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Animals III - Pseudocoelomates and Protostome Coelomates

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You should have a working knowledge of the following terms:

  • annelid
  • bivalve
  • cephalopod
  • foot
  • gastropod
  • hydrostatic skeleton
  • invertebrate
  • mantle
  • mantle cavity
  • mollusk
  • nematode
  • nephridium (pl. nephridia)
  • radula
  • roundworm
  • segmentation
  • septa
  • visceral mass

Introduction and Goals

This tutorial continues our discussion of the major features of animal evolution, concentrating on the bilateria. Previously we introduced three phyla: Porifera, Cnidaria, and Platyhelminthes. These groupings encompass representatives of the parazoa, radiata, and bilateria, respectively. We will begin by distinguishing among the organisms with body cavities, the pseudocoelomates and true coelomates. By the end of this tutorial you should have a better understanding of:

  • How specific organisms coincide with the major branch points in the phylogenetic tree in the evolution of animals
  • Characteristics of organisms in Phyla Nematoda, Mollusca, and Annelida
  • How certain roundworms cause diseases in humans

Figure. 1(Click image to enlarge)

Coelomates and Pseudocoelomates

The figure on the right illustrates the three basic body plans encountered in the bilateria: the acoelomates (e.g., flatworms, flukes, and tapeworms), the pseudocoelomates (e.g., roundworms, pinworms, and hookworms), and the coelomates (e.g., snails, clams, octopuses, earthworms, and leeches). Recall, pseudocoelomates differ from coelomates in that their body cavities are not completely lined by mesoderm-derived tissue.

Figure. 2 (Click image to enlarge)

Coelomates: Protostomes versus Deuterostomes

The true coelomates are categorized as either protostomes or deuterostomes. The distinction is based on differences in their early cell cleavage, coelom formation, and the fate of the blastopore.

Figure. 1(Click image to enlarge)

As mentioned earlier, one of the main differences between the two is in the origin of their guts. As shown in the bottom of this figure, the mouth develops first in protostomes, whereas in deuterostomes the anus develops first (the mouth forms secondarily). This distinction may seem irrelevant, but from an evolutionary standpoint it is a distinct bifurcation.

Figure. 3(Click image to enlarge)

Protostomes also have spiral patterns and determinate development during their first cell divisions, whereas deuterostomes have radial patterns and indeterminate development. Deuterostomes will be discussed later. Next we'll begin our discussion of the protostomes in the phylum Mollusca.

Protostome Coelomates: The Phylum Mollusca

Animals in the phylum Mollusca include clams, snails, octopuses, and sea slugs (pictured right). They are all protostome coelomates.

Figure. 8 (Click image to enlarge)

Mollusks are characterized by their soft bodies, which are usually protected by a hard calcium carbonate shell. However, this shell can be highly reduced or completely absent in some representatives of the phylum.

Class Gastropoda (meaning "stomach foot") includes snails, slugs, sea slugs, and nudibranchs. Most snails have a coiled shell, however, the shell is completely absent in nudibranchs (pictured right). Gastropods have terrestrial, marine, and freshwater representatives. Terrestrial gastropods (e.g., snails) lack the gills that are characteristic of other mollusks.

Figure. 9(Click image to enlarge)

Class Bivalvia includes mussels, oysters, and clams (right). Bivalves have a reduced head and two hinged shells connected by strong adductor muscles.

Figure. 10 (Click image to enlarge)

Class Cephalopoda includes octopuses, squid (right), cuttlefish, and chambered nautiluses. Cephalopods have a head surrounded by tentacles, which can be used for locomotion and grasping prey. The shells (also called pens) of squid and cuttlefish are reduced and internal.

Figure. 11 (Click image to enlarge)

The largest invertebrates (animals without backbones) are the giant squids, which are extremely elusive and typically only encountered washed-up on the shore. The largest, documented giant squid was 18 meters long. The individual in this photo (right) is 9 feet long. More recently, a 33 foot-long squid, weighing 990 pounds, was caught in the waters off New Zealand.

Figure. 12 (Click image to enlarge)

These courting cuttlefish (right), from the class Cephalopoda, are a significant food source in many cultures.

Figure. 13 (Click image to enlarge)

Class Polyplacophora includes the chitons (right 0. These flat organisms are often observed adhering to rocks in the intertidal zone.

Figure. 14 (Click image to enlarge)

Mollusk Anatomy

The internal anatomy of a representative mollusk (a snail) reveals how much more complex these organisms are than representatives from any of the preceding Animalia phyla discussed.


In general, the mollusk body plan consists of a muscular foot, a visceral mass, and a mantle. This drawing (right) depicts the basic anatomy of a mollusk. The foot is used for movement (especially in the gastropods) or as an anchor (as observed in chitons). The visceral mass houses most of the internal organs (e.g., the stomach, gonads, and heart). The mantle is the tissue layer that covers the visceral mass. In organisms that have shells, the mantle produces the shell. Underneath the mantle is a mouth at one end and a mantle cavity at the other. The mantle cavity houses the anus and gills.

Figure. 15 (Click image to enlarge)

Mollusks characteristically have a radula (a rasping structure that is used to scrape food particles from hard surfaces). For example, the radula of mollusks cleans algae off of the surfaces in aquariums.

Visit the marine aquarium in the HUB on the University Park campus and look for the radula on the underside of the snails cleaning the walls of the tank.

Figure. 16 (Click image to enlarge)

Protostome Coelomates: The Phylum Annelida

Animals in the phylum Annelida include the earthworms, leeches, and many marine worms. These protostome coelomates exhibit true segmentation. Segmentation refers to body plans that are divided into discrete units, which may be repeating or may each have a unique function in the body.

Figure. 1 (Click image to enlarge)

It is easy to detect segmentation in earthworms (right). The segments are physically separated internally by thin sheets of mesoderm-derived tissue termed septa (singular: septum; recall, the term septa was also used to describe structures that delimit cells in the hyphae of fungi, i.e., septate hyphae). Because of these separations, earthworms can contract muscles in some segments without affecting the hydrostatic pressure in adjacent segments. This ability is highly advantageous for movement, and via coordinated contraction and expansion of segments, earthworms are able to burrow into the soil with ease.

Figure. 17 (Click image to enlarge)

This figure (right) depicts some of the internal segmentation in an earthworm. The bodies of most annelids consist of a series of repeating segments, with repetition of organ systems (muscular, nervous, reproductive, circulatory, and excretory) in the segments. Therefore, most annelids do not capitalize on one of the main benefits of segmentation, the ability to specialize. Note, many segments have multiple hearts, and each segment has a pair of nephridia (an excretory organ). The next tutorial will discuss how more advanced animals show extreme specialization by exploiting segmentation.

Figure. 18 (Click image to enlarge)

Your recent lab exercise covered many of the main anatomical features of the earthworm. Note how the longitudinal blood vessels, nerve cords, and digestive tract all run the length of the body, whereas the pumping vessels and ganglia are all arranged in segments.


This tutorial continued our discussion of the bilateria branch of the kingdom Animalia. When you think about this character (bilateral symmetry), keep in mind that it is observed in animals that actively move through their environment. Bilaterally symmetrical animals not only have a single plane of symmetry, but their sensory and cephalic areas are usually displaced toward the anterior end of the animal.