Amphioxus or Lancelet

AMPHIOXUS or LANCELET, a small, marine creature, neither fish nor worm, but something between the two, which is the simplest example of the Vertebrate type of structure. It was first discovered on the coast of Cornwall, and a preserved specimen sent to Pallas, who was unacquainted with its lively movements, was described by him in 1774 as a slug, Limax lanceolatus. Couch rediscovered it in 1831, and his specimen was described and figured by Yarrell in 1836 under the title Amphioxus lanceolatus (Brit. Fishes, vol. ii.).

Characteristics.—The body of Amphioxus rarely exceeds 23-in. in length. It is laterally compressed and tapered at each extremity. From end to end above the alimentary canal runs a solid, elastic rod, the notochord, which plays an important part in the animal's movements. On each side of it run powerful longitudinal muscles, which are divided up into chevron-like segments, the myotomes, like the muscle-flakes of a whiting. Above the notochord lies a nerve-cord, corresponding to the spinal cord of Vertebrates, and giving origin, like it, to alternate pairs of sensory and motor nerves, which, however, remain dis tinct, and do not unite to form complex structures like the spinal nerves, with their double roots, in Vertebrates. There is, more over, no centralized brain in front, and no trace of paired olfac tory, optic and auditory sense-organs and nerves. Jaws, teeth and other prehensile organs are lacking, as well as skull, vertebral cartilages and paired limbs or fins. The circular, contractile mouth lies at the back of a preoral depression, the buccal cavity, on the underside of the snout. This cavity is fringed behind and on each side by stiff processes or cirri, provided with a solid, jointed, internal skeleton. They are capable of spreading out wards, thus freely opening the buccal cavity, or of bending in wards, when they overlap the entrance with a protective grating. A circlet of true, sensitive tentacles lies just inside the circular mouth. The cheeks of the buccal cavity are beset inside with parallel ciliated ridges, which are offsets from a primary pair of dorso-lateral ridges, diverging behind around the mouth, and converging forwards to surround an asymmetrical cilio-glandular depression known as Hatschek's pit. The entire organ is known as the wheel-organ. Its ciliary action is directed backwards towards the mouth. The alimentary canal is described below.

Classification.

There has been a tendency to include Amphi oxus within the Vertebrata as Acrania, but this procedure exagger ates the resemblances of Amphioxus to the Vertebrates, since in all the essentials of its life and organization, as distinct from superficial form, Amphioxus is much more intimately related to the Tunicata (q.v.), and constitutes with them a well-defined group, termed by Balfour Protochordata, the distinctive features of which lie in the feeding-mechanism described below. The differences between Amphioxus and the Tunicata are chiefly that Amphioxus retains its notochord and powers of locomotion throughout life, while in Tunicata these are mostly restricted to the larval stage, and the notochord itself is confined to the tail. Balanoglossus (q.v.), which in certain respects foreshadows Amphioxus, stands at a much lower plane of general organization, and, with the Pterobranchia (q.v.), possesses mere rudiments of the characteristic notochord and neural canal. Thus the position of Amphioxus within the phylum Chordata is indicated by the following classification: There are good reasons for regarding these three sub-phyla as marking successive stages in the progressive evolution of the Chordata, although in its possession of protonephridial excretory organs Amphioxus retains an archaic larval character (cf. Actino trocha) which has been lost by all other Chordata.

In the latest revision of the Cephalochorda (Franz, 1922) two genera only are recognized, Branchiostoma (=Amphioxus) and Asymmetron (see below). Which is primitive? Franz decides in favour of Amphioxus; but, if their relationship with Tunicates is borne in mind, the unpaired gonads, the small size (9-1 omm. ) at sexual maturity, and the frequent presence of a caudal appendage in Asymmetron enable us to recognize in this form a possible stage of transition from a neotenic Ascidian towards Amphioxus, the unsegmented caudal appendage (which supports a well-marked caudal fin in the pelagic larva) being a vestige of the Ascidian larval tail. The relations of the metapleural ridges and ventral fin-fold are consistent with this view, since it is generally admitted that the continuity of these structures in Asymmetron, though one-sided, is more primitive than the dis continuity in Amphioxus. The species of Asymmetron fall into two groups according to the presence or absence of the caudal appendage, the second group corresponding with the genus Hetero pleuron of Kirkaldy ; but the variations of a vestigial organ fur nish an insecure basis for a definite division of the genus.

1. ASYMMETRON, Andrews (Tattersall)—Gonads on right side only; left metapleur distinct, the right continuous with the ventral fin.

A—with a distinct caudal appendage. Species: lucayanum (62 71 myotomes), Bahamas, Zanzibar to Philippines; macricaudatunn (58-66), Florida; caudatunn (6o-64), Louisiade archipelago; orientate (63-69), Maldives.

B—without a caudal appendage. Species: cultellum (50-56 my., notochord unusually prominent behind), E. Africa to Philip pines and E. Australia; cingalense (61-64), Ceylon; parvum and agassizii (68-7o), Maldives; maldivense (7o-76), Maldives and Laccadives; bassanum (7o-78), S.W. Australia; liectori (84-85), New Zealand.

