380 MR. LISTER ON THE STRUCTURE AND FUNCTIONS 



onward to the hinder portion, leaving a narrow opake line of connexion with the 

 oesophagus ; the rest of the fore part, of which the apparent volume was nearly as 

 before, having an ochreous tint : this was inferred to be the liver, enveloping the 

 stomach above and on the sides, and accords with its place in other Ascidiae and 

 Mollusca. The line is continued by the intestinal canal, that rises and then bends 

 forward, taking the form of a reversed S, and terminates in an ascending rectum and 

 sphincter k. The faeces are considerable, as might be expected where the food is 

 taken with so little discrimination. Transparent vessels, that may be supposed lac- 

 teals, /, ramify along a part of the intestine, and meet at a collection of globular 

 bodies, from whence in the individual, fig. 2, two flattish lobes extend backward ; 

 in others these were wanting. From the meeting of the vessels two branches ran, 

 one downwards and backwards, which was lost under the stomach, the other for- 

 wards; and from the direction it took, I supposed it might communicate with a 

 main stream of blood near the heart. Some individuals had not the projection above 

 the vent observable in fig. 2. 



But the part that struck me as most remarkable in this creature was the circu- 

 lation, of which a good view can be obtained through the transparent coat ; for the 

 particles of the blood are numerous, and though not uniform in size or shape, are 

 mostly between -00025 and -0002 inch in diameter, and approaching to globular. 

 They are easily measured, as in the intervals between the spiracles they pass mostly 

 but one at a time (fig. 7). 



The creeping tube, which unites the individuals of a group, is the channel for two 

 separate currents of blood, an upward and a downward one, that are flowing at one 

 and the same time, and that send off each a branch to every peduncle : the blood 

 thus passes into the animal by one current, while another carries it back. One of 

 these canals communicates at the termination of the peduncle with the heart ; which 

 is placed, as has been mentioned, near the bottom of the branchial sac on the left 

 side, and consists of a transparent ventricle, or boyau, running forward and a little 

 sloping downward, in a channel hollowed to contain it. Along the whole length of 

 the boyau a part on one side of its axis seems fixed to the channel, the rest free and 

 contractile. 



When the blood entered the heart from the peduncle, contraction began at the 

 middle of the ventricle, impelling onward the contents of the fore part ; and the con- 

 traction of the back part followed in the same direction, so as for the whole to have 

 the effect of one pulsation : the heart was then filled again by a flow from the pe- 

 duncle. The intervals of the pulse were pretty regular in the same individual ; but 

 in different ones they varied from two seconds to one and a half second. Part of the 

 blood thus impelled formed a main upward stream along the front of the branchial 

 organ, branching oflf at each of the horizontal passages between the rows of spiracles, 

 and at one above them on a line with the junction to the mantle on each side. All 

 these again united, and formed a downward current behind. The horizontal chan- 



