Fig. 309. Amphioxus.

(1671). The branchial chamber is supported by a very singular sort of framework, first described by Retzius and Goodsir, and subsequently more in detail by Professor Muller. It consists of a considerable number (variable according to the age of the animal) of thin rib-like processes, which are united together superiorly, but quite free below, so that they constitute a series of semicircular arches, united together by transverse cartilaginous bands, so as to roof-over the branchial vault. This solid framework is lined internally with a kind of mucous membrane, which, however, is not continuous from rib to rib, and consequently does not fill up the intercostal spaces, but leaves a fissure between each pair of the cartilaginous arches, so that in adult specimens there are as many as a hundred of these branchial fissures or more; nevertheless, as the whole branchial chamber, as well as the margins of these fissures, which are extremely narrow, are closely set with vibratile cilia, it is very difficult to perceive their existence, which, indeed, was denied both by Eathke and Goodsir.

(1672). On placing a living Amphioxus in water coloured with indigo, and observing it with a microscope, it is apparent that the coloured particles that enter the branchial chamber are driven by the ciliary action, partly towards the alimentary canal, and enter the intestine, while another part traverse the branchial fissures, and thus enter the abdominal cavity, where there is no longer any ciliary movement, but the water which flows into it unceasingly through the branchial apparatus forms a continuous current, which finds an exit through the abdominal pore (c), the margins of which exhibit ceaseless movements of contraction and dilatation. Behind the abdominal pore, the cavity of the abdomen is impermeable to water, and closely embraces the terminal portion of the intestine.

(1673). The cavity in which the branchial apparatus above described is lodged contains likewise the greater portion of the alimentary tube, as well as the liver, the generative apparatus, and the kidneys; so that, in fact, it performs the functions both of a respiratory and abdominal cavity.

(1674). The digestive system of this singularly-organized being presents, in many respects, a very degraded type of structure. The branchial chamber above described terminates posteriorly in a short and narrow canal, which is the oesophagus. This opens into a wider intestine, which is always easily distinguishable, owing to the green colour of its parietes. A little beyond the termination of the oesophagus, there is appended to the intestine a long caecum (f), almost as capacious as the intestine itself, which is supposed by some to represent the liver, here reduced to its simplest possible condition. Muller, however, adds that the whole of the intestinal walls, which are lined with a greenish glandular structure, may be regarded as performing the functions of a hepatic organ.

(1675). The whole track of the intestinal tube, as well as the (so-called) hepatic viscus, is covered internally with vibratile cilia. The ciliary action, however, is more especially conspicuous in that part of the intestine which lies beyond the green-coloured portion; and it is here that excrementitious matter begins to be formed, which may be observed turning round and round with velocity in consequence of the surrounding ciliary movement.

(1676). At the posterior part of the respiratory chamber, and close to the abdominal pore, the microscope displays some small detached glandular bodies, which Muller thinks may be the kidneys; he, however, remarks that he could never discover them by dissection.

(1677). The ovaria consist of lax cellular tissue, surrounded with a delicate but strong membrane, which is closed on all sides. They are adherent by one side to the walls of the abdominal or, rather, thoraco-ventral cavity; elsewhere they are covered by the peritoneum. Costa, who first recognized these organs, observed that in the males the testes occupied the same situation as the ovaria. There are neither oviducts nor vasa deferentia; so that the products of the generative organs must necessarily pass through the abdominal cavity and escape through the abdominal pore, as is the case among the cyclostomous cartilaginous fishes.

(1678). The description given by Muller of the circulation of the blood in the Amphioccus is extremely interesting. The circulatory apparatus, while presenting a considerable resemblance to the normal arrangement met with in other fishes, exhibits an equally strong analogy with that of some of the Annelida in its division and distribution.

(1679). Muller enumerates *, as belonging to the circulatory apparatus of Amphioxus, the following parts: - 1. the arterial heart (das Arterienherz); 2. the bulbs of the branchial arteries (die Bulbillen der Kiemenarterien); 3. the aortic arch, which discharges the functions of a systemic heart (der herzartige Aortenbogeri); 4. the heart of the vena portae (das Pfortaderherz); 5. the heart of the vena cava (das Hohlvenen-herz), - the duties assignable to each being as follows: - The arterial heart (fig. 309, 11) is a thick vessel of uniform calibre throughout, situated in the median line, and running immediately beneath the branchial chamber, between the arches forming the framework of that cavity; posteriorly this vessel is continuous with the heart of the vena cava (n.) Before the moment of contraction, the arterial heart is seen to be filled with perfectly colourless blood; but when fully contracted, it is completely emptied (the interval between its contractions is about a minute.) From its sides are given off the bulbs of the branchial arteries (fig. 309, m m), which are little contractile cavities situated at the commencement of each branchial vessel, forming so many little hearts accessory to the preceding. Their number varies with that of the branchial arches, from five-and-twenty to fifty on each side, their office obviously being to distribute unrespired blood through the branchial apparatus. No branchial veins can be distinguished in the living animal; but by carefully detaching the branchial chamber and laying it on a strip of glass, it becomes apparent that the aorta, situated upon the dorsal aspect of the respiratory cavity, receives the veins supplied from each branchial arch.