2. AMPHIOXUS,

Yarrell (=Branchiostoma, Costa). Gonads in paired series; right and left metapleurs ending symmetrically behind the atriopore ; no caudal appendage. Species : lanceolatus (58-62 my.), European coasts; caribaeus (57-6o), Atlantic coasts of America; haeckelii (59-6i), Ceylon; belcheri (63-66), E. Africa to Japan and Queensland ; cali f orniensis (64-69), Califor nia; indicus (69-7r) (snout elongated), Indian Ocean; capensis S. Africa; elongates (77-8o), Pacific coast of S. America.

Habits.—Amphioxusburrows in loose sand, into which it dives with amazing rapidity. It projects its snout above the surface, and remains perfectly still unless disturbed, when it darts out of the sand, gyrates wildly about in zig-zag fashion for a few seconds, and then plunges head-first in again. Except for these brief moments of activity, its habits are as sedentary as those of a fixed Ascidian. Asymmetron has similar habits, but A. lucayanum appears to be actively pelagic at night.

Feeding

Mechanism.—Amphioxusagrees with the Tunicata, and differs from all other Chordata, in certain characteristic modifications of the front part of the alimentary canal or pharynx for the collection of minute floating organisms (micro plankton) as food. The canal itself in Amphioxus runs straight from mouth to anus, the latter being situated ventrally a short distance from the posterior extremity of the body, to the left of the ventral fin (in Tunicata also the anus is left-sided, but dorsal). The long, dilated pharynx is succeeded by a narrow gastro intestinal region, provided in front with a hepatic caecum pro jecting forwards on the right side. There is no distinct stomach. The entire side-walls of the pharynx, and the body-wall invest ing it, are pierced by a paired series of close-set U-shaped or tongue-barred gill-slits, each of which extends obliquely back wards, and is divided up into smaller perforations, firstly by fusion of the apex of the tongue-bar with the ventral wall of the slit, and then by subdivision of the half-gill-slits into stigmata by outgrowth of synapticulae from the tongue-bars. The edges of all the perforations are strongly ciliated and lash outwards through the body-wall. To protect the fenestrated walls from injury, the whole pharyngeal region is sunk beneath the general level, and enclosed within a peribranchial chamber or atrium by downgrowth of the muscular body-wall and ventral fusion of a pair of epibranchial flaps (the subatrial ridges). This fusion is incomplete behind, leaving an aperture, the atriopore, situated ventrally, midway between pharynx and anus. The metapleurs are a pair of ventro-lateral ridges on the outer surface of the atrial wall.

The narrow roof and floor of the pharynx are grooved longi tudinally. The ventral groove or endostyle is beset with rows of gland-cells, and between them runs a median tract of cells bearing long flagella. Its lateral margins are finely ciliated, and diverge in front to encircle the pharynx inside the mouth as a pair of peripharyngeal bands. These bands bend backwards dor sally and merge into the cilia tion of the upper or epipharyn geal groove, which leads into the oesophagus. It is in fact the food groove, and the rest of the pharynx is a collecting apparatus to supply it.

When Amphioxus is at rest, its mouth opens, and the cilia of its numerous gill-slits set up a current which flows steadily in through mouth and pharynx, and out through atrium and atrio pore. At the same time the en dostyle pours out a stream of mucilage, the greater part of which is flicked out of the groove right and left on to the side-walls of the pharynx, and is there swept upwards in the form of par allel longitudinal strings by the action of cilia coating the internal surface of the gill-bars. In this way the fenestrated walls of the pharynx are traversed by a moving succession of slimy threads. Food-particles entering with the water are sooner or later entangled in the mucilage and rolled upwards into the epipharyngeal groove which concentrates them and conveys them as a continuous food-rope backwards into the digestive part of the canal. The wheel-organ appears to assist this process in the buccal cavity by sweeping outlying particles on to the peripharyngeal bands.

Reproduction.—Thesexes are separate. The gonads arise as pouches from the lower ends of the myotomes, and project into the atrium. At the breeding season the pouches burst, and the sexual elements pass through the atrium to the exterior, where fertilization takes place.

account of its relative yolklessness, the fertilized egg of Amphioxus provides a typical example of total and regular segmentation, and of gastrulation by invagination. Notochord and neural canal (the latter enclosing the blastopore) are developed as in the lower Vertebrates. Paired mesoblastic somites arise as outgrowths, from the embryonic endoderm, and divide into the myotomes above and visceral elements below which unite to form the perivisceral coelom. A primitive tripartite arrangement of the coelom is recognizable, as in Balanoglossus, the myotomes and perivisceral elements being derivatives of the third primitive or trunk segment (MacBride). The first primitive segment is cut off from the blind extremity of the archenteron as a single vesicle, which divides into two, the right half expanding as the head-cavity, while the left, which acquires an opening to the exterior, becomes reduced to form Hatschek's pit. Its aper ture is clearly homologous with the proboscis pore of Balano glossus and the water-pore of Echinoderm larvae. It is surrounded in the larva by a peculiarly ciliated patch, known as the preoral pit, which subsequently develops into the wheel-organ of the adult. Goodrich has given good reasons for regarding this organ as equivalent to the anterior part, at any rate, of the Vertebrate hypophysis.

