Amphioxus

“ It has no legs, and neither head, skull, nor brain. Externally, the fore end of the body can be distinguished from the hinder end only by the open mouth. But still the Amphioxus in its internal structure possesses those most important features, which distinguish all Vertebrate animals from all Invertebrate animals, namely, the spinal rod and spinal marrow. ”

“ These first gill-slits in Amphioxus are later subdivided by the downgrowth of the tongue-bar from the dorsal edge.
Fig. 88.—Ventral aspect of three larvae of Amphioxus, showing the metapleural folds and the formation of the atrium. ap, Atriopore; k, gill-slits; lf and rf, left and right metapleural folds; m, mouth; w, pre-oral pit. (From Korschelt and Heider, after Lankester and Willey.)
The atrium is an ingrowth of the external space between the two ventral metapleural or atrial folds (Figs. 88 and 89) , paired lateral ridges of the body-wall, and so is lined by ectoderm. ”

“ In spite of various features rendering the development of the Frog more difficult of comprehension than that of most other vertebrates, it is easy to see that the step between it and Amphioxus is not a very great one, and will very likely be bridged over at some future time, when our knowledge of the development of other forms becomes greater. ”

“ Its gills are not like those of fishes, but its branchial apparatus is that of an Ascidian (Fig. 40) , confirming the view of the origin of the Amphioxus from the Ascidian Worms, suggested by their identical development. What is the Amphioxus? It seems to be an intermediate animal, a link connecting the Ascidian with the Fishes. The part of the body containing the mouth is usually regarded as the head, and is membranous in structure, which condition is found in fishes at certain periods of their existence. ”

“ The young amphioxus has, at this stage, which is called the gastrula stage, a curious parallelism with such a lowly form as the Hydra of our ditches. This latter creature, like the gastrula, consists essentially of two layers of cells, an outer protective and sensory layer, and an inner digestive one; it has a primordial intestine, or archenteron, and its mouth is sometimes regarded as being a blastopore. All animals that have little yolk, and start early in life for themselves, pass through a gastrula stage, substantially the same as this of amphioxus. ”

“ If we consider the fish in its most primitive varieties, we certainly seem led to the conclusion that it is the form to which the evolution of life would lead in any planet, as the basis of the higher development. In Amphioxus, for instance, we find the elongated bilateral animal simplified to an extraordinary degree; without external armor of any sort, with the simplest vital organs, with only the beginning of an internal skeleton, and with merely the suspicion of a fin, virtually a flattening of the skin. ”

“ Amphioxus, like the tunicates, does not possess the characteristics of other vertebrates. In all vertebrates above these forms the great characteristic is a well-defined brain-region from which arise nerves to organs of special sense, the eyes and nose. In Amphioxus no eyes exist, for the pigmented spot at the anterior extremity of the brain-region is no eye but only a mass of pigment, and the so-called olfactory pit is a very rudimentary and inferior organ of smell. ”

“ In the Mammalia the teeth, though restricted in number, attain the greatest possible variety of form, so that the jaws of different but allied species may be distinguished by their teeth.
[149] Let us now return to the lowest vertebrate of all, which has a large notochord and no bones. This is the Amphioxus, the Lancelet. Amphioxus has no bones whatever, and no head, in the sense in which we usually employ that term; that is to say, most of the structures which we see in the vertebrate head are undeveloped. The peculiarities of the structure of Amphioxus are many. ”

“ Amphioxus no doubt is specialized in many respects, and probably degenerate in others, just as, if we go to the other pole of the Craniates, we know that although the Anthropoid Apes are the nearest living representatives of the ancestor of man, they are specialized in many respects and almost certainly degenerate in other respects. ”

