CLASS TUNICATA. 53 



centre. This opening leads into a great chamber, which 

 may be termed the "respiratory sac" (fig. 21, B, p), 

 and which has a reticulated aspect, owing to its being 

 perforated by numerous small apertures. The Avater which 

 the animal takes in at a passes into this respiratory sac, 

 and then escapes through the openings in its walls into a 

 second chamber or sac, which opens on the surface at the 

 other neck ijj). In this way the creature both breathes 

 and obtains its food. From the bottom of the perforated 

 respiratory sac proceeds a gullet {g) which opens into a 

 stomach (.s), this in turn leading to an intestine {i\ which 

 terminates in a distinct vent (y) at the base of the second 

 chamber. 



There is a distinct heart {li) in the form of a tube open 

 at both ends, and alternately propelling the blood in op- 

 posite directions. The nervous system is in the form of a 

 little nervous mass («) placed between the two necks. 



Ascidia mentula attains a length of from one and a 

 half to three inches, and may be obtained by dredging in 

 from ten to thirty fathoms' depth. It can readily be kept 

 aUve with a little care ; and it is sufficiently transparent 

 to allow of the easy observation of the water-currents 

 which constitute such a striking feature in its vital fi;nc- 

 tions. 



Eecapitulation of Essential Characters. — Body 

 furnished with two openings. Conducting into two cham- 

 bers which occupy the greater portion of the cavity of 

 the animal. Internal organs enclosed in a double sac, 

 the outermost layer of which is more or less leathery, 

 whilst the inner is muscular. The nervous system is in 

 the form of a single nervous mass. There is an alimen- 

 tary canal, and a distinct heart. Respiration effected by 

 currents of water which enter at one of the openings of 

 the sac and are expelled from the other. These char- 

 acters distingTiish the Tunicata as a whole. 



