The intestinal canal is opaque, fuscous, or variously tinted, generally closely folded or convolute, and sometimes enveloped in the liver, forming altogether the " visceral nucleus." This, together with the heart, lies external to the mantle, between it and the test. The oesophageal aperture is in the antero-inferior part of the body, behind the heart, more or less conspicuous and variously modified; dissimilar in the alternate " proles " of the same species. In the solitary " proles" of S. pinnata and S. afignis it is stretched above the branchia. In the aggregate " pro les" of S. pinnala it is opened out longwise, and of a violet colour in the living specimen.
In S. cristata the intestinal canal has the following characters. The oesophagus is round, with a loosely plicated margin ; the sto mach has a contrary direction to the rest of the canal, being a cul-de-sac pointing ante riorly, and situated in the thickness of the antero-inferior protuberance of the test. It is membranous and transparent, and is de scribed by Cuvier as ordinarily containing a little greyish fluid. The intestine is a simple tube having a direction unusual among the Salpians ; it runs from the stomach towards the posterior extremity of the body, where it opens into the branchial cavity by a rather large anus. The faecal matter contained in it is greenish and vermiform. The liver (testicle, Krohn) in this species appears to be composed of large, parallel, longitudinal filaments, and terminates posteriorly in a delicate, tapering point. It differs also from the liver of other species in being of a whitish colour.
The intestine of the Salpians is usually twisted once or twice either around or within the liver, with the anus terminating nearly free of the latter, near the anterior attachment of the branchia. The anus is generally on the left side, opening posteriorly. The rectum never traverses the heart. In S. gibbosa and S. infundibulifornzis the intestine makes a little more than one turn, the two ends crossing one another a little. It has two caeca, one on each side, which are turned into the centre of the loop of the intestine. Eschricht describes the liver of S. zonaria as conspicuous, enve loping nearly all the alimentary canal, and consisting of a mass of ccecal tubes, each of which bear, near their free extremities and on one side only, a group of 2-6 minute, short ececal appendages.
The heart is with difficulty observed in dead specimens, but, from its pulsating move ments, is generally conspicuous in the living animals ; and in these only can the circulatory apparatus be traced out. It is situated in the antero-inferior region of the body, near the visceral nucleus, the anterior attachment of the branchia, and the generative organs. It is a somewhat long, pellucid, tubuliform ve sicle, enclosed in an immoveable pericardium. A longitudinal vessel or sinus (aorta, of Van Hasselt) traverses the inferior surface of the branchial cavity, and is continued into the in ferior lip of the posterior orifice, and into the base of the posterior prolongation (if present) of the test. Hence it is retroflexed and recur
rent. It gives off numerous branches at right angles on either side (fig. 787.) from which arise a great number of smaller branches, that sub divide, anastomose, and spread out in different parts of the body, forming a fine vascular net work. All the transverse canals open into a large dorsal vessel, which thus receives all the blood which has passed through the vessels of the branchial sac ; but, besides this, some blood is also received by it, which has not passed through this tissue, in consequence of the great dorsal vessel being connected at each extremity with the great thoracic or abdominal vessel by two considerable annular vessels. The blood returns downwards from the dorsal vessel through a canal lying on the dorsal surface of the abdomen (dorsal canal) to the opposite end of the heart. The vascular net-work is very conspicuous in S. aspera, and is sometimes particularly distinct in the appendages. These ramifications go off from one another at right angles, and afterwards are, for the most part, bent back archwise, as Chamisso and Van Hasselt have observed ; so that, with the exception of those running transversely, all these little vessels have a direction contrary to that of the principal vessels ; that is to say, they are directed from behind forwards, whilst the aorta runs from before backwards. At the anterior extremity of the heart are two ves sels, that answer to the pulmonary veins (dorsal sinus). They are equally distributed in the body of the animal, anastomosing with the branches of the principal sinus (aorta). The circulation of the blood among the vis cera is carried on by means of variable in terspaces occurring between these organs. The motions of the heart are made spirally, by a twisting of its parietes, and always begin from one or other of its extremities. As in other Tunicata, this action is oscillatory, having an alternately contrary direction, first impelling the blood in one direction, then stopping, contracting again, and soon impelling it in an opposite direction ; so that, after the blood has been flowing for some time from the heart to the aorta, to be distributed to the body, it stops, and then begins to run by the ar teries and the aorta to the heart, and from thence, by the pulmonary veins and their anas tomoses, it returns into the arteries and aorta. The contractions of the heart, in general very regular, diminish in rapidity at the ap proach of the periodic change in the circu lation, the blood stopping, and even retreat ing a little until a general contraction of the body determines it to take an opposite direction. The duration of the opposite cir culation is not always the same. Van Has selt saw the blood flow for three-fourths of a minute from the heart to the aorta ; and, dur ing this time he observed forty-two contrac tions of the heart ; and he saw it reflow from the arteries to the heart and the pulmonary veins for a third of a minute, and in this inter val he counted sixty-two pulsations.