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Animals IV - Arthropods, Echinoderms

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You should have a working knowledge of the following terms:

  • arachnid
  • arthropod
  • cephalothorax
  • chelicerae
  • chelicerate
  • compound eye
  • crustacean
  • cuticle
  • echinoderm
  • endoskeleton
  • jointed appendage
  • mandible
  • molting
  • pedipalp
  • thorax
  • trilobite
  • tube feet
  • uniramian
  • uniramous
  • water vascular system

Introduction and Goals

The previous tutorial concentrated on coelomates collectively known as the protostomes. This tutorial will complete our analysis of the protostome coelomates by examining the most diverse branch of animals, the arthropods. The second half of this tutorial will introduce the last major lineage, the deuterostomes.

Figure. 1 (Click image to enlarge)

As you progress through this tutorial, try to distinguish between the arthropods and other animals classified under the bilateria. How are these groups of animals similar? What morphological and developmental patterns do they have in common? How do they differ? Once you complete this tutorial you should be able to:

  • Explain why the animals in Phylum Arthropoda are thought to be so successful
  • Name and discuss the major classes and subgroups of Phylum Arthropoda
  • Describe the embryonic characteristics of deuterostomes
  • Describe the major characteristics of Phylum Echinodermata

Arthropod Diversity

In terms of numbers and the diversity of species, Phylum Arthropoda is the most successful group in Kingdom Animalia. Whether one looks on the land, in the air, or under the sea, they will find representatives from this phylum. Approximately one million arthropod species have been described to date. (Some estimate that the number of uncharacterized species may exceed 10 million.)

Figure. 2 (Click image to enlarge)

Characteristics of Arthropods

The extreme diversity observed in Phylum Arthropoda can be attributed to three main arthropod characteristics that have evolved into various forms to allow for adaptation to different environments: a hard chitinous exoskeleton, body segmentation, and jointed appendages.

Figure. 3 (Click image to enlarge)

The arthropod body is covered by a cuticle (structurally different from the cuticle of plants, which was described in tutorial 31, but there are some functional similarities), composed primarily of the polysaccharide chitin (the same type of polymer used by fungi as described here), but with a harder character; chitin is also described in the carbon and life tutorial. This exoskeleton endows arthropods with several adaptive features to the terrestrial environment. Its physical durability protects the animals from physical injury. It also provides structural support to the muscles that move their appendages. Lastly, the cuticle is waterproof and helps prevent desiccation in a dry terrestrial environment. The only cumbersome feature of the exoskeleton is that it confines growth, but arthropods deal with this problem by periodically shedding their exoskeletons in a process known as molting.

Arthropods are segmented, and generally there are distinct boundaries between the segments. For example, animals belonging to Class Insecta have three distinct segments: the head, the thorax (often grouped together with the head as the cephalothorax), and the abdomen. The rhinoceros beetle in the adjacent image has a head and thorax that are fused (a cephalothorax) and a large abdomen. Typically, these different body regions have distinct functions and often contain various types of jointed appendages. Jointed appendages afford the animal with a greater degree of movement. In addition to locomotion, the appendages may be adapted for other functions (e.g., feeding, sensory perception, copulation, and defense).

Figure. 4 (Click image to enlarge)

Some jointed appendages, such as the swimming appendages of this candy cane shrimp (a crustacean), are specialized to enable different species to adapt to their diverse environments.

Figure. 5 (Click image to enlarge)

This figure illustrates the body segmentation (cephalothorax and abdomen) and specialized appendages of a representative arthropod, a lobster (a crustacea). In addition to segmentation, arthropods have an open circulatory system.

Figure. 6 (Click image to enlarge)

Phylum Arthropoda: Classification

The arthropods are traditionally divided into four subgroups or subphyla: trilobites (all are extinct, but the fossil record indicates that they were once the dominant subgroup), chelicerates, crustaceans, and uniramians. As is the case for many taxonomic fields, however, newer molecular approaches are causing scientists to reconsider traditional relationships. As a student, this can create some confusion; hence, for clarity this course will use the traditional taxonomic schemes. However, this taxonomy will likely change as more molecular data provide a clearer insight into arthropod relationships.

