In the case of the ascidians these featuresdisappear in adult life when the larvae passthrough metamorphosis and most of thechordate features are lost. After the metamorphosis, the ascidianslarval stage is more biologically complex thanthe adult form of the animal.
Metamorphosis starts when the tunicatefinds a suitable rock/coral to affix to andcements itself in place. When they undergo metamorphosis, theylose their tail and other vertebratecharacteristics. The remainder of the body twiststhrough 180 degrees in order to become a smalltunicate.
What are the parts that are lost during metamorphosis?NotochordTailhollow neural tube running along the body (cerebral ganglion)Sensory vesicleOcellusstatolith
What are the parts that developed during metamorphosis?₰ Nervous system₰ Viscera₰ Incurrent and excurrent siphon₰ Tunic₰ Stolon
Adult Sea SquirtExternal features
• Tunic – acts as a kind of exoskeleton that protects the internal organs of the sea squirt ▫ Contains blood vessels and scattered cells ▫ This may be smooth, spiny or hairy for defense• Stolon/Base – is the point of attachment of a sessile adult to a permanent substrate
• Incurrent siphon – originated from the larval mouth ▫ Food particles come in along with the incoming water stream• Excurrent siphon – originated from the atriopore ▫ Expels digestive wastes, gametes and the feeding current carried out of the atrium
Digestive System• Pharynx – acts as a sieve for food ▫ Food particles are trapped ▫ Collects food, oxygen, and water then slowly ingests it• Endostyle – a groove in the pharynx that secretes mucus(movement is caused by action of cilia) that traps food particles and conveys them into the digestive tract
• Oesophagus – passageway of food particles towards the stomach• Stomach – where the entrapped food particles from the endostyle go and become digested, after being swept by cilia and oesophageal papillae (found in the oesophagus)
• Intestine – food from the stomach passes through and emerges into the anus• Anus – where the digestive wastes pass through and are expelled out through the excurrent siphon
Nervous system• Cerebral ganglion – serves as a primitive brain
Reproductive system• Gonads – are hermaphroditic and consist of ovary and testis on each side• Genital ducts – sperm and egg cells are discharged separately towards the excurrent siphon
Circulatory system• Atrium – a fluid filled collecting chamber surrounding the pharynx ▫ Where atrial muscles pumped out and discharged the deoxygenated water to the exterior via the excurrent siphon• Heart – where blood is pumped alternately forward for a few beats, then backward, pausing before reversing the flow.
Digestive Process• Water, along with any food particles (mainly plankton) it contains enters the incurrent siphon.• Mucus, secreted by the endostyle, traps food particles as water is pushed out of the gill slits.• Food passes through the oesophagus and into the stomach
Digestive Process• The stomach secretes enzymes that digest the food.• Digested material travels through the intestine, which opens up at a cloaca at the base of the excurrent siphon.
Pathway Water Incurrent(w/ food particles) Siphon Pharynx Oesophagus Stomach Water (through Intestine body cavity) Cloaca Water Excurrent (w/ excretions) Siphon