3i 8 MANUAL OF ZOOLOGY SECT. 



to drive floating particles that come within their influence outwards 

 towards the oral aperture, the latter secreting and discharging a viscid 

 mucous matter. Anteriorly the endostyle is continuous with a ciliated 

 ridge which runs circularly round the anterior end of the pharynx. In 

 front of this circular ridge, and running parallel with it, separated from 

 it only by a narrow groove, is another ridge of similar character; these 

 are termed the peri-pharyngeal ridges, the groove between them is the 

 peri-pharyngeal groove. Dorsally, i.e., opposite the endostyle, the 

 posterior peri-pharyngeal ridge passes into a median, much more 

 prominent, longitudinal ridge, the dorsal lamina, which runs along the 

 middle of the dorsal surface of the pharynx to the opening of the 

 oesophagus. The mucus secreted by the gland cells of the endostyle 

 forms viscid threads which entangle food-particles (microscopic organ- 

 isms of various kinds) ; the cilia of its ciliated cells drive these forwards 

 to the peri-branchial groove, around which they pass to the dorsal 

 lamina, and the cilia of the cells of the latter drive them backwards to 

 the opening of the oesophagus. 



Some little distance in front of the anterior peri-pharyngeal ridge, at 

 the inner or posterior end of the oral siphon, is a circlet of delicate 

 tentacles (Fig. 198, tent). 



The oesophagus leads from the pharynx (near the posterior end of 

 the dorsal lamina) to the stomach, which, together with the intestine, 

 lies embedded in the mantle on the left-hand side. The stomach is a 

 large fusiform sac. The intestine is bent round into a double loop, and 

 runs forwards to terminate in an anal aperture, situated in the atrial 

 cavity. There is no liver; but the walls of the stomach are glandular, 

 and a system of delicate tubercles which ramify over the wall of the 

 intestine is supposed to be of the nature of a digestive gland. 



The Ascidian has a well-developed blood system. The heart is a 

 simple muscular sac, situated near the stomach in a pericardium forming 

 part of the primitive ccelom. Its mode of pulsation is very remarkable. 

 The contractions are of a peristaltic character, and follow one another 

 from one end of the heart to the other for a certain time; then follows 

 a short pause, and, when the contractions begin again, they have the 

 opposite direction. Thus the direction of the current of blood through 

 the heart is reversed at regular intervals. 



The nervous system is of an extremely simple character. There is a 

 single nerve-ganglion, which lies between the oral and atrial apertures, 

 embedded in the mantle. This is elongated in the dorso-ventral direc- 



