In 1883-1886 Bateson showed by his embryological researches that the Enteropneusta exhibit chordate (vertebrate) affinities in respect of the coelomic, skeletal and nervous systems as well as in regard to the respiratory system, and, further, that the gill-slits are formed upon a plan similar to that of the gillslits of Amphioxus, being subdivided by tongue-bars which depend from the dorsal borders of the slits.
In accordance with this view there would be also some probability in favour of regarding the collar nerve-tube of the Enteropneusta as the equivalent of the cerebral vesicle only of Amphioxus and the Ascidian tadpole, and also of the primary forebrain of vertebrates.
The branchial bars which constitute the borders of the clefts are of two kinds: - (i) Septal bars between two contiguous clefts, corresponding to the primary bars in Amphioxus; (2) Tongue bars.
The chief resemblances ------ between Balanoglossus an Amphioxus in respect of `'?'
While in Amphioxus the tb, tongue-bars.
From serving primitively as the essential organ of the cleft the tongue-bar may have undergone reduction and modification, becoming a secondary bar in Amphioxus, subordinate to the primary bars in size, vascularity and development; finally, in the craniate vertebrates it would then have completed its involution, the suggestion having been made that the tongue-bars are represented by the thymusprimordia.
The respiratory current of water is therefore conducted to the exterior by different means from that adopted by Amphioxus, and this difference is so great that the theory which seeks to explain it has to postulate radical changes of structure, function and topography.
The vascular system does not readily lend itself to morphological comparison between such widely different animals as Balanoglossus and Amphioxus, and the reader is therefore referred to the memoirs cited at the end of this article for further details.
In his memoir on Amphioxus, Berlin, 1844) in the way of making out its complete structure than the ablest of his contemporaries or successors could do with a plethora.
Kolliker (Development of Cephalopods, 1844), Remak (Development of the Frog, 1850), and others had laid the foundations of this knowledge in isolated examples; but it was Kovalevsky, by his accounts of the development of Ascidians and of Amphioxus (1866), who really made zoologists see that a strict and complete cellular embryology of animals was as necessary and feasible a factor in the comprehension of their relationships as at the beginning of the century the coarse anatomy had been shown to be by Cuvier.
- In ?/?/ i / the Acrania (Amphioxus) the ???
Des sciences de St Petersbourg (1866), and " Entwickelungsgeschichte des Amphioxus lanceolatus," ibid.
AMPHIOXUS, or Lancelet, the name of small, fish-like, marine creatures, forming the class Cephalochorda, of the phylum Vertebrata.
The theoretical interest of Amphioxus depends upon a variety of circumstances.
Although it is true that there is a certain amount of gradation in the degree of development to which these organs have attained in the various orders, yet it is hardly sufficient to enable the imagination to bridge over the gap which separates Amphioxus from the lowest fishes in regard to this feature of organization.
On account of the absence of anything in the nature of a skull, Amphioxus has been regarded as the type of a division, Acrania, in contrast with the Craniata which comprise all the higher Chordata.
The ordinal name for the genera and species of Amphioxus is Cephalochorda, the term referring to the extension of the primary backbone or notochord to the anterior extremity of the body; the family name is Branchiostomidae.
With regard to its habits, all that need be said here is that while Amphioxus is an expert swimmer when occasion requires, yet it spends most of its time burrowing in the sand, in which, when at rest, it lies buried with head protruding and mouth wide agape.
Amphioxus favours a littoral habitat, and rarely if ever descends below the 50-fathom line.
- Amphioxus lanceolatus, Yarrell (Branchiostoma lubricum, Coste).
Amphioxus is a small fish-like creature attaining a maximum length of about 3 in., semitransparent in appearance, showing iridescent play of colour.
Alimentary, Respiratory and Excretory Systems. - Although the function of the two latter systems of organs is the purification of the blood, they are not usually considered together, and it is therefore the more remarkable that their close association in Amphioxus renders it necessary to treat them in common.
It is important to emphasize the fact that in Amphioxus the excretory tubules are co-extensive with the gill-clefts.
- Amphioxus lanceolatus laid open ventrally.
These may be called the spinal eyes, and it is said that they are disposed in such a way as to receive illumination preferentially from the right side, although this fact has no relation with the side upon which Amphioxus may lie upon the sand.
The development of Amphioxus possesses many features of interest, and cannot fail to retain its importance as an introduction to the study of embryology.
- Diagram of embryo of Amphioxus seen from above in optical section.
One point must not be omitted, namely, the homogeny of the endostyle of Amphioxus and the thyroid gland of Craniata.
Boveri, "Die Nierencanalchen des Amphioxus," Zool Jahrb.
1897; Amphioxus, p. 333; T.
Garbowski, "Amphioxus als Grundlage der Mesodermtheorie," Anal Anz.
Hesse, "Die Sehorgane des Amphioxus," Zeitschr.
Lankester, "Contributions to the Knowledge of Amphioxus lanceolatus (Yarrell)," op. cit., xxix.
MacBride, "The early Development of Amphioxus," Quart.
P. Hazen, "The Gastrulation of Amphioxus," J.
Schneider, "Einiges fiber Resorption and Excretion bei Amphioxus lanceolatus," Anat.
Sobotta, "Die Reifung and Befruchtung des Eies von Amphioxus lanceolatus," Arch.
Weiss, "Excretory tubules in Amphioxus lanceolatus," Quart.
Willey, Amphioxus and the Ancestry of the Vertebrates (1894); "Remarks on some recent Work on the Protochorda," Quart.
Burchardt, "Finer Anatomy of Amphioxus," with bibliography, Jena Zeitschr.