Animalia: A Progression of Complexity

As we begin our Tiptoe through the Taxa, keep in mind the following characteristics as they change across phylogenetic groups...

• tissues
• organs and organ systems:
  • integumentary system - protection against mechanical injury, infection, desiccation
  • digestive system - food processing
  • nervous system - rapid coordination of body activities; response to environmental stimuli
  • muscular system - movement
  • reproductive system - the obvious

  • excretory system - removal of nitrogenous waste from body fluid
  • skeletal system - structural support
  • circulatory system - internal distribution of materials
  • respiratory system - gas exchange (O₂ in, CO₂ our)
  • immune and lymphatic systems - body defense against pathogens and cancer
  • endocrine - slower coordination of body activities, response to environmental stimuli

• body symmetry
• internal body cavity
• embryonic development and morphogenesis

Subkingdom Parazoa, Phylum Porifera: The Sponges

Sponges, as we already have mentioned, lack true tissues or a true plane of body symmetry, though some forms are superficially radially symmetrical.

Four types of cells:

• choanocytes - "collar cells"
• pinaocytes - form the epithelium
• porocytes - barrel-shaped cells form the incurrent pores
• amoebocytes - roving scavenger cells that participate in feeding and in secreting the skeletal elements, SPICULES

Most of these cells remain TOTIPOTENT throughout the life of the animal. If you disrupt the cells of a sponge, they can re-aggregate and form a new sponge!

Let's have a LOOK.

The Poriferan Bauplan:

• Totipotent cells
• Aquiferous System
  • incurrent pores (OSTIA) draw water into spongocoel
  • water exits via the OSCULUM
• The external "skin," composed of pinacocytes, is called PINACODERM
• The internal surface, lined with choanocytes, is called CHOANODERM
• Between these two layers is the gelatinous MESOHYL where amoebocytes roam from choanocyte to choanocyte, collecting nutrients from them and distributing them to other cells.
• Internal space is called the SPONGOCOEL (or sometimes the "atrium")
• General anatomy comes in three flavors, each a more complex version of the previous:
  • asconoid - sac shaped
  • syconoid - folded asconoid
  • leuconoid - folded syconoid
• Most are hermaphroditic, producing both sperm and eggs (sperm derived from choanocytes and eggs from specialized amoebocytes called archaeocytes)
• Sponges have a vast variety of sexual reproductive means, with sequential hermaphroditism, dioecy, and constant alternation of sex found in various species.
• Larval form is a ciliated ball of cells (a blastula) called a parenchymula (most common, though there are two other types).
• This remains free-living for a while, and then settles on the substrate on one end and develops into an adult sponge

• No evidence of any form of nervous system or sense organs
• However, sponges can respond to environmental conditions by opening and closing incurrent and excurrent canals, constricting canal lumen size, reversing water flow, etc.
• These responses are mediated at the cellular level, and there is cooperation among cells.

Three Classes of Sponges:

• Class Calcarea - Calcareous Sponges make spicules of calcium carbonate
• Class Hexactinellida - Glass Sponges make spicules of silicon dioxide
• Class Demospongiae - Bath Sponges make spicules of silicon dioxide, and also have skeletal framework of collagen fibers arranged in a network called SPONGIN. (Commercially valuable forms have lost their silica spicules) (95% of all sponge species are in this Class)

• There are about 9000 species

• Almost all species are benthic and marine, though some are freshwater.

• Highly sensitive to pollution; unpolluted waters harbor the greatest diversity of sponges.

• Sponges living in littoral (more shallow, coastal regions with high light levels) are ENCRUSTING.

• Benthic forms are often upright and exhibit superficial radial symmetry.

• Come in a rainbow of colors, with poisonous/stinging ones often being bright orange or red. These pigments are often due to photosynthetic or bacterial symbionts.

• Calcareous sponges most common in warmer, shallower waters where it is easier to precipitate calcium carbonate (which takes more energy to make than silicon dioxide)
• Glass and Demosponges are more common at great depths where water is cold, and calcium carbonate is too difficult to precipitate, and silica becomes less soluble.

Subkingdom Eumetazoa, Superphylum Radiata, Phylum Cnidaria: The Cnidarians

The name of the phylum derives from the Greek knide meaning "nettle."

