NOTES FOR BIOLOGY 1002
SECTIONS 004, 005, 006
Spring 2006
DR. STEVEN POMARICO
CHAPTER 25
ANIMALS: THE INVERTEBRATES
The main distinguishing features of the Kingdom Animalia are
-Eukaryotic
-Multicellular
-Heterotrophic
-Aerobic
-Sexual reproduction
-Most mobile with rapid response to stimuli
-Life cycle with embryonic development
-Ingestive method of digestion
-No cell wall
The embryonic development includes germ layers.
Germ layers are the layers of cells in the early embryo that lead to all tissues and organs.
Diploblastic vs. Triploblastic
The three possible germ layers are:
endoderm - interior lining of the organs
mesoderm - muscles, skeleton and circulatory system
ectoderm - outer coverings
In addition to these characteristics, as animals evolved, they developed more complex and organized bodies.
Some of these increases in complexity and organization are:
-cellular specialization
-body plan
vertebrate vs. invertebrate
-cephalization
-body cavity
-segmentation
-digestive system
In addition to these characteristics, as animals evolved from the animal-like protists, they developed more complex and organized bodies
Many animals have a body plan which includes symmetry.
Symmetry indicates that a geometric plane could be drawn through an organism such that the halves of the organism are “mirror” images of each other.
Two types of symmetry are seen (See fig 25.3):
Radial symmetry - round body plan
Animals have a top and bottom but no back/front or left/right.
Bilateral symmetry - body plan with a left side and a right side.
Also means a back (dorsal) and front (ventral) surface can be determined
With bilateral symmetry cephalization also appears.
Cephalization is the concentration of nervous tissue (including a “brain” and sensory organs) into a defined region (head) of the body.
The gut or gastrovascular cavity is were digestion occurs in many animals.
The design of this gut varies from digestion in a simple sac (See fig 25.4) to digestion in a tube open at both ends and running through the body.
In addition to the gut cavity many animals have an another cavity known as the coelom (See fig. 25.4).
Coelom is the body cavity that is between the digestive system and the body wall.
There are three main types of body cavities:
-No coelom
-Pseudocoelom in which a cavity is present but is not derived from the mesoderm.
-True coelom has a cavity derived from mesoderm.
Another aspect of body design is segmentation
Segmentation is a body design in which similar repeating units are present.
The origin of animals as a group are unclear. Many hypotheses propose that animals may have come from protistians. If this is the case, then as animals evolved they developed more complex and organized bodies
One major aspect of this evolution of the animal kingdom is the increase in cellular organization and cellular specialization.
Cellular specialization leads to the development of tissues, which then can lead to the combination of tissues into an organ. Organs may then become arranged into organ systems.
Sponges - Phylum Porifera (See fig 25.6)
Characteristic of sponges
-Sessile - non-moving
-Filter feeders
-May reproduce sexually or asexually by budding and fragmentation
-Most species have an asymmetric body plan designed for water filtration
-Lack tissue structure but have specialized cells (See fig 27.7)
Three types of specialized cells are found in the sponge phylum
Epithelial cells -outer covering layer (like a skin)
Make up and regulate the pores
Collar cells - Inner layer which pumps water through the sponge using
flagella and filters out the food
Amoeboid cells - mobile cells between the two layers that ingest the
collected food
-Secretion of glass-like structures known as spicules.
The first group to show tissues and symmetry are the Cnidarians
The Cnidarians include:
Hydra, Corals, Anemones, and Jellyfish (See fig 25.9)
Characteristics of the Cnidarians:
-Mostly marine
-Tissues present - including a nerve net
-May reproduce sexually or asexually by budding
-Simple sac-like body with a gastrovascular cavity
two types of body plan
Polyp or medusa
-Tentacles armed with nematocyst (See fig. 25.8)
-Only two germ layers form in the early embryo
The Cnidarians are missing the mesoderm
The two germ layers present in Cnidarians lead to the inner and outer body layers.
Between these body layers is the mesoglia which acts as a hydrostatic skeleton.
All the remaining phyla in the Kingdom Animalia have bilateral symmetry and three germ layers.
Some species (Sea Urchins and Starfish) have radial symmetry as adults but have bilateral symmetry in the larval stages.
The first group to show bilateral symmetry are the Acoelomates or Flatworms - Phylum Platyhelminthes
The characteristics of the Flatworms are:
-Bilateral symmetry
-Concentration of ganglia and sensory organs in the head of the non-parasiticspecies.
