Lecture 16 Flashcards
Two major classes of deuterostomes
Echinoderms
- invertebrates such as sea stars
Chordates
- Vertebrates such as fish, reptiles and mammals as well as some invertebrates
Chordata
Members of deuterostome with 4 derived traits
- notochord
- Dorsal hollow nerve cord
- Pharyngeal slits or clefts
- Muscular post anal tail
Invertebrates (lanclets and tunicates) as well as all vertebrates
Notochord
A longitudinal, flexible rod found between the digestive tube and the nerve cord
- skeletal support for the body
- Not a calcified - hardened skeleton
In vertebrates, the backbone develops around the embryonic notochord
- In many species, the backbone almost completely replaces the notochord
- In humans, embryonic notochord reduces and becomes part of the intervertebral discs
Dorsal hollow nerve cord
- Nerve cord of chordate embryo develops from a specific surface of the ectoderm (neutral plate) that rolls and internalizes into a neural tube
- Neural tube is dorsal to the notochord
The nerve cord of the vertebrate develops into
the brain/spinal cord
- Other animal phyla has a solid nerve cord that are in many cases ventral
Pharyngeal slits or clefts
Pharynx: Region just posterior to the mouth (towards the back of the mouth)
Pharyngeal clefts: Chordate embryos has arches and grooves that forms along the outer surface of the pharynx
For some chordates pharyngeal clefts develop into pharyngeal slits to
Allow water to pass through the body without passing through the entire digestive system
Feeding for many invertebrate chordate
Develops into gill slits of fish
Pharyngeal clefts do not develop into
open slits/gills in adult tetrapods
In humans, pharyngeal clefts
differentiate and form various muscle, nervous and skeletal tissues around the face and neck
human embryo at ….. weeks have pharyngeal clefts
4
Muscular post anal tail
Chordates have a muscular tail that extends post anus
- tail is greatly reduced/lost during embryonic development
Non chordates have digestive tract which extends nearly to the whole length of the body
Lanclets
Invertebrates (no backbone)
Have all features of a chordate
- notochord
- dorsal hollow nerve cord
- pharyngeal slits
- post anal tail
Adult has cilia around their mouths
- burrow their tail ends into sand and use cilia to draw water into their mouths to capture food
Tunicates
More closely related to vertebrates than lancelets
- resembles a ‘normal’ chordate with all 4 chordate traits
Larvae settles at a suitable location and goes under radical metamorphosis
- notochord and tail resorbed
- nervous system degenerates
Filters water through the enlarged pharynx slits to get food
The 4 derived trats of chordates are absent in
adults of many members of this clade - humans and tunicates
These organisms are still chordates and in most cases the 4 traits are still observed in the embryos of chordate animals
Human embryo has
all 4 derived features of chordates
during development:
- Notochord gets replaced by the vertebra
- Nerve chord develops into spinal chord and the brain
- Pharyngeal slits develop into various tissues of face and neck
- Post anal tail recedes
