214 CIRCULATION IN THE TUNICATA. 



form the ventricle, which terminates in an artery that goes 

 to the boring-shell. The heart is loosely attached ; its action 

 is distinctly seen through the external covering, and in some 

 instances continued to act after it was laid bare. 



" The first contraction is in the two auricles, which are 

 shortened in that action. This enlarges the ventricle before 

 it contracts. The great artery from the ventricle goes di- 

 rectly to the head, and the vessels that supply the auricles 

 are seen to come from the gills. The auricles are lined 

 with a black pigment, so that their contents cannot be 

 seen through their coats, and the ventricle, from its thick- 

 ness, is not transparent ; but the muscles of the boring-shells 

 are of a bright red, and all the parts between the heart 

 and head are supplied with red blood." 



In the Teredo, then, every part of the blood " passes 

 through the vessels of the gills, and then through the cavi- 

 ties of the heart. As this animal is to work a machine 

 capable of boring a very hard substance, and to go on work- 

 ing during the whole of that period of life in which its 

 growth is continued, to make room for the increased bulk, 

 so it requires that the blood be more highly aerated, and 

 supplied with greater velocity to these active organs. The 

 heart, also, to give it greater advantage in these respects, 

 is placed near to the boring-shells, so that the blood which 

 goes to them is of the brightest colour. 



" In this circulation the first action of the heart is to 

 supply the different parts of the body with aerated blood : 

 upon this the activity of the heart is wholly exerted ; the 

 blood is returned more slowly through the gills, and remains 

 there a longer time, so as to receive a greater degree of the 

 influence from the air contained in the water. "* 



In the single Mollusca Tunicata the circulating apparatus 

 is simpler than in any of the other orders. The heart of 

 the Ascidia is an organ with a single cavity, situated near 

 the stomach, f and presents a less distinct muscular struc- 

 ture than it does in cephalous mollusca. It is of an oblong 

 or spindle shape, and the two extremities are prolonged 

 into two vessels, almost equal to itself in their diameter. 



* Lect. Comp. Anat. iii. 162, 163. 



t According to Cuvier the position of the mouth determines the position 

 of the heart ; but according to Milne-Edwards, in all Tunicata the position 

 is regulated by the situation of the ovaries. " L& ou les ovaires sont thora- 

 ciques, le creur Test e*galement ; 1& ou les ovaires sont loges avec le tube 

 digestif dans un abdomen simple, le coaur est placd aussi a cote de 1'intestin 

 dans cette meme cavite' ; enfin, la ou 1'ovaire est infere et se trouve dans un 

 post-abdomen, le cosur est egalement rele"gue dans cette portion terminale 

 du corps." Obs. Sur les Ascid. Comp. p. 7. 



