TUNIC ATA. 475 



cording to the direction of the circulating currents the one trunk- 

 vessel will be an artery, the other a vein, and the circulation itself 

 will be pulmonic or systemic. 



The nervous system must be first sought for in the interspace 

 between the two openings of the muscular tunic ; there you will find 

 a ganglion {g), from which it is not difiicult to trace filaments 

 diverging to each aperture of the sac where the circular disposition 

 of the muscular fibres prevails ; other branches accompany the 

 longitudinal fibres, and supply the respiratory sac ; two contiguous 

 filaments are continued to the oesophageal orifice. 



Eight pigment- or eye-specks have been detected at the entrance 

 of the respiratory tube ; and six, of a deep yellow colour, at the 

 entrance of the anal tube. 



In the animal manifesting this organisation, which is much richer 

 unquestionably than the amorphous and rugged exterior would seem 

 to promise, the only vital actions obvious to ordinary vision are an 

 occasional ejection of water from the orifices of the tunic by a sudden 

 contraction, succeeded by a slow and gradual expansion. Such con- 

 tractions and expansions, aided by the ciliary currents, and the 

 peristaltic movements of the alimentary, circulating, and secerning 

 tubes, are all the actions which the organic machinery has to perform 

 in the living ascidian. 



The respiratory currents of sea water with the nutrient molecules 

 in suspension are introduced by the ciliary action through the 

 branchial orifice {e, fig. 179) into the pharyngeal respiratory sac, 

 from which the oesophagus (a) selects the appropriate food. The 

 alimentary excretions and the generative products are expelled 

 through the anal outlet (c) by the contraction of the muscular tunic. 



In consequence of the space between this and the outer tunic being 

 closed, that tunic accompanies the muscular tunic in its contraction, 

 through the influence of the surrounding pressure ; when the muscle 

 ceases to act, the elasticity of the outer coat begins to restore the 

 fibrous sac to its former capacity, and the surrounding water flows 

 into its cavity, either directly or by distending the branchial sac. 

 We shall find other instances of the economising of muscular force 

 by the substitution of elasticity as we ascend in the survey of the 

 molluscous organisation. 



In the small compound Ascidians the organisation is essentially 

 like that of the solitary species, but the viscera are somewhat dif- 

 ferently disposed : the cavity of the body is longer and narrower, the 

 entire animal viewed singly being more vermiform. In their natural 

 organic association they are arranged in different modes, and under 

 different forms ; some, as the beautiful Diazona, diverging like the 



