Protostome Evolution

The Protostomes are divided into two groups:

1. The Lophotrochozoa - which includes the segmented worms, molluscs, lophophorates and several smaller phyla.
2. The Ecdysozoa - which includes the arthropods and several other phyla that periodically molt.


The name Lophotrochozoa comes from the names of the larval type of the two major animal groups included: the Lophophorata and the Trochozoa.


The Trochozoa contain many worm-like forms, including the segmented worms (Phylum Annelida) but it also includes the Mollusca (snails, squid, octopods, clams, etc). It might seem strange at first to group earthworms and squid together. They don't look much alike as adult but many annelids and molluscs share patterns of development in early embryonic stages. When these larvae hatch, each is a microscopic swimmer known as a trochophore larva:

The larvae of these two groups is nearly identical: with two bands of cilia around the middle that are used for swimming and for gathering food, and at the "top" is a cluster of longer cilia.

The Annelids - A review of their basic body plan

The annelids are the segmented worms (earthworms, leeches, and polychaetes).

In annelids all of the body, except the central nerve tracts, main blood vessel, and digrestive tract are segmented.
Each segment is, limited by septa dividing it from neighbouring segments, and has a fluid-filled coelom. Structures such as the excretory, locomotory and respiratory organs are generally repeated in each segment. Segments are formed sequentially in annelids and are established during development from growth zones located at the posterior end of the body; so the youngest segment in the body of an annelid is always the most posterior. The only parts of the annelid body that are not segmental are the head and a terminal post-segmental region called the pygidium.

A second feature found in all annelids are structures called chaetae or setae. These are bundles of chitinous, thin-walled cylinders held together by sclerotinized protein. .

Segments together with chaetae allow more efficient locomotion and permits specialization of body parts.

More efficient locomotion: Each segment has its own separate set of muscles (run circular and longitudinal in each segment; no cross-striations) that can bend or stretch each segment independently.

In Annelids, the nervous system consists of a "brain" or dorsal ganglion and two circumpharyngeal nerve cords that course down the ventral surface of the body.

The body is covered in thin, flexible, permeable chitinous cuticle that stretches as the animal grows.

Larger body size means more organ systems.

Annelids probably originated in the Precambrian but the first definite fossils are known from the Cambrian (the earliest part of the Proterozoic) fossil deposites in Canada known as the Burgess Shell.

Annelida are into three major groups; Polychaeta (marine worms), Oligochaeta (earthworms etc.) and Hirudinea (leeches). Earthworms and leeches are the familiar annelids for most people, but polychaetes comprise the bulk of the diversity of Annelida and are found in nearly every marine habitat, from intertidal algal mats downwards.

Phylum Mollusca

The molluscs include a variety of familiar animals such as snails , clams , abalone, octopus and squid. There are about 70,000 described species within this phylum.

The coelom is reduced to a small space around the hearts , gonads , and metanephridia ( kidney -like organs). The principal body cavity is a blood-filled sac surrounding the organs.

Molluscs have a mantle, which is a fold of the outer skin that produces the calcium carbonate shell, and a muscular foot that is used for motion. Many have a feeding structure, the radula , mostly composed of chitin .

There are a variety of body forms:

The Molluscs are thought to have evolved just before the Cambrian and diversified throughout the early Proterozoic. One of the major groups, the Ammonites, were major predators in the sea before the rise of jawed fish.


AMMONOIDS are similar to the modern chambered nautilus. These extinct marine animals thrived in the Paleozoic and Mesozoic Eras, some 400 to 65 million years ago. Ammonites were able to swim, thanks to the unique construction of their shell, which was divided into a series of air chambers. The air in the chambers provided buoyancy for the animal to float; like modern cephalopods, they probably moved through the water using jet propulsion. Because ammonites lived exclusively in marine environments, their presence also indicates the location of prehistoric seas.



Mostly marine; sedentary or sessile.

Because they are sessile, you might gues that they are filter feeders and you would be right -- they have a set of ciliated tentacles (the lophophore) that surround a U-shaped digestive tract:


The Lophophorates include:

1. Pterobranchs - lophophorates living in small colonies of secreted tubes in the sand or mud or attached to rocky substrates.
Bryozoans - lophophorates living in large colonies in secreted houses
Brachiopods (phylum Brachiopoda) superficially resemble bivalve molluscs but contain a lophophore.
  1. do not extrude the lophophore - water is sucked in and food particles removed.
  2. were plentifl in Paleozoic and Mesozoic, but today are only about 350 species.