Except for sponges, animal cells are arranged into tissues (a tissue is a group of cells alike in structure and function… such as muscle tissue or brain tissue). Tissues are necessary to produce organs and organ systems.
The dominant generation in the life cycle is the diploid generation. Their gametes are heterogametes (different sizes); eggs are larger than sperm. Gametes are produced by meiosis. A typical animal life cycle is shown below.
Most animals, collectively called the eumetozoans , have closely functioning tissues. Some organisms are diploblastic , have only two cell layers… such as the hydra and jellyfish. Some organisms are triploblastic and have three cell layers. The three “ germ layers” are the ectoderm , mesoderm and endoderm . There is a group of animals, the parazoans (sponges) that are not organized into true tissue and lack organs.
The three layers of tissues that become established during early embryonic development are called germ layers. They give rise to the body tissues. These three layers are called….. 1. ectoderm , 2. mesoderm , and 3. endoderm .
The Embryonic Germ Layers develop from the Blastula….. Gastrula Hollow ball of cells stage following the blastula, consisting of a hollow, two-layered sac of ectoderm and endoderm surrounding an archenteron (opening) archenteron Gastrula
The body parts of a radially symmetrical animal are arranged around a central axis so that each part extends from the center. Animals that exhibit radial symmetry tend to be sessile (immobile). Radial symmetry allows them to reach out in all directions.
3. Cephalization The term “Cephalo” means “head”.
In animals with bilateral symmetry, there is a greater increase in the nerve tissue concentrated in the anterior end (the head) as animals increase in complexity. For example, brains have formed with accessory organs for seeing, hearing, tasting, etc.
G astro V ascular C avities (GVC) are areas where food is digested. If they have only one opening, the processing is limited. Two openings designate a digestive tract allowing food to be digested more thoroughly.
Many animals have segmented body parts. In some cases the parts repeat over and over again, as with earthworms. In other animals, the segments are modified, such as with insects… they essentially have 3 segments…. the head, thorax and abdomen.
The Gastrula will become the gut (digestive tract) of the mature animal. In species that have a separate mouth and anus, the tube will eventually extend through the length of the embryo and fuse with the opposite side. One opening will become the mouth, the other will become the anus.
( Blastopore becomes the anus) ( Blastopore becomes the mouth) Blastopore Blastopore
7. Phylum Annelid (Segmented Worms: includes leeches, earthworms and polychaetes)
Earthworms ingest soil, extract nutrients in the digestive system and deposit undigested material (mixed with mucus from the digestive tract) as casts through the anus. Important to farmers as they till the soil and castings improve soil texture.
Phylum Annelid (Segmented Worms: includes leeches, earthworms and polychaetes) Fanworm Polychaete Worm
8. Phylum Arthropoda ( spiders, insects, crustaceans, and various related organisms)
Arthropods have two kinds of life cycles…(1) complete metamorphosis , (2) incomplete metamorphosis .
Arthropods have: * jointed appendages, * a well-developed nervous system, * specialized body segments , and * an exoskeleton made of chiton .
The specialized body segments are the: Head, Thorax and Abdomen
Incomplete Metamorphosis (immature forms are often called nymphs) Nymphs resemble the adult in form except for being smaller and lacking fully developed wings and sexual organs. Life Cycle: Egg nymph adult
Complete metamorphosis Immature forms are called larvae (larva, singular). The pupal stage is a transition stage, when the larva is transformed to the adult. Pupa molts to the adult form. Life Cycle: Egg --> larva --> pupa --> adult
Pharyngeal Gill Slits provides channels across the pharynx to the outside of the body. In some chordates, the slits become gills for oxygen exchange, or for filter feeding, while in others, the slits disappear during embryonic development.