This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
* "Ueber den Bau und die Lebenserscheinungen des Amphioxus lanceolatus." Berlin Trans. 1842.
The blood of the Amphioxus is not, as in fishes, entirely supplied to the aorta through the medium of the branchiae, but is partly conveyed immediately into that vessel through two large trunks, the representatives of the ductus arteriosus (fig. 309, k), which directly unite the median arterial heart with the aorta, and are, to a certain extent, continuations of the heart itself. They are, however, themselves contractile organs, and are actively employed in the propulsion of the blood, as is the aorta itself (fig. 309, i), which, doubtless, performs the functions of a heart.
(1681). The heart of the vena portae (fig. 309, o) is a long vessel, which runs along the under surface of the intestine as far as the hepatic caecum; its contractions are readily observable in the living animal, the intervals between each being exactly the same as in the other hearts above mentioned.
(1682). The heart of the vena cava (fig. 309, n) is placed opposite to the preceding - that is to say, on the dorsal aspect of the hepatic caecum; it is at first of small size, but gradually becoming larger, ultimately empties itself into the arterial heart (I), which it supplies with blood.
(1683). The contractions of the vessels, or hearts, above described succeed each other in such a manner that each in turn becomes gradually filled, while others contract. The systole of the arterial heart does not commence before the act of contraction has been completed in all the rest of the system. Moreover each trunk contracts in succession with so much energy that it seems to empty itself entirely, and remains for some little time undistinguishable, from which circumstance it necessarily results that any given portion of the blood will have passed through the entire round of the circulation in the time which elapses between the consecutive contractions of the same portion of the vascular system - a space of time which observation shows to be in Branchiostoma about a minute.
(1684). But, perhaps, the most remarkable feature in the anatomy of this singularly-organized being is the apparently complete absence of a brain. The medulla spinalis, slightly thickened towards the central part of the body, tapers off posteriorly as it approaches the tail, where it terminates in a point; and towards the anterior part of the body, as appeared to MM. Batlike * and Goodsir 1, a similar disposition was observable: hence they conceived that the central axis of the nervous system in this fish was reduced entirely to the parts representing the spinal cord in other Vertebrata. Subsequent researches have, however, shown that this is not strictly the case, but that, although there is no cerebral enlargement corresponding to the encephalon of ordinary fishes, the anterior extremity, inasmuch as there exist distinct olfactory 2 and optic organs, must be regarded as essentially encephalic in its nature.
(1685). In tracing the modifications observable in the construction of the vertebral column of fishes, we have a beautiful illustration of the progressive advances of ossification in this the central portion of the osseous system. The spine of the Lamprey, although at first sight apparently entirely soft and cartilaginous, presents already, in the arches which compose the spinal canal, and in the soft cord that represents the bodies of the vertebra, slight indications of an incipient division into distinct pieces: rings of ossific matter are distinguishable, encircling at intervals the soft spinal cartilage, upon which they perceptibly encroach; so that, on making a longitudinal section of the cord, it offers the appearance sketched in the adjoining figure (fig. 310, a.) In a more advanced form of a fish's skeleton, as for example in the Sturgeon, these ossified rings are found to have enlarged considerably, and penetrate still more deeply into the cartilaginous mass (fig. 310, b.) As the bony rings thus developed approximate the centre, it becomes more and more evident that they represent the bodies of so many vertebrae; but even in the majority of fishes the central part remains permanently unossified, so that a cartilaginous axis traverses the vertebral column from one end to the other (fig. 310, c); and it is not unusual to find the central aperture perfectly obliterated, as delineated in the fourth sketch (d).
(1686). Fishes, being continually resident in an element nearly of the same specific gravity as their own bodies, require little firmness or solidity in the construction of their spinal column: a free and unfettered power of flexion in certain directions, so as to permit an ample sweep of their expanded tail, which forms the principal agent in propelling them forwards, is far more essential to their habits. Thus the cartilaginous spine of the feeble Lamprey is sufficient for all needful purposes; and even in the most perfectly ossified fishes, from the manner in which the vertebrae are united to each other, the greatest possible flexibility is ensured. The body of each vertebra presents two conical cups, the apices of which are nearly or quite continuous; the margin of each cup-like depression is united by elastic ligament to the corresponding margin of the contiguous vertebra, and thus between the bodies of each pair of vertebrae a wide cavity is formed (fig. 310, d), which is filled up with a semigelatinous substance; so that, by this beautiful contrivance, the mobility of the whole chain is abundantly provided for.
Fig. 310. Development of vertebral column.
* "Benierkung uber den Bau des Amphioxus lanceolatus" Monatsberichte der Akad. der Wissenschaffcen, 1841.
1 Transactions of the Eoyal Society of Edinburgh, vol. xv. 2 Vide Kolliker, Muller's Archiv, 1843.