“ It is true that the Amphioxus is far below all other living vertebrates. It is true that it has no separate head, no developed brain or skull, the characteristic feature of the other vertebrates. It is (probably as a result of degeneration) without the auscultory organ and the centralised heart that all the others have; and it has no fully-formed kidneys. Every single organ in it is simpler and less advanced than in any of the others. Yet the characteristic connection and arrangement of all the organs is just the same as in the other vertebrates. ”

“ In the Teleostei a very shallow depression along the axis of the keel is the only indication of the medullary groove of other forms.
In Amphioxus (fig. 190) , the Tunicata, Petromyzon (?) , Elasmobranchii (fig. 182) , the Urodela and Mammalia (fig. 187) , the epiblast of the medullary plate is only formed of a single row of cells at the time when the formation of the central nervous system commences; but, except in Amphioxus and the Tunicata, [Pg 303] it becomes several cells deep before the completion of the process. ”

“ There are only two pairs of cerebral nerves, both of which are sensory (Willey, Amphioxus, 1894) . In the Cyclostomata, of which the lamprey (Petromyzon) is an example, the minute brain is much more complex, though it is still only a very slight enlargement of the anterior end of the cord. The single cavity seen in Amphioxus is here subdivided into three: an anterior or prosencephalon, a middle or mesencephalon, and a hinder or rhombencephalon. The rhombencephalon has a very slight transverse thickening in the fore-part of its roof, this is the rudimentary cerebellum (Cer.) ”

“ Haeckel regard the formation of this floor as part of an involution to which the lower layer cells owe their origin, and consider the involution an equivalent to the alimentary involution of Batrachians, Amphioxus, &c. To this question I hope to return, but it may be pointed out that my observations prove that this view can only be true in a very modified sense; since the invagination by which hypoblast and alimentary canal are formed in Amphioxus is represented in Elasmobranchii by a structure quite separate from the ingrowth of cells to form the floor of the segmentation cavity. ”

“ The mode of formation of the mesoblast is not certainly known in Amphioxus; we shall find, however, that for all other vertebrates it arises from the cells which are homologous with the involuted cells of this animal.
Since food material is a term which will be very often employed, it will be well to explain exactly the sense in which it will be used. ”

“ All who have worked with Amphioxus must be aware how difficult it is to conserve the animal in a satisfactory state for making sections. The spinal cord, especially, is apt to be distorted in shape, and one of its ventral corners is frequently produced into a horn-like projection terminating in close contact with the sheath. In such cases the connective tissue fibres of the sheath frequently present the appearance of a nerve-like prolongation of the cord ”

“ For all these reasons the lowest form of fish to be investigated, in the hopes of finding out the nature of the earliest formed fish, is not Amphioxus, but Ammocœtes, the larval form of the lamprey—a form which, as I hope to satisfy my reader after perusal of subsequent pages, more nearly resembles the ancient Cephalaspidian fishes than any other living vertebrate. ”

“ IX. On the Spinal Nerves of Amphioxus [61] . During a short visit to Naples in January last, I was enabled, through the kindness of Dr Dohrn, to make some observations on the spinal nerves of Amphioxus. These were commenced solely with the view of confirming the statements of Stieda on the anatomy of the spinal nerves, which, if correct, appeared to me to be of interest in connection with the observations I had made that, in Elasmobranchii, the anterior and posterior roots arise alternately and not in the same vertical plane. ”

“ From Kowalewsky’s investigations, published in 1867, on the individual development of the Amphioxus and the adhering Sea-squirts (Ascidia) belonging to the class of mantled animals (Tunicata) , it has been proved that the ontogenies of these two entirely different looking animal-forms agree in the first stage of development in a most remarkable manner. The freely swimming larvæ of the Ascidians (Plate XII. Fig. A) develop the undeniable beginning of a spinal marrow (Fig. 5 g) and of a spinal rod (Fig. 5 c) , and this moreover in entirely the same way as does the Amphioxus. ”