Figure. 7 (Click image to enlarge)

The majority of modern chelicerates (e.g., spiders, scorpions, ticks, and mites) are terrestrial arthropods belonging to Class Arachnida. As with the trilobites, most of the marine chelicerates are extinct, even though a few marine members (e.g., the horseshoe crab) have survived. The arachnids (like this tarantula) are best distinguished by their claw-like feeding appendages (chelicerae), from which this subgroup gets its name. Another characteristic of the chelicerates is the presence of two body segments (a cephalothorax and an abdomen). The cephalothorax has six pairs of appendages, including four pairs of walking legs, one pair of chelicerae, and one pair of pedipalps. The pedipalps have either a feeding or sensory function. They lack antennae and have simple eyes.

Figure. 8 (Click image to enlarge)

The crustaceans make up the second largest arthropod subgroup. Extant species are mainly aquatic animals, although some terrestrial species (e.g., pill bugs and wood lice) are classified within this group. Animals in this class include crabs, lobsters, crayfish and shrimp, and they are the only arthropods with two pairs of antennae. Segmentation is obvious and extensive in these animals. In contrast to the chelicerates, crustaceans have jaw-like mandibles and compound eyes. The fiddler crab (shown here) is a typical crustacean.

The uniramians, the largest extant subgroup, represent so many individual species that this subgroup accounts for the majority of all known (and probably most of the still undiscovered) animal species on the planet. The uniramians have mandibles and compound eyes (as do the crustaceans). Uniquely, they have only one pair of sensory antennae and their appendages are unbranched or uniramous, hence their name. Uniramians include Class Insecta, with its twenty-six described orders. Insects first appear in the fossil record about 400 million years ago. Insects have the characteristic uniramian features, and additionally, they have three distinct segments (head, thorax, and abdomen). Two pairs of wings and three pairs of legs are typical. Scientists who study insects are called entomologists. Class Insecta is the largest class in the phylum Arthropoda, and their diversity is unmatched in the kingdom Animalia.

Figure. 9(Click image to enlarge)

The Other Coelomates: Deuterostomes

To understand the evolutionary history of animals, scientists rely on several types of data including, but not limited to, the following: the overall morphology of the organism, DNA sequences and similarities at the molecular level, and the developmental sequence of events in the embryo.

The sequence of events during embryonic development is the main characteristic used to distinguish protostomes and deuterostomes. Review this figure, which depicts the major developmental characters that distinguish a protostome from a deuterostome. Recall that in protostomes the mouth forms first, whereas in deuterostomes it forms secondarily. Another important distinction is how the coelom forms; in protostomes it forms from a splitting of the mesoderm (schizocoelous), whereas in most deuterostomes the coelom forms from mesodermal outpocketings of the archenteron (enterocoelous).

Figure. 10 (Click image to enlarge)

There is some debate about which phyla belong in the deuterostome lineage. In this and the next tutorial we will focus on two phyla that clearly fall into this lineage, Echinodermata and Chordata.

Phylum Echinodermata

Representatives of the phylum Echinodermata are common inhabitants of coastal tide pools (or even our local saltwater aquarium at the HUB). Sea lilies, sea stars (starfish), and most other echinoderms are sessile or slow-moving animals. Some representatives are instantly recognized by their five-fold symmetry (having rays and/or arms in fives or multiples of five). Even though the radial symmetry of these adult animals would lead one to reject them as belonging to the bilateria, the bilateral symmetry of the larval stage of these animals indicates that they really belong to the bilateria lineage. Furthermore, these animals are also known as coelomates since they have a large, fluid-filled cavity lined with mesoderm and usually a complete gut. Many species of sea stars have the ability to turn their stomachs inside out by everting them through their mouths. This allows them to initiate the digestion of their prey before introducing it to their body cavities.

