TUNIC ATA. 



87 



body, the muscles, nervous system, the digestive apparatus, the 

 generative and circulatory organs, lie embedded in a kind of body 

 cavity. 



The nervous system is confined to a simple ganglion, the position 

 of which near the inhalent aperture marks the dorsal surface. The 

 nerves which radiate from the ganglion branch and pass, some to 

 the muscles and viscera, some to the sense organs such as eyes, 

 auditory and tactile organs which are found principally in the free- 

 swimming Tunicates. 



The muscular system is chiefly developed around the respiratory 

 cavity, and serves for the dilatation and contraction of this space as 

 well as for closing the inhalent and exhalent pores. In the Ascidians 

 there are three layers of muscles, an external and internal longi- 



Kl 



Nu. 



End M(l 



FIG. 559. Salpa democratica from the side, somewhat diagrammatic. 0, Mouth ; Ph, pharyngeal 

 cavity; Kl, cloaca: A, exhalent opening; Br, gills; N, nervous centre; Ma, mantle; 

 M, muscular rings; Z, languet ; Wb, ciliated arc; End, endostyle; Wr, ciliated groove 

 Nu, nucleus ; C, heart. 



tudinal and an internal circular layer, while iu the Salps there are 

 band-like rings of muscles embedded in the substance of the body- 

 wall, and effecting not only the renewal of the water used in 

 respiration, but also the movements of the free-swimming barrel- 

 shaped body. A special organ of locomotion is present in the small 

 Appendicularia and the free-swimming Ascidian larvse ; it is placed 

 on the ventral surface as indicated by the position of the heart, and 

 consists of a vibratile whip-like caudal appendage supported by a 

 notochordal rod (urochord). 



The alimentary canal begins in all cases with a wide pharyngeal 

 cavity, which functions as a respiratory organ. The anterior mantle 

 opening, which must be looked upon as the mouth, leads into this 

 cavity. The cesophageal opening is placed at a distance from the 