“ Septal bars between two contiguous clefts, corresponding to the primary bars in Amphioxus; (2) Tongue-bars. The chief resemblances between Balanoglossus and Amphioxus in respect of the gill-slits may be stated briefly as follows:— (α) the presence of two kinds of branchial bars in all species and also of small crossbars (synapticula) in many species; (β) numerous gill-slits, from forty to more than a hundred pairs; (γ) the addition of new gill-slits by fresh perforation at the posterior end of the pharynx throughout life. ”

“ Thus, for instance, in the development of the human embryo, the transition from the invertebrate to the vertebrate may be represented in the reality by the isolated amphioxus, which remains stationary where vertebrate man begins, and can make no step forward, while the human embryo advances farther and farther till it reaches its highest limit. ”

“ The peculiar odor, resembling that of iodine, which is a noticeable feature of the 'Amphioxus sand' in the Bay of Naples, was not recognized at any of the collecting places examined by us. Incidentally in our dredgings for Amphioxus, it was noticed that there are many sandy bottoms and beaches which are inhabited by large numbers of a rather small Balanoglossus. The western portion of Castle Harbor contains brain corals (Meandra) in great abundance, many of them attaining an enormous size and weight. ”

“ At the chordonian stage of its development, the human embryon is equally destitute of a true heart, brain, and limbs, thus corresponding to a sub-type of the vertebrates called by Haeckel, Acrania, of which the amphioxus is the best-known representative. There is, nevertheless, in this heartless, brainless, limbless, and almost shapeless mass of but slightly differentiated protoplasm, that wonderful impulse of evolution by which its destiny, as an individual of the highest organic rank, is assured. ”

“ The brain, however, must have been expanded, and there must have been paired organs of smell, two lateral eyes and probably two dorsal eyes, and a large paired auditory apparatus. The mesoblastic system of muscles and fibrous skeleton was highly and regularly segmented, but in the anterior region cephalization had proceeded to a considerable extent. The resemblances between such a creature and Amphioxus are so close that they cannot be dismissed. ”

“ If, as I have pointed out, the atrial cavity of Amphioxus has been closed in Ammocœtes by the apposition of the pleural fold with the branchial body-surface, then the remains of the position of the atrial chamber must exist in Ammocœtes as that extraordinary space between the somatic muscles and the branchial basket-work filled with blood-spaces and modified muco-cartilage. It is in this very space, close against the gill-slits, that the thymus glands of Ammocœtes are found, in the very place where the nephric tubules of Amphioxus would be found if its atrial cavity were closed completely. ”

“ In the middle line of its ventral side we find the hypobranchial groove. The mantle or cloaca-cavity (the atrium) that surrounds the gill-crate is also formed in the same way in the Ascidia as in the Amphioxus. The ejection-opening of this peribranchial cavity corresponds to the branchial pore of the Amphioxus. In the adult Ascidia the branchial gut and the heart on its ventral side are almost the only organs that recall the original affinity with the vertebrates. ”

“ I can only conclude that both they and Amphioxus arose late, after the vertebrate stock had become well established, so that in their degenerated condition they give indications of their vertebrate ancestry and not of their more remote arthropod ancestry. ”

“ A full regulation of the intimate structure of isolated blastomeres to a new whole, has been proved to exist in the highest degree in the eggs of all echinoderms, medusae, nemertines, Amphioxus, fishes, and in one class of the Amphibia (the Urodela) ; it is facultative only among the other class of Amphibia, the Anura, and seems to be only partly developed or to be wanting altogether among ctenophora, ascidia, annelids, and mollusca. ”

“ But the hinder spot below (C) presents rays, and is the commencement of the infra-caudal lobe.
The hinder end of the body tapers to a point, as with Amphioxus, the extremity being slightly bent downward. At the junction of the middle with the hinder third of the body commences a delicate median fin, colorless, and without rays for the most part, and extending around the tip of the tail forward to the vent, thence forward to about the middle of the body. Such a primordial median fin exists in the young of all fishes whose development has been studied. ”