-Tissues organized into organ systems (See fig 25.11)
-One-way digestive system
-Food enters through the pharynx
-Waste leaves through specialized excretory cells (flame cells)
-Many free-living forms like tubellarians but some parasitic types
Tapeworms (See fig 25.12), Hookworms, Liver flukes
The first group in which a body cavity appears are the Roundworms - Phylum Nematoda (See fig 25.13)
The characteristics of the Roundworms are:
-Large number of species 20,000 identified (may be as many as 500,000)
-Inhabit almost any moist or aquatic environment
-Pseudocoelom with a hydrostatic skeleton
-Complete digestive system with a mouth and an anus
-Some parasitic forms:
Trichinella (vectors through pork) (See fig 25.16)
Heartworms (vectors through mosquitos)
The rotifers - Phylum Rotifera have many characteristic of the roundworms and since this is a relatively minor phylum and we will not cover any of the details.
Most of the remaining Phyla of the animal kingdom have a true coelom
The coelom offers organisms the advantage of better internal support of the digestive system.
The difference in the development of the coelom separates two major branches of the Animal Kingdom. This developmental difference is apparent only during embryonic development and defines the Protostomes vs. Deutrostomes.
The separation between protostomes and deutrostomes corresponds with some other developmental differences between these two major branches in the Animal Kingdom.
Spiral/determinate cleavage vs. Radial/indeterminate cleavage
The embryo forms of all the phyla we have discussed so far, as well as the protostomes, go through spiral/determinate cleavage.
By the time they get to about the four-cell stage the cells have lost their totipotency and their fate is already fixed.
The embryos of the deutrostomes go through radial/indeterminate cleavage. The cells present after the four-cell stage retain their totipotency (stem cells) and the ability to develop into any type of cell or even a complete embryo.
The protostome branch of the Animal Kingdom branches into 3 separate phyla:
Mollusks - Phylum Mollusca
Annelids - Phylum Annelida
Arthropods - Phylum Arthropoda
Phylum Mollusca - Mollusks (See fig 25.19)
Class Gastropoda - Gastropods
-Snails
Class Bivalvia - Bivalves
-Oysters, Clams, Mussels, and Scallops
Class Cephalopodia - Cephalopods
-Squids and Octopuses
Characteristics of the Mollusks are:
-Mostly marine
-Soft bodies with hard shells (some internalized or missing)
-More ganglia concentrated in head
-Body plan featuring (See fig 25.18):
Mantle and foot
Mantle is a dorsal covering which in some species secretes a shell
Foot is a muscular extension used for sensing, moving, grabbing, and digging.
The increases in ganglia in the head of some Mollusks (especially the Cephalopods)
gives them the ability to “learn.”
The increase in ganglia also has lead to increased senses (taste and especially sight).
The Annelids (along with the Arthropods) exhibit segmentation.
Segmentation allows an organism to increase body size (length) without developing a new type of body part.
The Annelids also show improvements in the design of some major systems.
The characteristics of the Annelids are:
-True coelom
-Segmentation
-Closed circulatory system with 5 hearts (See fig 25.25)
-Advanced excretory system with kidney-like nephridia
-Examples of segmented worms are:
Earthworms, Polychaetes and Leeches
Nephridia filter blood and the fluid of the coelom to remove waste material.
The Annelids and all the previous Phyla have all lacked a developed respiratory system.
The exchange of gases was accomplished by diffusion at the surface of cells that are in direct contact with the external environment.
The development of specialized respiratory tissues increased the rate of gas exchange and allowed an increase in the size of the organisms.
The improved respiratory system functions by increasing the surface area for gas exchange.
The first groups to show the development of a respiratory system are the Arthropods - Phylum Arthropoda
Annelids - Class Insecta - Insects
- Class Arachnida - Arachnids
Spiders, mites, ticks, scorpions
- Class Crustacea - Crustaceans
Crawfish, shrimp, crabs, lobsters
The characteristics of the Arthropods are:
-Largest animal phylum
about 1 million described species
possibly 9 million total species
-Fused and modified segmentation
-Exoskeleton
-Advanced cephalization
increase in senses including compound eyes
-Specialized respiratory system including
Gills, Book lungs, or Trachea
Exoskeleton is a hard external skeleton made of chitin that must be periodically shed or molted.
In addition to providing support the exoskeleton also allow for more precise movements.
The disadvantage to living in a “suit-of-armor” is that to grow you must change from the smaller suit to a larger one.
Insects may also go through metamorphosis during development.
Metamorphosis is any change in shape or proportion.
This can be either incomplete metamorphosis or complete metamorphosis
Incomplete involves a change in size and proportion
The usual progression is egg > nymphs > adult
Complete involves a change in body shape.
The usual progression is egg > larva > pupa > adult
The remaining 2 phyla are on the deutrostome branch of the animal kingdom and show radial/indeterminate cleavage.
The first of these phyla are the Echinoderms - Phylum Echinodermata (See fig 25.39).
The Echinoderms include the Sea stars, Sea urchins and Sea cucumbers
These organisms lack some of the characteristic (especially as adults), which we have seen in previous groups:
-No cephalization
-No segmentation
-No circulatory system
-No excretory system
-Primitive nervous system (with nerve net)
-Mostly marine
-Endoskeleton of CaCO3