Anatomy & Physiology
Basic body plan
The body of adult pyura sp is covered by a thick leathery tunic (test) secreted by the ectoderm (Kott, 1989). The tunic is made of tunicin, a kind of cellulose and contains blood vessels as well as test and blood cells (Kott, 1989). The tunic is unique in that it is living tissue and grows with the ascidian, meaning that ascidians are the only animals with an exoskeleton that do not moult (Ruppert et al., 2004). In this species the tunic is attached ventrally to the substrate by secreted adhesive material. Underneath the tunic is the body wall, a connective tissue layer that contains the muscles, blood vessels, and amoeboid cells (Kott, 1989). The tunic has two apertures, the buccal (incurrent) siphon and the atrial (excurrent) siphon, which are openings into a cavity called the atrium (Kott, 1989). The buccal siphon opens into the pharynx or branchial sac which is used for filter feeding, and connects to the digestive system situated in the body wall (Kott, 1989). The anus empties into the atrium below the atrial siphon (figure 1). Ascidians are bilaterally symmetric and lack a coelom. Musculature is controlled by a neural ganglion embedded in the body wall between the two siphons (Zeng & Swalla, 2005).
.jpg)
Figure 1. Diagram of the internal anatomy of Pyura sp based on a dissection and diagrams from Ruppert et al., (2004)
Tadpole larvae and the Chordate hallmarks
Adult ascidians lack most of the hallmark chordate features however these are all present in the tadpole larvae; notochord, dorsal hollow nerve cord, pharyngeal gill slits, endostyle and postanal tail.
The pharynx, digestive system and feeding
This ascidian is a filter feeder and uses it’s pharynx to filter food particles from the water.
Water is drawn into the pharynx through the buccal siphon by the beating of cilia lining the gill slits (branchial stigmata). The water passes through the wall of the pharynx and food particles are collected by a mucous sheet lining the pharynx, secreted by the endostyle (Kott, 1989). Cilia on the pharyngeal lining move the mucous net into the middle of the pharynx where it is then conveyed into the oesophagus (Ruppert et al., 2004). The U-shaped digestive system is made up of the oesophagus, the stomatch at the base of the U, the intestine, rectum and anus. The stomach secretes digestive enzymes and is the site of extracellular digestion (Ruppert et al., 2004). In this species, the stomach lining is folded, characteristic of stolidobranchs (figure 2; Kott, 2005). The pyloric gland which secretes digestive enzymes and plays a part in blood toxin removal is situated on the outer wall of the intestine and opens into the stomach (Ruppert et al., 2004). Absorbtion of nutrients takes place in the intestine and waste is excreted via the anus into the atrium where it is expelled in the water current out the atrial siphon (Barrington, 1965). The pharynx is also used for gas exchange (Barrington, 1965).
.jpg)
Figure 2. From left to right: the intestine, stomach and oesophagus of a dissected Pyura sp
The structure of the pharynx varies greatly between different ascidian taxa. In this species, the pharynx has both transverse and longitudinal vessels and is pleated and folded to increase the surface area for filtration and gas exchange (figure 3).
Figure 3. Pharyngeal lining from a dissected Pyura sp. A shows the transverse and longitudinal vessels and individual pleats B shows many pleats with longitudinal vessels in one fold
Internal transport
Ascidians have an open circulatory system, with blood flowing in a circuit through vessels and small sinuses (Ruppert et al., 2004). The heart is a simple cylinder that pumps blood by peristaltic waves of muscle contraction. The blood does not contain any respiratory pigments such as haemoglobin so oxygen and carbon dioxide are transported in the blood plasma (Barrington, 1965). Periodically the heart will undergo heartbeat reversal and start pumping the blood in the reverse direction (see video). This is to ensure an even supply of nutrients to tissues at either end of the circulatory system (Barrington, 1965).
Heartbeat reversal can be seen here as blood moves through the vessels in the pharynx of the ascidian Pyura sp
Excretion
Ascidians lack nephridia and most likely dispose of nitrogenous waste via diffusion. The pyloric gland is thought to play a role in blood toxin removal (Barrington, 1965).
Nervous system and musculature
The dorsal hollow nerve chord is lost in adult ascidians, leaving only the cerebral ganglion and the neural gland of the central nervous system (Barrington, 1965). The cerebral ganglion is the brain of the ascidian and is located between the two siphons. The nerves from the anterior end of the ganglion control the buccal siphon and muscles associated while the nerves from the posterior end control the atrial siphon and the rest of the organs and muscles (Barrington, 1965).
|