dorsal surface, supplying blood to the visceral organs, and smaller vessels commonly run from both sides into the tunic. Nitrogenous waste, in the form of ammonia, is excreted directly from the blood through the walls of the pharynx, and expelled through the atrial siphon. Unusually, the heart of sea squirts alternates the direction in which it pumps blood every three to four minutes. There are two excitatory areas, one at each end of the heart, with first one being dominant, to push the blood through the ventral vessel, and then the other, pushing it dorsally. There are four different types of blood cell: lymphocytes, phagocytic amoebocytes, nephrocytes and morula cells. The nephrocytes collect waste material such as uric acid and accumulate it in renal vesicles close to the digestive tract. The morula cells help to form the tunic, and can often be found within the tunic substance itself. In some species, the morula cells possess pigmented reducing agents containing iron (hemoglobin), giving the blood a red colour, or vanadium (hemovanadin) giving it a green colour. In that case the cells are also referred to as vanadocytes. Nervous system The ascidian central nervous system is formed from a plate that rolls up to form a neural tube. The number of cells within the central nervous system is very small. The neural tube is composed of the sensory vesicle, the neck, the visceral or tail ganglion, and the caudal nerve cord. The anteroposterior regionalization of the neural tube in ascidians is comparable to that in vertebrates. Although there is no true brain, the largest ganglion is located in the connective tissue between the two siphons, and sends nerves throughout the body. Beneath this ganglion lies an exocrine gland that empties into the pharynx. The gland is formed from the nerve tube, and is therefore homologous to the spinal cord of vertebrates. Sea squirts lack special sense organs, although the body wall incorporates numerous individual receptors for touch, chemoreception, and the detection of light. Life history Almost all ascidians are hermaphrodites and conspicuous mature ascidians are sessile. The gonads are located in the abdomen or postabdomen, and include one testis and one ovary, each of which opens via a duct into the cloaca. Broadly speaking, the ascidians can be divided into species which exist as independent animals (the solitary ascidians) and those which are interdependent (the colonial ascidians). Different species of ascidians can