ASCIDIA. 69 



oesophagus ; the former, penetrating the mantle in the 

 dorsal region, goes associated with the third branch 

 from the dorsal extremity of the heart to ramify in the 

 test. Tims originate the double vessels which carry 

 the nourishing fluid to and from the outer tunic. The 

 stem that goes towards the oesophagus turns aside to 

 join the great ventral branchial channel, the dorsal 

 sinus of Milne-Edwards, which communicates with the 

 ventral extremity of the transverse branchial channels 

 (Plate III, figs. 2 and 3). These latter channels, or 

 primary branchial channels, communicate also with the 

 visceral and pallia! plexuses through the agency of the 

 branchial suspenders. Thus every portion of the 

 organism is brought under the influence of the heart. 



\\Hieii the heart pulsates in the direction of the 

 dorsal extremity the blood is at once thrust into the 



e/ 



test and branchial organ; a portion of it will then find 

 its way by the suspenders into the pallia! plexuses of 

 both sides and into the visceral plexus, and thus to the 

 ventral end of the heart, before reaching which point, 

 however, it will mingle with that from the great 

 ventral branchial sinus and that returned from the 

 test. Thus the blood which reaches the heart is only 

 partially aerated, as neither that which circulates in 

 the test nor that which comes directly from the visceral 

 and pallia! plexuses can to any extent be aerated. 

 When the action of the heart is in the opposite or 

 ventral direction, the reverse of this will take place, 

 but then the only unaerated portion of the blood- 

 return is that which lias supplied the test. 



The pulsations of the heart are not of equal number 

 in each oscillation ; there is usually a little variation 

 in this respect. In a young individual of A. sordid a 

 in which the pulsations were counted there were loS 

 in one direction and 120 in the other. 



The minute structure of the branchial sac varies a 

 little in the different species of this genus. The 

 simplest form of the structure is found in A. -renosa. 

 In this species the transverse or primary vessels or 



