Animals

Main characteristics of animals:

• Multicellular heterotrophic eukaryotes

• Feed via ingestion, with a few exceptions

• Capable of movement at some point in life

• Cells lack cell walls

• Most are diploid and form gametes by meiosis

 Animal Phylogeny: See figures and key points below

 Figure 1. Eukaryotic supergroups and animal phylogeny. Image source: Urry et al.

Animals:

• Are Opisthokonts in the eukaryotic supergroup Unikonta

• Closest living relatives are member of the protistan clade Choanoflagellates.

• Are a monophyletic group (= clade)

Animal phyla 

Phylum Porifera (sponges)

• Are metazoans (multicellular animals)

• covered with pores for water intake

• most have no tissues (i.e. they are not eumetazoans)

• most are asymmetrical

• body structure provided by spicules made of CaCO₃, silica, or a modified collagen called spongin

• Feeding: Water is drawn in through pores and out osculum by choanocytes, which capture food.

Figure 2. Water flow in a sponge. Image source: Urry et al.

Phylum Cnidaria (jellies, hydrozoans, corals and anemones)

• radially symmetrical diploblasts (possess only an endoderm and ectoderm embryonic germ layer during development)

• occur as a polyp (sessile) or medusa (free-swimming); many have both stages in different points of their life cycles (polyp = asexual stage; medusa = sexual stage)

• have a gastrovascular cavity (incomplete digestive tract with 1 opening)

• key synapomorphy: possess cnidocytes, stinging cells for paralyzing prey

Figure 3A. Hydra

Figure 3B. Moon jellies, Aurelia

Figure 3C. Sea anemones

Bilaterians:

Bilaterally symmetrical animals have distinct dorsal and ventral sides and anterior and posterior ends:

Figure 4. Body planes in animals. Image source: Urry et al.

• There are two major clades of bilaterally symmetrical animals:

  • Protostomes (Platyhelminthes, Molluscs, Annelids, Arthropods, Nematodes)

    • In many, the blastopore becomes the mouth

  • Deuterostomes (Chordates and Echinoderms)

    • In many, the blastopore becomes the anus

In this week's first of two animal labs, we will look at three Protostome phyla: Platyhelminthes, Mollusca, and Annelida

 Phylum Platyhelminthes (flatworms)

• acoelomate triploblasts (lacking a body cavity, or coelom, but have three embryonic germ layers during development)

• Free-living flatworms (turbellarians), parasitic tapeworms, and parasitic flukes

• gastrovascular cavity in most; tapeworms have no digestive tract

Figure 5A. A planarian, an example of a turbellarian

Figure 5B. Tapeworm

Figure 5C. Flukes

Phylum Mollusca (clams, mussels, scallops, snails, squids, octopuses, e.g.)

• Coelomate triploblasts

• The body of a mollusk has 3 main parts (figure 6):

1. Muscular foot

• located ventrally

• function: locomotion

• sometimes is modified into arms/tentacles

2. Visceral mass

• contains digestive, excretory, and reproductive organs, and the heart

3. Mantle

• heavy fold of tissue enclosing and covering the visceral mass

• secretes shell in organisms that have one

• Mantle cavity encloses the respiratory organs

Figure 6. Molluscan body plan. Image souce: Urry et al.

Class Gastropoda: slugs, snails

• most have a shell

• terrestrial and aquatic

Class Cephalopoda: squids, octopuses, cuttlefish, chambered nautiluses

• muscular foot modified into tentacles/arms

• all marine

Class Bivalvia: clams, scallops, mussels, oysters

• shells in two halves

• marine and freshwater aquatic

Figure 7A. Land snails are examples of gastropods.

Figure 7B. Squids are examples of cephalopods.

Figure 7C. Mussels are examples of bivalves.

Phylum Annelida (segmented worms)

• Coelomate triploblasts

• segmented body

• earthworms and relatives; marine annelids; leeches

• many species have setae (bristle-like structures, also called chaetae, made of chitin)ك

• coelomates - fluid-filled body cavity; acts as hydrostatic skeleton