Chapter 46 Animal development
Animals are characterized by:
- body symmetry
- number of embryonic germ layers
- type (or absence) of body cavity
- characteristics of embryonic development.
Body symmetry
Asymmetry = no symmetry
Animal tissues develop from embryonic germ layers
Triploblastic animals have three germ layers (ectoderm, mesoderm, and endoderm)
Germ layer Tissue
endoderm lining of digestive tract or gut
mesoderm muscles and most other organs
ectoderm outer covering of animal
Diploblasts have tissues derived from two embryonic germ layers (endoderm inside,
ectoderm outside)
Triploblasts have tissues derived from three embryonic germ layers (endoderm inside,
ectoderm outside, and mesoderm in the middle)
The three basic body plans of triploblasts relate to the presence or absence of a body cavity
- Acoelomates: no internal body cavity
- Pseudocoelomates: mesoderm on the outside and endoderm on the inside
- Coelomates: surrounded by mesoderm
, Characteristic True coelom Pseudocoelom
Acts as hydrostatic skeleton yes yes
Cushions internal organs yes yes
Allows independent movement of organs yes yes
Derived from mesoderm only yes no
Derived from mesoderm and endoderm no yes
Coelom: internal body cavity in which internal organs are suspended
Definitions protostome and deuterostome
- Protostomes, the mouth arises from the blastopore, the first opening into the
archenteron (the hollow center of the early embryo).
- Deuterostomes, the anus arises from the blastopore, and the mouth is a secondary
opening.
Embryonic cleavage
Protostome: spiral cleavage, determinate
Deuterostome: radial cleavage, indeterminate
Coelom formation
In protostomes, solid blocks of mesoderm on either side of the archenteron split and
form the coelom.
In deuterostomes, the coelom is formed from bulges of mesoderm that pinch off
from the archenteron.
Animals are characterized by:
- body symmetry
- number of embryonic germ layers
- type (or absence) of body cavity
- characteristics of embryonic development.
Body symmetry
Asymmetry = no symmetry
Animal tissues develop from embryonic germ layers
Triploblastic animals have three germ layers (ectoderm, mesoderm, and endoderm)
Germ layer Tissue
endoderm lining of digestive tract or gut
mesoderm muscles and most other organs
ectoderm outer covering of animal
Diploblasts have tissues derived from two embryonic germ layers (endoderm inside,
ectoderm outside)
Triploblasts have tissues derived from three embryonic germ layers (endoderm inside,
ectoderm outside, and mesoderm in the middle)
The three basic body plans of triploblasts relate to the presence or absence of a body cavity
- Acoelomates: no internal body cavity
- Pseudocoelomates: mesoderm on the outside and endoderm on the inside
- Coelomates: surrounded by mesoderm
, Characteristic True coelom Pseudocoelom
Acts as hydrostatic skeleton yes yes
Cushions internal organs yes yes
Allows independent movement of organs yes yes
Derived from mesoderm only yes no
Derived from mesoderm and endoderm no yes
Coelom: internal body cavity in which internal organs are suspended
Definitions protostome and deuterostome
- Protostomes, the mouth arises from the blastopore, the first opening into the
archenteron (the hollow center of the early embryo).
- Deuterostomes, the anus arises from the blastopore, and the mouth is a secondary
opening.
Embryonic cleavage
Protostome: spiral cleavage, determinate
Deuterostome: radial cleavage, indeterminate
Coelom formation
In protostomes, solid blocks of mesoderm on either side of the archenteron split and
form the coelom.
In deuterostomes, the coelom is formed from bulges of mesoderm that pinch off
from the archenteron.