A thin skin overlaying a hard, yet flexible, endoskeleton composed of calcium carbonate plates and spines characterizes the exterior of echinoderms. These plates have a very complex arrangement. Electron microscopy of the surface of a sea urchin demonstrates this elegant arrangement, or sponge-like mesh, which creates a plate that allows for special structures to protrude through the endoskeleton for locomotion, feeding, gas exchange, and protection. These protrusions, typically described as tube feet (often suction-cup or sucker-like appendages), enable echinoderms to move and provide them with the ability to grip and manipulate objects or prey.

Figure. 11 (Click image to enlarge)

Echinoderms also have a unique water vascular system (a network of hydraulic canals extending in from the tube feet and around the gut of the organism). This expansive network uses hydraulic pressure to manipulate the extension of the tube feet as the animal breathes, feeds, or moves across the ocean floor.

Phylum Echinodermata: Classification

Members of the phylum Echinodermata are diverse. In the adult state it is often unclear how these animals are related. However, a close inspection of their anatomy and embryology reveals their common evolutionary history. Shown below are members from four echinoderm classes (you will not be tested on these classes).

Class Asteroidea (sea stars)

Figure.12 (Click image to enlarge)

Class Echinoidea (sea urchins and sand dollars)

Figure. 13 (Click image to enlarge)

Class Holothuroidea (sea cucumbers)

Figure. 14 (Click image to enlarge)

Class Ophiuroidea (brittle stars)

Figure. 15 (Click image to enlarge)

What Do Humans Have In Common With Sea Urchins?

The correlation between humans and sea urchins may not be obvious, however, common embryonic features reveal their common ancestry. As you should have learned, stages of embryonic development have a significant impact on the classification of animal species. Whether it is the fate of the blastopore, the number of germ layers (diploblastic versus triploblastic), or the formation and origin of the tissue lining the body cavity, embryonic origins provide information about the commonalities between all members of the kingdom Animalia.

The phylogeny that places humans (and other vertebrates) and echinoderms in close proximity is also supported by nucleic acid sequence data. As more data are collected, these relationships may be reconsidered, but to date the evidence showing embryonic similarities between the species of these groups is compelling. Some scientists have even discussed placing Phylum Echinodermata closer to Phylum Chordata, or even within Phylum Chordata, due to the discovery of some early echinoderms that might have possessed pharyngeal slits and a tail (diagnostic chordate features that are discussed in the next tutorial). Furthermore, undiscovered species may also provide information on the relatedness of chordates and echinoderms. From what you should have learned, would a modern representative of these groups shed more light on this overall question, or could just as much be learned from the fossil of an adult animal? Why or why not?


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

Arthropods fall into one of four subgroups. Segmentation, and subsequent specialization, is one of the hallmarks of each of these four subgroups.

Trilobites were a very successful group that became extinct about 250 million years ago. Although these animals were segmented, they had very little diversification in their segments.

The chelicerates include spiders. These animals have two major body segments: the cephalothorax and the abdomen. Their appendages are all clustered in the cephalothorax. Typically there are six pairs of appendages. These include the chelicerae, which are involved in feeding, and the pedipalps, which may have a feeding function. The balance of the appendages is legs. This group does not have antennae but does have simple eyes.

Crustaceans, the second largest extant subgroup, include mostly aquatic species, however, there are some terrestrial species. They have two pairs of sensory antennae, jaw-like mandibles, and compound eyes.

The uniramians are the largest extant subgroup. Uniramians have jaw-like mandibles, compound eyes, one pair of sensory antennae, and unbranched (uniramous) appendages. This subgroup includes Class Insecta, which have distinct segments of variable numbers.

All arthropods are protostomes. That is, during early development the primitive mouth forms first (before the anus). However, in the deuterostomes the mouth forms secondarily to the anus. The polarity of the embryonic gut is clearly an ancestral trait that links organisms that look very different. The deuterostomes include the echinoderms and the chordates, and in the next (and last) tutorial you will learn more about the chordates.