XIII 



PHYLUM CHORDATA 15 



on one side is a second of similar character. The former aperture 

 is termed the oral, the latter the atrial. A strong current of 

 water will be noticed, by watching the movements of floating 

 particles, to be flowing steadily in at the former and out of the 

 latter. When the animal is removed from the 

 water both apertures become narrowed, so as 

 to be almost completely closed, by the con- 

 traction of sphincters of muscular fibres which 

 surround them. At the same time the walls of 

 the body contract, streams of water are forced 

 through the apertures, and the bulk is con- 

 siderably reduced. 



Body-wall and Atrial Cavity. The outer 

 layer of the body-wall is composed of a tough 

 translucent substance forming a thick test or 

 tunic (Fig. 716, test.). This proves when analysed 

 to consist largely of a substance called tunicine, 

 which is apparently identical with the cellulose, 

 already referred to (Vol. I., p. 14) as a charac- 

 teristic component of the tissues of plants, and 

 of rare occurrence in the animal kingdom. 

 The test of an Ascidian is frequently referred 

 to as a cuticle, and it is a cuticle in the sense 

 that it lies outside the ectoderm and is derived 

 from that layer in the first instance. The cells, from the right side. 

 however, by the action of which its substance is 

 added to in later stages, seem to be chiefly de- 

 rived, not from the ectoderm, but from the underlying mesoderm, 

 from which they migrate through the ectoderm to the outer surface. 

 These formative cells of the test are to be found scattered through 

 its substance. Running through it are also a number of branching 

 tubes lined with cells, each terminal branch ending in a little 

 bulb-like dilatation. The interior of each tube is divided into 

 two channels by a longitudinal septum which, however, does not 

 completely divide the terminal bulb. Through these tubes (which 

 are of the nature of looped blood-vessels) blood circulates, passing 

 along one channel, through the terminal bulb, and back through 

 the other channel. 



When the test is divided (Fig. 716) the soft wall of the body or 

 mantle (mant.), as it is termed, comes into view ; and the body is 

 found to be freely suspended within the test, attached firmly to the 

 latter only round the oral and atrial apertures. The mantle (body- 

 wall) consists of the ectoderm with underlying layers of connective- 

 tissue enclosing muscular fibres. It follows the general shape of 

 the test, and at the two apertures is produced into short and wide 

 tubular prolongations, which are known respectively as the oral and 

 atrial siphons (Fig. 718, or. siph., atr. siph.). These are continuous at 



