Topic 13- Animal Origins

Question 1

What are the 3 lines of evidence that choanoflagellates are closely related to animals?

Answer 1

1. cell morphology
2. cell morphology unique to animals
3. DNA sequence homology (almost identical!)

Question 2

why do sponges represent a separate lineage, distinct from all other animal phyla?
A. they are sedentary and resemble plants
B. They do not have specialized cell types
C. they have only 2 layers of cells
D. they lack true tissues

Answer 2

D. they lack true tissues!

therefore they don’t gastrulate!

Question 3

what’s the hypothesis of evolution of multicellularity

Answer 3

ball-shaped colony of choanoflagellate’s may have evolved into a simple animal with endo and ectodermal layers

Question 4

what caused such a rapid increase in diversity during the cambrian period? list 4 extrinsic (environmental) and 2 intrinsic (biological)

Answer 4

extrinsic

• higher O2 levels
• evolution of predation
• new niches beget more new niches
• modified genes, modified bodies

intrinsic

• rapid change in genes regulating (hox genes and predator-prey arms race)
• genetic causes (evolution of hox genes complex= developmental flexibility)

Question 5

porifera are ___ except for the larval stage where they’re ___

Answer 5

sessile

mobile

Question 6

porifera lack __ genes and are ___

Answer 6

hox

asymmetrical

Question 7

structural support in sponges comes from ___

- some only have ___

Answer 7

spicules

collagen

Question 8

explain the 2 cell types of porifera (sponges)

Answer 8

1. choanocytes: flagellated for creating steam of water for feeding
2. Amoebocytes: motile- moves nutrients between cells, can also lay down spicules for support

Question 9

define mesohyl

Answer 9

gelatinous acellular layer between the outer ‘skin’ and the choanocyte layer
* not a tissue b/c it’s acellular

Question 10

suspension feeding sponges use ____ and ____ digestion (within cells)

Answer 10

phagocytosis

intracellular

Question 11

explain the process of sponge reproduction

Answer 11

mostly hermaphroditic

• simultaneous hermaphrodite: male & female @ same time
• sequential hermaphrodites: first one sex, then change to the other
• they do not have ovaries or testes!
• eggs arise from modified amoebocytes and sperm arise from modified choanocytes

Question 12

explain the eumetazoan design

Answer 12

specialized cells aggregated into distinct tissues; more advanced designs, tissues combine–> organs such as kidneys, liver, lungs, gut etc

Question 13

how many layers & what symmetry do cnidarians have (jellyfish)

Answer 13

diploblastic

radial

Question 14

explain the 2 different basic body forms of cnidarians

Answer 14

1. polyp: sessile- oral end upwards, attached to a substrate

2. medusa (motile form)- oral end downwards, moves freely through the water via hydrostatic skeleton

Question 15

cnidarians have unique cells called ____ which are used for ___ and ___

Answer 15

cnidocytes

feeding and defense