XVI. INTRODUCTION. 



branchial siphon, and are then entangled in the strings 

 of mucus which are formed in the endostyle and are 

 driven forward by ciliary action, and then round to right 

 and left by means of the " peripharyngeal groove" so as to 

 reach the front of the dorsal lamina, which in its turn guides 

 the mucus-entangled food downwards to the oesophagus 

 (PI. A, fig. 4, ce., see also PL B, fig. 10). 



The oesophagus is a narrow tube which leads to the 

 stomach, a large thick- walled organ lying on the left-hand 

 side of the branchial sac (PL A, fig. 4). From the opposite 

 end of the stomach arises the intestine, a large curved tube 

 which ends by opening into the cloacal portion of the atrium, 

 from which the faecal pellets are carried to the exterior 

 through the atrial aperture by the water current. 



The heart is placed alongside the stomach, and is notable 

 in all Tunicata for its remarkable property of periodically 

 reversing the direction of contraction, so that the course of 

 circulation is at one time in one direction, and about a 

 minute later is in the opposite direction (PL B, fig. 10). 

 In a common British species of Ascidia the reversal of the 

 blood current takes place about every ninety seconds. 



The nervous system consists of a single ganglion placed 

 near the front end of the dorsal edge of the body (fig. 4), 

 between the branchial and atrial apertures. Underneath 

 the ganglion is placed the neural, or hypophysial, gland 

 (fig. 6), the duct of which has a curiously-curled or other- 

 wise complicated opening into the branchial sac at the 

 anterior extremity of the dorsal lamina, and which, from its 

 position and its prominence, is known as the dorsal tubercle 

 (figs. 6 and 7, ^.O. This tubercle is probably a sense organ. 

 The only other sense organs are the pigment spots between 

 the lobes of the branchial and atrial apertures, the tentacles 

 at the base of the branchial siphon, and possibly some parts 

 of the dorsal lamina. 



