Circulating System. - 41 



thin, dark-coloured membrane; the auricles open by con- 

 tracted valvular orifices into two white strong tubes ; these, 

 united, 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 directly 

 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 thickness, 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 cavities 

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

 of boring a very hard substance, and to go on working during 

 the whole of that period of life in which its growth is con- 

 tinued, 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." {Lectures on 

 Comp. Anatomy, vol. iii. p. 162, 163.) 



In the 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, and 

 presents a less distinct muscular structure 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. One of these vessels receives, 

 as it is believed, all the blood from the branchiae, and is in 

 consequence named the branchial vein ; the other, of greater 

 length, is an aorta to distribute the blood through the whole 

 system. {Savigny, Mem. sur les Animauoc sans Vertebres, vol. ii. 

 p. 1130 



