Internal Anatomy

Tunic, Body Wall and Musculature

The body wall of ascidians is overlain by a tunic composed of proteins, carbohydrates and tunicin fibres. Tunicin is a cellulose-like material unique to the tunicates. These fibres run parallel and occur in successive sheets of different orientation, this acts to give the tunic strength - just like the successive layers in plywood. Although the tunic is a form of exoskeleton, ascidians are the only animals that grow with these protective coverings and so eliminate the need to moult. The reddish tint is largely owing to scattered blood capilliaries in the body wall and bright red pigments in the vascular ampullae (attachment organs). Hemocytes also feature in the tunic, adding to its colour. Herdmania momus also has calcareous spicules of about 1-2um long that are symmetrically tapered on each end and have short, sharp spines that project backwards encircling the spiracle shaft.

Underneath the tunic, lies a single layered skin or epidermis. This encloses a basal lamina and a thick jelly-like connective tissue in which muscles, nerves, ameboid cells and blood vessels occur. The body wall is thin and composed of striated muscles and spicules. Musculature is composed of outer circular and inner longitudinal bands of smooth muscle. Circular bands, those which run along a dorso-ventral (back-belly) axis forms tight rings around the branchial siphon as well as running up the atrial siphon. These muscles predominate on the siphons, regulating the size of their openings. Longitudinal muscles running along the anterior, posterior-axis extend along the body to the siphons and when contracted act to withdraw the siphons.

Pharynx and Atrium

The interior end of buccul siphon opens into the pharynx where the mouth is surrounded by a ring of buccal tentacles which prevent entry of large unwanted particles by projecting in to the water flow and obstructing their path.  The pharynx walls are perforated by ciliated gill slits that allow water to pass from the pharynx to the surrounding atrium. Running along the other side of the pharynx (the ventral side) is a mucous secreting groove called the endostyle. Where the pharynx meets the buccul siphon the endostyle splits into two bands that encircle the opening and then converge at the midline to form a dorsal lamina which is lined with tentacle-like languets. The water-filled atrium surrounds the pharynx and opens to the exterior through the atrial siphon. The cavity inside the atrial siphon receives waste and gametes and so is termed the cloaca.  

Digestive system

Down from the pharynx is Herdmania’s D-shaped gut loop composed of the oesophagus (the decending arm), the stomach (an enlargmenet at the base of the U) and the ascending arm (the intestine) and then the terminal end (the rectum and anus). The oesophagus connects to the branchial sac below the dorsal lamina so that food caught by the branchial sac may be channeled to the stomach (Degnan 1991). The stomach, much like our own, is lined with cells that secrete digestive enzymes and the intestine is likely the site of absorption. A network of tubules called the pyloric gland covers the outer wall of the anterior intestine and opens into the base of the stomach, secreting digestive enzymes, removing blood borne toxins and storing glycogen. Cauliflower shaped liver lobules protrude from the pyloric portion of the gut and the gut loop is loosely associated with the body wall (figure 1). Excretory organs are lacking and organisms must rely on diffusion of ammonia across the pharynx. Other metabolic by-products are stored in nephrocytes which accumulate in tissue and are only released at death. This is known as storage excretion.

Hemal System

Like all ascidians, Herdmania momus has a well developed hemal system that includes a heart, vessels and sinuses through which blood rich in hemocytes but lacking in respiratory pigments  flows through in a definitive cirtcuit. Gas is transported in the plasma. The heart is a small, tubular cylinder found at the base of the digestive loop. Heart contraction is caused by filaments of the myocardium in a manner that pushes blood forward and prevents flow backward. Blood vessels are simple channels in the connective tissue and although circuit is not clearly established the flow pattern and placement of major blood vessels is well understood. Blow flows from the heart into a large ventral aorta below the endostyle that supplied blood to the gills. Capilliaries here join a mid dorsal vessel and aorta that carries blood to the gut and other viscera. Vessels from these visceral eventually return blood back to the heart and complete the circuit. Heartbeat is myogeneic and pacemakers at each end alternate in dominance to make opposite beat directions.

Reproductive system

There is a single long gonad in the connective tissue on each side of the body wall, containing both eggs and sperm and attached on the ovary side. The gut loop is encloses the left gonad at the posterior end, it has a long deep orange ovary is long overlain by a light orange test (figure 1) (Degnan 1991). The oviduct and sperm duct are separate, run parallel to the intestine and open into the atrium near the anus. Eggs and sperm travel to the cloaca releasing small amounts nightly through a process called trickle spawning.

Nervous System

The dorsal hollow nerve cord of all ascidians is lost during larval metamorphosis leaving the adult body plan secondarily simplified and divergent from other chordates. Remnants of the neural tube form the brain and the neural gland. The brain, termed the cerebral ganglion, is located in the connective tissue between the two siphons. The nerves here control body, pharynx and siphon contraction . The neural gland lies beneath the brain but actually has no nerves. Sensory organs are also lost during the transition from larvae to adult but sensory cells remain on the surface of the siphons, buccal tentacles on the atrium which are used for controlling water flow through the pharynx.