00 TUNICATA. 



mouth inside this respiratory cavity, which in the Ascidians has 

 the form of a latticed branchial sac. A ciliated groove bounded by 

 two folds extends along the middle ventral line of the pharyngeal 

 cavity, between the mouth and the opening of the O3sophagus. The 

 glandular walls of this ventral groove are distinguished as the endo- 

 style (figs. 558 and 559, End). It begins with two lateral ciliated 

 arcs, which unite to form a complete ring near the inhalent aperture 

 (mouth), and somewhat in front of the ganglion pass over a small 

 cone projecting into the pharyngeal cavity. 



The digestive canal which follows the pharyngeal cavity consists 

 of a ciliated oesophagus, which is usually narrowed into the form of 

 a funnel ; of a stomach, usually provided with a liver ; and of a small 

 intestine, which bends round, forming a loop, and opens into the 

 cloacal cavity. 



There is always a heart, which is placed on the ventral side of the 

 intestine and is surrounded by a delicate pericardium. The con- 

 tractions, which are active and regular, pass from one end of the 

 heart to the other. 



The sudden change in the direction of the contractions (discovered 

 in the Salps by Hasselt), by which after a momentary period of rest 

 the direction of the blood stream in the heart is reversed, is worthy 

 of note. The vascular trunks (lacunae) passing from the heart lead 

 into a system of spaces in the body wall through which the blood 

 passes. In the Ascidians there are also vascular loops in the mantle, 

 in that diverticula of the body wall, containing blood and covered 

 with epidermis, project into the mantle. Two principal channels 

 for the blood are placed in the middle line one on the dorsal side, 

 and the other on the ventral beneath the ventral groove ; they are 

 connected by transverse channels placed in the wall of the branchial 

 cavity. The latter communicate with the blood spaces of the 

 variously-shaped branchia, which is formed by the walls of the 

 pharynx, and over the surface of which the water is continually 

 renewed by means of the vibratile cilia which cover it. In the 

 Ascidians almost the entire wall of the pharynx takes part in the 

 formation of the gill. In these animals the pharynx has the form 

 of a sac with net-like walls i.e., its walls are perforated by a number 

 of slits, which lead from the pharynx into a chamber which is de- 

 veloped round it. This chamber is derived from the cloacal cavity, 

 and is known as the peribranchial chamber. The branchial sac or 

 pharynx is fixed to the walls of the peribranchial cavity along the 

 whole length of the endostyle, and by numerous short trabeculse 



