The large short tubes of the branchial and anal orifices have each five or six obscure marginal indentations, and can be drawn in and closed at the will of the animal. Within and at the bottom of the branchial tube are nu merous, simple, tentacular filaments of different lengths. The particles drawn into the bran chial sac by the current of water are seldom stopped by these tentacles, but lodge some where on the branchial net-work. A lively animalcule will sometimes disengage himself by struggling, and dart about in the cavity until he lodges on some other part ; or if a morsel is found unsuitable, it is ejected by the funnel's being closed, and the branchial sac suddenly contracted vertically. Mostly, however, whatever part the food lodges on, it travels from thence horizontally with a steady slow course towards the front of the cavity, where it reaches a downward stream of similar materials ; and they proceed to gether, receiving accessions from both sides, and enter at last, at the bottom, the cesopha gus ; which is a small flattened tube, carrying them, without any effort of s wallowing, towards the stomach : the oesophagus takes a sharp curve upwards and backwards before arriving there. " It is extraordinary;' observes Mr. Lis ter, "that the particles pass along so near to the spiracles, with their cilia in full activity, with out being at all affected by them." " I have, says he, " in some positions, seemed to catch a glimpse of a membrane suspended within, too transparent to be commonly seen. One may imagine the water to pass to the spiracles strained through the meshes of such a mem brane, and the food to be carried along it by invisible villi ; but this is mere conjecture." The stomach runs backward horizontally. When seen from the side, its anterior portion has an inflated appearance, and, when from below, it seems to possess two lateral lobes. The liver has an ochreous tint, and envelopes the anterior portion of the stomach. The in testine on leaving the stomach rises, and then bends forward with a sigmoid flexure, and ter minates in an ascending rectum and sphincter.
Transparent vessels ramify along a part of the intestine, and meet at a collection of globular bodies, from whence, in one indivi dual, two flattish lobes were observed to extend backwards. These globular and lobate bodies probably constitute the generative organs. From the meeting of the above-mentioned vessels two branches run ; one downwards and backwards, but under the stomach, the other forwards. From their direction Mr. Lis ter supposed them to communicate with a main stream of blood near the heart.
The circulation in these animals is very interesting, and easily discerned through their transparent tissues. The blood circulating in one individual of the group descends by the peduncle into the common root-like stem, and penetrates into the next member of the group, so that there exists in these Ascidians a com mon circulation, having as many centres and motive organs as there are animals growing on the same stem. The blood-globules are very 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 stigmata they pass mostly but one at a time. The creeping tube, which unites the individuals of a group, contains two channels for two separate currents of blood, an upward and a downward one, that are flowing at the same time, and that send off each a branch to every peduncle. The blood then 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 before mentioned, near the bottom of the branchial sac on the left side, and consists of a transparent ventricle or tube, running for ward and a little downward, in a channel hollowed to contain it. Along the whole
length of this tube, a part on one side of its axis seems fixed to the channel, the rest is free and contractile. Mr. Lister observed that when the blood entered the heart from the peduncle, contraction began at the middle of the ventricle, impelling onward the contents of the forepart ; and the contraction 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 peduncle. 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 off at each of the horizontal passages between the rows of spiracles, and traversing the space above them on a line with the junction of the branchial sac and the mantle on each side. All these streams again united, and formed a downward current behind. The horizontal vessels were connected also by the smaller vertical channels between the spiracles ; the set of the current in the latter being up wards for the two lower rows, and downwards for the two upper rows.
Another large portion of the blood, on leaving the heart, immediately divided into many ramifications, that spread like a net-work over the stomach and intestines, and over the mantle. Of these, a part ran into the hori zontal passages above the branchial sac, a part into the descending dorsal vessel. A large proportion, after leaving the intestines, took a short course, and collecting into one channel, flowed into the dorsal vessel near the bottom, and all, united, then entered the peduncle, and constituted the returning current that went to circulate in other animals of the group. After this circulation had gone on for a while, the pulsations became fainter for a few beats, and the flow slower, and suddenly, with but slight pause, the whole current was reversed, the heart gave the opposite impulse. The vessel in the peduncle that before poured in the blood, now carried it back, and the other channel the contrary, and every artery became a vein. These changes continued to succeed each other alternately. The average time of the currents being the same in both directions, but the period of each varying within a single observation as much as from thirty seconds to two minutes. Mr. Lister points out the ana logy between this phenomenon and the very similar circulation that obtains in the stem of the Sertularia, described by him in the same volume of the Philosophical Transactions. This acute observer goes on to say, that some times, when the creeping tube, or the peduncle, has been injured, the circulation of an indi vidual is in consequence insulated, but without appearing to impair any of its functions. In one animal which was severed from the pe duncle, the pulsation ceased for a few seconds. It then began irregularly, and with consider able pauses, increasing in steadiness as it went on. At first the impulse given by the heart was towards the front ; and the downward back stream, instead of flowing out at the wound, was poured into the hinder end of the ventricle ; the cut end of the vessel leading from the heart being nearly opposed to the bleeding dorsal vessel cut through at the same place, and the two vessels, in their undisturbed state, lying across each other at this point. But when the current was reversed, part of the blood was driven for a time through the stump of the peduncle into the water ; how ever, it soon staunched, and all the vital actions went on as before the separation, ex cept that at the beginning of every pulsation there was a slight recoil.