444 THE ANIMAL KINGDOM. 



and in Salpa it is reduced to the dorsal lamina with a great gap on either 

 side of it. 



The digestive portion of the alimentary canal is formed as an out- 

 growth of the archenteron. It is ciliated in part or wholly, especially the 

 intestinal section. It lies either behind the pharynx, or to the left side of 

 it (some A. Simplices and Compositae\ or is reduced in size and placed 

 quite to the ventral side, as in Pyrosoma, Doliolum, and Salpa, and in the 

 last it forms, with the other viscera, a small mass termed ' nucleus.' The 

 first portion of it (' oesophagus ') is narrow, and variable in length. The 

 stomachal region is more or less dilated, and the intestinal tubular. The 

 anus opens on the ventral surface between the stigmata, or on the right 

 side in Larvacea. In other Urochorda the intestine has an adoral and 

 dorsal twist, and the anus opens just within the atrial aperture, i.e. is dorsal 

 in position. When this aperture is terminal and posterior, the course of 

 the intestine is still towards the dorsal aspect. In Larvacea there are no 

 specialised gland cells at all in the digestive tract. Glandular caeca coat 

 the stomach in many A. Simplices and Compositae, and in the Molgulidae 

 form a lobed mass. In most Urochorda a system of clear tubes ramifies 

 over the intestine and sometimes the stomach as well, occasionally anasto- 

 mosing freely (some Salpae) and ending in ampullae (Ascidia, Perophora 

 Salpa, Pyrosoma). They open by one or two large tubes into the posterior 

 part of the stomach. Their function is not clear, but cilia have been 

 detected in them in Perophora and Doliolum. 



A heart is absent only in Kowalewskaia. In other Larvacea it lies 

 more or less ventrally close to the stomach, is elongated transversely, and 

 at either end of the longer axis is a large cell. Between the two cells 

 stretch muscle fibres. In other Urochorda the heart is tubular, with circular 

 muscle cells, and is contained in a pericardial sheath. It lies ventrally on 

 the right or left side, close to the stomach, in front of it, along it, or, in 

 Clavellinidae and many A f Compositae, behind it. In all Urochorda the 

 direction of its beats undergoes periodic reversal. In Larvacea there are 

 no vessels and no blood corpuscles nothing but irregular and small spaces 

 representing a coelome. The direction of the currents can only be detected 

 when the blood is infested with parasitic organisms. Other Urochorda 

 possess vessels in connection with both ends of the heart. A main blood- 

 channel lies below the endostyle, another above the dorsal lamina. The 

 two are connected by transverse vessels encircling the pharynx, which in 

 their turn are connected by minute longitudinal interstigmatic vessels. The 

 latter are absent in Pyrosoma. There are sometimes large longitudinal 

 vessels running on the inner surface of the pharynx, connected to the 

 transverse vessels where they cross them, e.g. in Pyrosoma, many Ascidiae, 

 &c. Branches from the pharynx pass into the body walls and test. The 

 visceral vessels are connected to the dorsal vessel of the heart and pharynx. 



