AMPHIOXUS
385
cultellus (Fig. 1) inhabits Torres Strait, and has also been found at Ternate. Asymmetron lucayanum is the Bahaman representative of the family, with a sub-species, A. caudatum, in the- South Pacific from New Guinea to the Loyalty Islands. The Peruvian species, Branchiostoma clongatum, with nearly 80 myotomes, cannot at present be assigned to its proper sub-genus. External Form.—The following description, unless otherwise stated, refers to A. lanceolatus, which has been already figured in side view, in section, and in dissection in the article T ertebrata (Ency. Brit. vol. xxiv.). Amphioxus is a small fish-like creature attaining a maximum length of about 3 inches, semi-transparent in appearance, showing iridescent play of colour. The body is narrow, laterally compressed and pointed at both ends. The main musculature can be seen through the thin skin to be divided into about sixty pairs of muscle-segments (myotomes) by means of comma-shaped dissepiments, the myocommas, which stretch between the skin and the central skeletal axis of the body. These myotomes enable it to swim rapidly with characteristic serpentine undulations of the body, the movements being effected by the alternate contraction and relaxation of the longitudinal muscles on both sides. Apparently correlated with this peculiar locomotion is the anatomical fact of the alternation of the myotomes on the two sides. Symmetrical at their first appearance in the embryo, the somites (from which the myotomes are derived) early undergo a certain distortion, the effect of which is to carry the somites of the left side forwards through the length of one half-segment. For example, the twenty - seventh myotome of the left side is placed opposite to the twenty-sixth myocomma of the right side. The back of the body is occupied by a crest, called the dorsal fin, consisting of a hollow ridge, the cavity of which is divided into about 250 compartments or fin-chambers, into each of which, with the exception of those near the anterior and posterior ends of the body, projects a stout pillar composed of characteristic laminar tissue, the fin-ray. The dorsal crest is continued round both extremities, becoming expanded to form the rostral fin in front and the caudal fin behind. Even in external view, careful inspection will show that the body is divisible into four regions, namely, cephalic, atrial, abdominal, and caudal. The cephalic region includes the rostrum or prseoral lobe and the mouth. As already stated, the notochord extends beyond the mouth to the tip of the rostrum. The mouth consists of two portions, an outer vestibule and an inner apertura oris ; the latter is surrounded by a sphincter muscle, which forms the so-called velum. The vestibule of the mouth is the space bounded by the oral hood; this arises by secondary down-growth of lip-like folds over the true oral aperture, and is provided with a fringe of tentacular cirri, each of which is supported by a solid skeletal axis. The oral hood with its cirri has a special nerve-supply and musculature by which the cirri can be either spread out, or bent inwards so that those of one side may interdigitate with those of the other, thus completely closing the entrance to the mouth. The velum is also provided with a circlet of twelve tentacles (in some species sixteen) which hang backwards into the pharynx; these are the velar tentacles. The atrial region extends from the mouth over about two-thirds of the length of the body, terminating at a large median ventral aperture, the atriopore; this is the excurrent orifice for the respiratory current of water and also serves for the evacuation of the generative products. This region is really the branchiogenital region, although the fact is not apparent in external view. The ventral side of the body Fig. 2. — AmpMoxus lanceolatus ^ Vfc iui open ventrally. (After in the atrial region is broad and con- laid slightly altered.) w, vex, so that the body presents the Ratlike, mouth appearing as an elonappearance of a spherical triangle in gated slit when relaxed (as in Fig. 1.—Epigonichthy,'; cultdlus from below and from the left side. (Slightly transverse section, the apex being the lamprey); p, perforated altered from Kirkaldy.) rm and Im, right and left metapleur; ai, atriopore ; pharynx; endostyle; Q, an, anus ; e, “ eyespot ” at anterior end of neurochord projecting beyond the formed by the dorsal fin and the gonads; l, e,liver; at, level of myotonies (my); n, notochord; rgo, gonads of right side only showing angles bordered by two hollow folds, atriopore; i, intestine; an, through by transparency; go20, the last gonad; dfr, dorsal hn with 1m- the metapleural folds, each of which anus. In this species the chambers and fin-rays ; vfc, ventral fin-chambers. is produced as an asymcontakis a continuous longitudinal atrium metrical blind pouch behind Chesapeake to La Plata. A. californiensis occurs on the coast of lymph-space, the metapleural canal. the atriopore as far as the anus. Branchiostoma the In the genus — — California, and A. belcheri extends its area of distribution from Queensland through Singapore to Japan. A recently described metapleural folds terminate symmetrically shortly behind the species, Dolichorhynchus indicus, characterized by the great length atriopore, but in Heteropleuron the right metapleur passes uninterinto the median crest of the ventral fin (Fig. 1). _ In this of the praeoral lobe or snout, has been dredged in the Indian Ocean. ruptedly Paramphioxus bassanus occurs on the coast of Australia from Port connexion it may also be mentioned that in all cases the right half fin (Fig. 2). Phillip to Port Jackson ; P. cingalensis at Ceylon. Epigonichthys of the oral hood is directly continuous with the rostral S. I. — 49
partly recapitulated in the coilrse of the embryonic stages of craniate vertebrates. In comparative morphology it provides many illustrations of important biological principles (such, for example, as substitution and change of function of organs), and throws new light upon, or at least points the way to new ideas of the primitive relations of different organic systems in respect of their function and topography. One of the most puzzling features in its structure, and, at the same time, one of the greatest obstacles to the view that it is essentially primitive and not merely a degenerate creature, is the entire absence of the paired organs of special sense, olfactory, optic, and auditory, which are so characteristic of the higher vertebrates. 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. Classification.—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 Craniota 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 Branchiostomidce. The amount of generic divergence exhibited by the members of this family is not great in the mass, but is of singular interest in detail. There are two principal genera—1. Branchiostoma Costa, having paired sexual organs (gonadic pouches) ; 2. Heteropleuron Kirkaldy, with unilateral gonads. Of these, the former includes two subgenera, Amphioxus (s. str.) Yarrell and Dolichorhynchus Willey. The species belonging to the genus Hcteropleuron are divided among the three subgenera Paramphioxus Haeckel, Epigonichthys Peters, and Asymmetron Andrews. The generic characters are based upon definite modifications of form which affect the entire facies of the animals, while the specific diagnoses depend upon minor characters, such as the number of myotomes or musclesegments. Habits and Distribution.—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. Its food consists of microscopic organisms and organic particles ; these are drawn into the mouth together with currents of water induced by the action of the vibratile cilia which are abundant along special tracts on the sides and roof of the vestibule of the mouth and in the walls of the perforated pharynx (“ciliary ingestion”). Amphioxus favours a littoral habitat, and rarely if ever descends below the 50-fathom line. Species occur in all seas of the temperate, tropical, and subtropical zones. The European species, A. lanceolatus, is found in the Black and Mediterranean Seas, and on the coasts of France, Great Britain, and Scandinavia, w'hile a closely allied species or sub-species, A. caribccus, frequents the Caribbean region from
