This Subphylum is more familiar to us and it should be as we are part of this amazing Subphylum. There are still the 5 chordate features which are:
- Notochords – this structural and offers support
- Dorsal tubular nerve cord
- Pharyngeal pouches and slits – these are homologous to gills
- Endostyle or thyroid gland – has the thyroid hormone as a result
- Post anal tail – extends beyond the tail
In evolution these animals tend to be more active hence there is more sense organs and a larger brain. They are also very muscular with a higher metabolic rate.
Cephalisation includes having a 3 part brain, a cranium which protects the brain and there are more complicated sensory organs.
MUSCLES
There is an endoskeleton which is either made of cartilage or bone. This endoskeleton includes a cranium, vertebrae and there are muscles which are attached. The vertebrae could be considered a replaced notochord as they both provide similar structure and support.
There is a lot of extensions which help provide increased surface area to allow muscle attachments. Movement has now become more versatile and flexible due to the W segmented myomeres (muscles). This also allows finer control.
PHYSIOLOGY UPGRADES
The pharynx of a vertebrate has the ability to work with filter feeding and assist with respiration too. The muscular pharynx is located near the vascularised gills (for gas exchange and oxygen), so the muscles of the pharynx helps drive water through the slits.
There is also a new addition of a liver and a muscular gut which will be super duper handy.
CIRCULATORY
There is now a 3 chambered heart with different parts such as a sinus venosus, atrium and ventricles. There is also red blood cells (erythrocytes) which contain hemoglobin. This is used to grab oxygen and distribute it across the body.
PEE TIME?
There are pairs of glomerular kidneys which have become more refined for uptaking certain nutrients and releasing others.
In the fishes lampreys and amphibians there are mesonephric tubules with glomeruli and pee is excreted througha mesonephric duct.
In the reptiles, birds and mammals there is metanephric kidney pairs as well as a ureter.
NERVOUS SYSTEM
This has now become more advanced with a tripartite brain (forebrain, midbrain and hindbrain) protected by a cranium. There is also integrated neural signals from the sensory receptors.
NEURAL CREST
This is a new development of cells to allow development for the head to provide internal communication. These cells can give rise to:
- cranium
- pharyngeal skeleton
- tooth dentine
- cranial nerves
- some ganglia
- Schwann cells
- some endocrine glands
These cells can also help regulate development of neighbouring tissues at the tooth enamel and pharyngeal muscles and retina.
ECTODERMAL PLACODES
This also have cells which allow development of the sense organs such as:
- olfactory epithelium
- lenses
- inner ear epithelium
- ganglia
These sensory organs help bring information to the nervous system.
Interesting information?
There is a Hox gene replication which is believed to control development of body segments. An example is given in the brachiosoma which has 1 copy, and the gnathostomes (jawed fishes) which have 4 copies which helps provide more complexity.
Now that I have attempted to cover the basics I will now talk about in as much detail as I can about different cool groups. The first one is going to be on Agnathas which are the jawless fishes.
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