Lecture 15 Invertebrates 2: Intro to Animals II

Question 1

Three types of symmetry

Answer 1

1. Asymmetry-No axis divides body into equal halves. ex: some sponges
2. Radial symmetry-As long as one cuts through the centre, the body can be divided into equal halves (from many angles)
3. Bilateral symmetry
   * single axis divides body into equal halves
   * midsagittal plane (between eyes)

Question 2

Cephalization is

Answer 2

the concentration of sense organs, nervous control at the anterior end of the body, forming a head and brain.

Question 3

Gemmule is

Answer 3

the asexual reproductive structure in some plants (as the buds of mosses) and the sponges and hydra. A mass of cells capable of remaining dormant then later develops into a new individual.

Question 4

Fertilization and zygote and embryo are

Answer 4

Fertilization = the process of combining a single sperm cell with a single egg cell
Embryo = young animal, still contained within a protective structure (e.g., egg shell or uterus)
Zygote = diploid cell resulting from union of two haploid gametes; 1st cell of next generation

Question 5

3 major steps of Embryonic development (zygote to multicellular organism)

Answer 5

1. Cleavage = multiple rounds of rapid cell divisions (mitotic)
   * process forming many cells from one cell (morula, blastula)
   * Overall size, volume not changing because cytoplasm not replenishing
   * Fastest division of lifetime
   * Developed undifferentiated cells called Blastomere (bud parts)
   * Blastomere receives different cytoplasm (Cytoplasmic determinant)–distinct gene regulation cascades–cell differentiation
2. Gastrulation = in-folding, invagination
   * process forming embryonic tissue layers (2-3)
   * Body needs a well defined head-tail axis at this stage (Hox genes)
   * Cnidarians stop here, while other groups’ blastula keep expanding.
3. Cellular differentiation = process in which the immature cells take on individual characteristics and reach their mature (specialized) form and function.

Question 6

Structures of late gastrula (3)

Answer 6

Question 7

Three layers of gastrula are

Answer 7

Ectoderm-“outer skin”, into epidermis, nervous system
Endoderm-“inner skin”, into digestive and respiratory tracts
Mesoderm-“middle skin”, into most internal organs (muscles, skeletal system, part of gonads), for animals with bilateral symmetry

*Organisms with 2 germ layers are diploblastic (2 buds), and triploblastic with 3 germ layers.

Question 8

Animal phylogeny update with embryonic cell layers

Answer 8

Question 9

Introduce sponges (porifera) (8)

Answer 9

• Porifera means “pore bearing”
• Earliest animals to appear in the fossil record (700 MYA) late Precambrian
• ~9,000 species, mostly marine
• Inhabit a wide range of habitats
• Sessile as adults, motile as larvae (dispersal)
• Primitive features (specialized cells but no true tissues or organs).
• Digestion still intracellular–exception in animals. Respiration and excretion through diffusion.
• Endoskeleton: spicules and a network of elastic fibres for support

Notes: Sponge morphology (symmetry, size, color, shapes) is highly variable. Flat sponges in water with waves while finger-like lives in calm water. Large barrel sponges found deep in ocean.

Question 10

Anatomy of a Sponge - Filter Feeding

Answer 10

Use water currents for food, oxygen and reproduction.
Coordinated movement of flagella would suck water in

Morphological similarities between choanocytes of sponge (left) and choanoflagellate (right). Considered closest living relatives of animals

Question 11

More detailed anatomy of sponge

Answer 11

Question 12

Cross-section of sponge wall (support)

Answer 12

High spongin content - “bath sponge”
High SiO2 content - “glass sponge”

Question 13

Asexual reproduction of sponge

Answer 13

• Fragmentation (external budding)
  – Not self-induced
  – Brought about by waves or predators
• Internal budding (gemmules)
  – Clone of parent
  – Types of cells enclosed by protective covering (dormant)
  – Important under harsh conditions where adults may not survive

The asexual reproduction implies that multicellular organisms might have evolved from single-celled aggregation.

Question 14

Sexual reproduction of sponges

Answer 14

Hermaphroditic (unisex)
* Sperm and eggs are produced in choanocytes or amoebocytes at different time, not self fertilizable
* Broadcast sperm
* Sperm is trapped by the choanocytes of a “female” sponge, delivered by amoebocytes
* Eggs fertilized, develop in extracellular matrix.
* Fertilization happens in mesohyl
* Larvae released (motile). Able to swim with cilia.
* Settle on suitable substrate