Amphioxus hatches at about the eighth hour after fertilization as a ciliated embryo, with the neural folds, notochord and first few mesoblastic somites established, but without mouth or anus. The ciliated embryo elongates rapidly with the formation of additional somites, and at about the 36th hour passes into the first larval stage, characterized by perforation of the mouth, first gill-slit and anus. This stage is curiously asymmetrical, the mouth and single gill-slit being situated opposite one another to left and right. The mouth becomes enormously distended along the left side, while new gill-slits arise on the right side up to as many as 14. Subsequently (metamorphosis stage) the last six of these, as well as the first one, close up, and the remainder rotate below to their permanent position on the left side, while a new series arises above the first to replace it on the right. At the same time the mouth rotates forwards to a median position.

Various theories have been put forward to account for this temporary distortion. The present writer suggests that it is the consequence of a secondary reduction of yolk in the egg, accom panied by premature hatching, which has entailed the improvisa tion of a temporary feeding mechanism, the basis of which appears to be the enlargement and special ciliation of the mouth and an adaptation of the club-shaped gland for the supply of mucilage.

Of great interest are the origin of the endostyle as a V-shaped loop of the peripharyngeal band, thus recalling the adoral band of Echinoderm larvae, and the development immediately behind the endostyle, of the club-shaped gland, with two successive in ternal orifices on opposite sides of the endostyle, and a single external orifice below the left-sided mouth. Willey regarded this gland as a modified gill-cleft. The present writer identifies it with certain pharyngeal glands of Appendicularians, and inter prets all of them as highly modified vestiges of the epicardium or budding-organ of Tunicata (q.v.).

Evolution.--If

these views are correct, Amphioxus must have been derived from sessile ancestors of a primitive Ascidian stock, and its elongated form is the result of loss of fixation and retention of larval habits, modified by secondary adaptation to an, arenic olous life. We have already seen that Asymmetron retains a caudal appendage which can be little else than a remnant of the Ascidian larval tail, and this accords with its possession of un paired gonads as in Ascidians. The secondary symmetrization of the gonads in Amphioxus is paralleled in Ascidians themselves (Stolidobranchia). The main axis of Chordate evolution may thus be regarded as a line of sedentary plankton-feeding organ isms, which began with an external apparatus of ciliated tentacles and food-grooves, and ended with an endopharyngeal mechanism of gill-slits and endostyle. From this Pterobranch-Ascidian line Balanoglossus, Amphioxus and the Appendicularians have been given off at different levels as successive paedomorphic offshoots. The line may be carried still farther back to the primitive stalked Echinoderms, to inheritance from which we must ascribe such features as the larval form of Balanoglossus (Tornaria), Hat schek's pit and the V-shaped endostyle of Amphioxus. Assuming that the ancestral Echinoderm handed on to its Hemichordate descendants a primitive type of Dipleurula larva, the Proto chordate tadpole may readily have been derived from it by the substitution of muscular for ciliary means of progression. The adoral band may thus have been the forerunner of the endostyle, and the circumoral ciliated band, with its underlying nervous system, of the ciliated neural folds of Vertebrates, which at the outset have identical relations to mouth and blastopore.

That the Vertebrata have been derived from Protochordata, and not vice versa, is shown by the retention in Vertebrate em bryos of unequivocal remnants of the Protochordate feeding mechanism of endostyle and tongue-barred gill-slits, as shown by the details of development of the thyroid and thymus glands.

BIBLIOGRAPHY.-E. A. Andrews, "On Asymmetron lucayanum," Bibliography.-E. A. Andrews, "On Asymmetron lucayanum," Stud. Biol. Lab. Johns Hopkins Univ., v. (1893) ; A. Willey, Amphioxus and the Ancestry of the Vertebrates (1894) ; J. W.

Kirkaldy, "A Revision of the Genera and Species of Branchio Stomidae," Q.J.M.S., xxxvii. (1895) ; E. W. MacBride, "The Forma tion of the Layers in Amphioxus, etc.," Q.J.M.S., liv. (1 goo) ; E. S. Goodrich, "On the Structure of the Excretory Organs of Amphioxus," Q.J.M.S. liv. (1909) ; J. H. Orton, "The Ciliary Mechanisms on the Gill and the Mode of Feeding in Amphioxus, etc.," Jour. Mar. Biol. Ass. x. (1913) ; V. Franz, "Systemat. Acranier-Studien," Zoo/. Anz. liv. (192 2) ; W. Garstang, "The Morphology of the Tunicata and its bearings on the Phylogeny of the Chordata," Quart. Jour. Micr. Sci. (1928) ; V. Franz, Zeit f. d. Ges. Anat. (1927). (W. GA.)