The Ascidiacea are the Sea Squirts or Tunicates and they make up the bulk of the species found within the Urochordata. They are all sessile (non-moving or staying in one place) as adults. Most species are common coastal animals occurring in rock pools and out into deeper water to about 400 metres depth, though there are species which have been found living ad depths up to 5,000 metres. They can be either solitary or colonial and the colonial species may share a common exhalent siphon. Many species are translucent or whitish in colour but some species are much more colourful and can be red, brown, yellow and even blue. The coastal habitats of solitary species, and the fact that their tunics are the most solid of all the Urochordates often means that they found intact along the seashore after storms and rough seas. They may still contain water and if trodden on or squeezed this water will shoot out of the exhalent siphon. It is this that gives them their common name of Sea Squirts. The name Tunicates arises from the existence of the tunic.
The Ascidiacean tunic is composed mostly of an acellular (not made of cells) matrix of tunicin, a polysaccharide similar to cellulose, there are some living cells of various sorts within this matrix but they are well spaced out. Typically this tunic is attached to the substrate by a small holdfast and stands upright. It has two openings, an inhalant (water comes in) siphon and an exhalent (the water goes out) siphon. On the inner surface of the tunic is a thin epidermis, it is this which secretes the tunic. Inside of the epidermis there is a thicker dermis and then bands of circular and longitudinal muscle, these muscles can squeeze the tunic causing a jet of water to leave exhalent siphon, this action may help deter predators.
Inside this muscular band is the space where the main body of the animal exists, this space is not a true coelom, because it arises in a different way, the coelom of the tunicates has become degenerate (it doesn't exist any more, though it did in their ancestors). Instead tunicates have a pericardial cavity which surrounds the heart. Some authorities consider this to be a vestigial (having been reduced in size and extent and rendered mostly functionless be evolution) coelom. The rest of the body organs (except the pharynx) are enclosed in a membrane called an epicardium and surrounded by mesenchyme (a loose collection of cells in a jelly like matrix). This is generally called the visceral cavity, as it is in many animals, the viscera being a collective term for the digestive, excretive and reproductive organs combined.
The visceral cavity however is not large and the bulk of the space within the tunicate body is taken up by the atrial cavity. This contains the enlarged pharynx, sometimes called branchial sac. This pharynx has numerous small pores or slits in its walls through which water can pass. It is connected to the digestive tract at one end and to the inhalant (sometimes called the buccal) siphon at the other, this is effectively the animals mouth.
Tunicates feed by drawing water in through the inhalant siphon. This water passes through the pharynx where small particles are filtered out, it then leaves through the exhalent siphon. The water current is maintained by beating cilia, though they can force water out of the atrial cavity by muscular contraction of the tunic if frightened. The small particles, plankton etc, are trapped on a continually moving layer of mucous. This mucous is secreted by special cells and is moved across the surface of the pharynx by the beating of many small cilia, eventually it is passed in the digestive tract where both it and the particles caught up in it are digested.
Tunicates have an interesting blood system. Their blood is pumped around their body through numerous small spaces in the mesenchyme surrounding the viscera. These spaces are called sinuses, they are not true blood vessels, although they perform the same function. The tunicate heart has the unusual habit of beating about 100 beats during which the blood flows in one direction. Then it stops beating for a short time, then it starts beating again but this time the blood is flowing in the opposite direction. The blood of tunicates is normally clear and often contains extremely high quantities of vanadium, a rare element normally occurring in very small quantities in sea water. Nobody yet seems to know why it should collect this vanadium.
Tunicates are mostly hermaphroditic, meaning they are both male and female at the same time. Generally they avoid self fertilisation by either having the eggs and sperm chemically designed to reject each other, or by having the eggs and sperm mature at different times. Sperm are released into the sea but the eggs are retained within the body where they are fertilised by sperm brought in with incoming water. The eggs are brooded within the body until they hatch.
The larvae resemble tadpoles and are far more obviously members of the phylum Chordata than the adults. The larvae swim towards light and thus the surface of the sea at first then after a short while they reverse direction and swim down towards the sea floor, often in less than one day. Tunicate larvae do not feed and are essentially a dispersal form. Soon they find a suitable spot on the sea floor and the settle in a head down, tail up position. They attach themselves to the sea floor (substrate) using special adhesive glands in the front of their head. Once settled they undergo an amazing metamorphosis during which the symbols of the phylum Chordata, the post-anal tail and the notochord it contained are lost. The remainder of the body twists through 180 degrees in order to become a small tunicate. Most tunicates are thought to live about one year as adults.