parasitic tapeworms

Triploblastic--Three layers germ layers

Ectoderm—gives rise to integument and nervous system, often source of the cells for reproduction

Endoderm—gives rise to gut and out pocketings from gut

Mesoderm—gives rise to muscles, excretory structures and matrix of material surrounding reproductive cells

Acoelomates as the Mesoderm is a parenchyma (packed mass of cells)

Digestive tract is incomplete or lacking (tape worms)

One of the simple forms are the acoel worms that essentially are small-worm like creatures lacking a “real gut”. The worms have a cilliated epidermis

The syncitial theory looks towards the Ciliophoran protists as the source group.

Trichoplax

Embryology