Three Classes of Cnidarians:

• Class Hydrozoa - Hydras & their relatives (polyp and medusa alternate)
• Class Scyphozoa - jellyfish (medusa is dominant stage)
• Class Anthozoa - sea anemones, corals, sea pens, etc. (polyp is dominant stage)

• true plane of symmetry: radial (sometimes modified as biradial or quadriradial)

• true tissues
  • ectoderm (becomes the epidermis in the adult)
  • endoderm (becomes the gastrodermis in the adult)
  • mesogloea (sometimes partly cellular mesenchyme)
• true organs and organ systems
  • integumentary system - YES
  • digestive system - YES
  • nervous system - YES
    The nervous system consists of a simple nerve net composed of naked, largely nonpolar neurons. There is no brain.
  • muscular system - YES
    The muscular system is formed by epitheliomuscular cells derived from both ectoderm and endoderm.
  • reproductive system - YES

  But not the following:

  • excretory system - NO
  • skeletal system - NO (hydrostatic only)
  • circulatory system - NO
  • respiratory system - NO
  • immune and lymphatic systems - NO
  • endocrine - NO

The Cnidarian Bauplan:

• Two true tissue layers, endoderm and ectoderm
• Radial symmetry
• Gastrovascular cavity (enteron) is the only cavity in the body
• No head; primary body axis is oral --> aboral
• Many species are DIMORPHIC: alternating forms of POLYP (asexually reproducing, feeding form) and MEDUSA (sexually reproductive form)
  

• Unique cells called CNIDOBLASTS (or cnidocyte) containing stinging capsule, the NEMATOCYST.
• Larva is a ciliated gastrula called a PLANULA. In sessile forms, this little guy settles on the substrate and develops into the fully formed adult
  

Subkingdom Eumetazoa, Superphylum Bilateria, Phylum Platyhelminthes: The Flatworms

The name of the phylum derives from the Greek platy meaning "flat" and helminth meaning "worm."

Three Classes of Platyhelminthes:

• Class Turbellaria - The Free-Living Flatworms
• Class Trematoda - The Flukes (All parasitic)
• Class Cestoda - The Tapeworms (All parasitic)

In these most primitive bilaterians, we see the origin of:

• true bilateral symmetry and cephalization

• more complex true tissues
  • ectoderm (becomes the epidermis in the adult)
  • endoderm (becomes the gastrodermis in the adult)
  • mesenchyme (celluar)

The Platyhelminth Bauplan

The following organ systems are present, some seen for the first time in animals:

• integumentary system - YES
  Parasitic forms (flukes and tapeworms) have a TEGUMENT in addition to the epidermis. This is formed of nonciliated cytoplasmic extensionsof large cells whose bodies lie in the mesenchyme. It offers protection against the host, as well as allowing gas exchange.
• digestive system - YES
  • well developed incomplete digestive tract in Turbellarians
  • reduced digestive tract in Trematoda
  • digestive tract lost in Cestodes
• nervous system - YES
  • well developed in Turbellarians
  • reduced in Trematodes
  • even more reduced in Cestodes
    In the more primitive turbellairan flatworms, the nervous system consists of a simple nerve net with a minor concentration of neurons in the head.
    More derived forms have a "ladderlike" nervous system reminiscent of the segmentation seen in much more derived taxa, and a well-developed cerebral ganglion.
    • chemoreceptors are often present in flaplike extensions of the head called "auricles"
    • Photoreception is made possible with simple ocelli, little more than a pigment spot in an pigmented cup. This pigment cup casts a shadow, allowing the animal to determine the direction of incoming light.
    • statocyst (balance cells) are located near the cerebral ganglion
    Note that the nervous system and sense organs are often reduced or absent in parasitic forms.

• muscular system - YES
  • Longitudinal, circular and transverse muscle cells allow contraction in every direction.
  • Muscular system well developed in turbellarians and flukes, vestigial in cestodes.
• reproductive system - YES
  hermaphroditic
• excretory system - YES
  In free-living turbellarians, the excretory system consists of a tubular PROTONEPHRIDIAL system. It is compoesed of anucleate FLAME CELLS attached to collecting tubules. The network of flame cells and tubules rings the external margins of the body, and there are NEPHRIDIOPORES located along the sides of the animal where nitrogenous wastes are excreted.

  The following organ systems are not yet present:

  • skeletal system - NO (though some forms have a few tiny, calcareous plates)
  • circulatory system - NO
  • respiratory system - NO
  • immune and lymphatic systems - NO
  • endocrine - NO

Symbiosis

The Turbellarians are all free-living, and the trematodes and cestodes are 100% parasitic, with often complex life cycles.

There are as many life cycles as there are parasites, but a a few definitions will serve us well:

• definitive host - organism in which the adult parasite resides
• intermediate host - organism in which various larval/developmental stages of the parasite exists until they is passed to the definitive host and metamorphose into the adult

Most species of parasite are relatively host-specific, but some can inhabit more than one different species of definitive host.

In many life cycles, more than one intermediate host is required for the full life cycle to be completed.

Let's look at some more pictures...