• The stomach of Botrylloides rotifera(fzg.783.) is pyriform and divided into seven or eight lobes by furrows that pass horizontally from the car diac to the pyloric extremity. The intestine is sigmoid, and is divisible into three portions ; the first part is smooth and transparent, the second is surrounded with a granular tissue„ and the third is membranous like the first. A glandular mass, apparently the hepatic organ, is situated at the commencement of the third portion of the intestine, and gives rise to many minute excretory ducts that soon unite into a single trunk, which appears to empty itself into the intestine near the pylorus. Be low this organ and• more behind, is another glandular mass, which apparently belongs to the generative organs, and gives origin to a little duct passing upwards to the cloaca. The heart is situated laterally, reposing on the cardiac side of the stomach. In B. rubrum the ovary is double, one part being situated on either side of the thoracic chamber. The testicle is seldom very distinct in the Didenz nina and Botryllina, but has been occasionally observed.
The nerve-ganglion between the two orifices, or rather on the dorsal side of the buccal orifice, is generally more or less distinct in the Botryllido.
The circulation of the Botryllide does not materially differ from that of the other Asci dians, and has the same peculiar periodical change in the direction of the blood-currents. In the Polyclinina the heart is placed quite at the inferior extremity of the post-abdomen. It is invested with a thin, transparent pericar dium, and has the form of a large contractile tube, bent on itself; and tapering at the ex tremities. In the Didenznina the heart is shorter, and instead of being situated beneath the ovary, it is lodged with that organ, by the side of the intestinal loop, this condition approaching that existing in the Clavellini&e. Lastly, in the Botryllina, it ascends still higher, being seated near the stomach, nearly at the base of the branchial sac. Milne-Edwards, to whom we are indebted for the preceding facts, remarks that these different positions of the heart always coincide with analogous changes in the position of the ovarium. It is also the same, says he, in the simple Asci dians ; and Cuvier was, without sufficient reasons, led to consider that the heart fol lowed in its position that of the mouth.
If, says M. Edwards, we separate from the common test of any of the Polyclinian species a lively individual, and place it under the mi croscope with a little sea-water, the move ments of the heal• may be easily studied.. The heart's contractions succeed each other somewhat regularly, but they are not brisk and extending at once through the organ, as in the generality of animals. The contraction
commences at one of its extremities, and the narrowing of the tube is propagated in an undulatory manner towards the opposite ex tremity, in a manner somewhat similar to the peristaltic movements of the intestines in the higher animals. For some time the contrac tions follow each other somewhat rapidly, and have all the same direction ; but suddenly they are arrested, and then recommence in a con trary direction. The blood thus sent sometimes from behind forwards, and sometimes vice versa, ascends towards the thorax ; but does not appear to be conducted thither by vessels. It is poured out between the inner tunic of the abdomen and the viscera lodged in that cavity; and here it forms currents, which vary in their position according as the movements of the animal, or any other mechanical causes, op pose their passage. In general, however, the chief portion of the blood ascends by the dorsal or the ventral surface of the abdomen ; and after having bathed the surface of the viscera, it gains the base of the branchial sac. When the heart's contraction is from behind forward, the ascending current passes along the anterior side of the abdomen, and the blood enters a large vertical canal, on the front of the respiratory cavity, termed by Milne-Edwards the great thoracic or ventral sinus. This median sinus gives rise on each side to a series of large transverse vessels, which intercommunicate by means of a num ber of minute vertical vessels, and which, after havino. formed a kind of vascular net work, spread over the walls of the branchial cavity, and terminate in another vertical canal parallel to the ventral sinus, but situated on the opposite side of the thorax. A portion of the blood arrives at the same time in this dorsal sinus without having traversed the branchial net-work, by means of a vessel that arises from the superior extremity of the great ventral sinus and surrounds the base of the branchial orifice. Lastly the blood spreads out between the viscera and the internal tunic of the body, descends along the dorsal side of the abdomen, and again reaches the heart. If the circulation were constant in the above direction, it would somewhat resemble that of the other Acephalans. The heart might then be compared to an aortic ventricle, the tho racic sinus to a great branchial artery, and the dorsal sinus to a branchial vein. But owing to the contrary directions of the blood-cur rents, from the periodically varying impulses of the heart, the vessels that fulfil at one time the functions of veins, at another become arteries, and vice versa.