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AMPHIOXUS

been definitely shown on embryological and physiological The abdominal region comprises a short stretch of body between has to be the simplest persistent form of the vertebrate liver. atriopore and anus, the termination of the alimentary canal. grounds Nervous System.—As has been already indicated, a solid subIt is characterized by the presence of a special development cylindrical elastic rod, the notochord, surrounded by a sheath of of the lophioderm or median fin-system, namely, the^ ventral jin, connective tissue, the chordal sheath, lies above the aliwhich is composed of two portions, a lower keel-like portion, laminar canal in contact with its dorsal wall, and extends beyond which underlies an upper chambered portion, each chamber con- mentary both in front and behind to the obtusely pointed extremities of taining typically a pair of gelatinous fin-rays, finally, the it miwW m/wm comprises the post-anal division of the trunk. The the body. This notochord represents the persistent primordial axis which, in the higher Craniota (though not so in the keel of the ventral fin is continued past the anus into the expanded skeletal gives way by substitution to the segmented vertebral column. caudal fin, and so it happens that the anal opening is displaced lower), Immediately above the notochord there lies another subcylindrical from the middle line to the left side of the fin. In Asymmetron the chord, surrounded by a sheath of connective tissue. This caudal region is remarkable for the curious elongation of the noto- chord isalso neither elastic nor solid, but consists of nerve tissue, chord, which is produced far beyond the last of the myotomes. Alimentary, Respiratory, and Excretory Systems.—Although the fibres, and ganglion cells, surrounding a small central canal. For sake of uniformity in nomenclature this nerve chord may be function of the two latter systems of organs is the purification of the the neurochord. It is the central nervous system, and conthe blood, they are not usually considered together, and it is called within itself the elements of the brain and spinal marrow of therefore the more remarkable that their close association in tains higher forms. The neurochord tapers towards its posterior end, Amphioxus renders it necessary to treat them in common. The alimentary canal is a perfectly straight tube lined throughout by where it is coextensive with the notochord, but ends abruptly in ciliated epithelium. As food particles pass in through the mouth front, some distance behind the tip of the snout. The neurochord they become enveloped in a slimy substance (secreted by the attains its greatest thickness not at its anterior end but some way endostyle) and conveyed down the gut by the action of the vibratile behind this region ; but the central canal dilates at the anterior cilia as a continuous food-rope, the peristaltic movements of the extremity to form a thin-walled cerebral vesicle, in the front wall of gut-wall being very feeble. The first part of the alimentary canal which there is an aggregation of dark pigment cells constituting consists of the pharynx or branchial sac, the side walls of which an eye-spot, visible through the transparent skin (Fig. 1). There are perforated by upwards of sixty pairs of elongated slits, the are two pairs of specialized cerebral nerves innervating the prseoral gill-clefts. Each primary gill-cleft becomes divided into two by a lobe, and provided with peripheral ganglia placed near the tongue-bar which grows down secondarily from the upper wall of termination of the smaller branches. Corresponding with each the cleft and fuses with the ventral wall. New clefts continue to pair of myotomes, and subject to the same alternation, two pairs form at the posterior end of the pharynx during the adult life of of spinal nerves arise from the neurochord, namely, a right and the animal. The gill-clefts open directly from the cavity of the left pair of compact dorsal sensory roots without ganglionic pharynx into that of the atrium and so give egress to the respira- enlargement, and a right and left pair of ventral motor roots tory current which enters the mouth with the food (Fig. 2). The composed of loose fibres issuing separately from the neurochord atrium or atrial chamber is a peripharyngeal cavity of secondary and passing directly to their termination on the muscle-plates of origin eifecting the enclosure of the gill-clefts, which in the larva the myotomes. The first dorsal spinal nerve coincides in position opened directly to the exterior. The atrium is thus analogous to with the myocomma which separates the first myotome from the the opercular cavity of fishes and tadpoles, and, as stated above, second on each side, and thereafter the successive dorsal roots pass remains in communication with the exterior by means of the through the substance of the myocommata on their way to the atriopore. The primary and secondary bars which separate and skin ; they are therefore septal or intersegmental in position. The divide the successive gill-clefts from one another are traversed by ventral roots, on the contrary, are myal or segmental in position. blood-vessels which run from a simple tubular contractile ventral In addition to the cerebral eyebranchial vessel along the bars into a dorsal aorta. The ventral spot there are large numbers of branchial vessel lies below the hypobranchial groove or endostyle, minute black pigmented bodies and is the representative of a heart. As water for respiration beside and below the central streams through the clefts, gaseous interchange takes place canal of the neurochord, combetween the circulating colourless blood and the percolating mencing from the level of the water. The pharynx projects freely into the atrium ; it is sur- third myotome. It has been rounded at the sides and below by the continuous atrial cavity, determined that these bodies but dorsally it is held in position in two ways. Firstly, its dorsal are of the nature of eyes wall (which is grooved to form the hyperpharyngeal groove) is (Becheraugen, Hesse), each closely adherent to the sheath of the notochord ; and secondly, the consisting of two cells, a cupcell and a tripharynx is attached through the intermediation of the primary shaped bars. These are suspended to the muscular body-wall by a double angular retinal cell. These membrane, called the ligamentum denticulatum, which forms may be called the spinal eyes, at once the roof of the atrial chamber and the floor of a per- and it is said that they are sistent portion of the original body-cavity or coelom (the dorsal disposed in such a way as to mi ccelomic canal on each side of the pharynx). The ligamentum den- receive illumination prefery ticulatum is thus lined on one side by the epiblastic atrial entially from the right side, epithelium, and on the other by mesoblastic coelomic epithelium. although this fact has no reNow this ligament is inserted into the primary bars some lation with the side upon distance below the upper limits of the gill-clefts, and it there- which Amphioxus may lie fore follows that, corresponding with each tongue-bar, the upon the sand. When kept in atrial cavity is produced upwards beyond the insertion of the captivity the animal often lies ligament into a series of bags or pockets, which may be called the upon one side on the surface atrial pouches. At the top of each of these pouches there is a of the sand, but on either side minute orifice, the aperture of a small tubule lying above each indifferently. Over the cerepouch in the dorsal coelom. These tubules are the excretory bral eye there is a small orifice tubules or nephridia. They communicate with the coelom by placed to the left of the base several openings or nephrostomes, and with the atrium by a single of the cephalic fin, leading into opening in each case, the nephridiopore. It is important to em- a pit which extends from the phasize the fact that in Amphioxus the excretory tubules are co- surface of the body to the surex'tensive with the gill-clefts. The perforated pharynx terminates face of the cerebral vesicle ; some distance in front of the atriopore. At the level of its posterior this is known as Kolliker’s end a pair of funnel-shaped pouches of the atrium are produced olfactory pit. forwards into the dorsal coelom. These are the atrio - coelomic Reproductive System. — The Fig. 3.—Diagram of embryo of Amphi. funnels or brown funnels, so called on account of the characteristic sexes are separate, and the oxus seen from above in optical section. pigmentation of their walls. There are reasons for supposing male or female gonads, which (Adapted from Hatschek.) pc, prsehead - cavity of embryo; cc, that these funnels are vestiges of an ancient excretory system, are exactly similar in outward chordal collar-cavity (first somite); my, mesowhich has given way by substitution to the excretory tubules appearance, occur as a series dermic somites (myocoelomic or archdescribed above. In the same region of the body, namely* close i of gonadic pouches projecting enteric pouches) ; ch, notochord with neural tube (neurochord) lying behind the pharynx, a large diverticulum is given off from the into the atrial cavity at the the it; np, anterior neuropore; ne, ventral side of the gut. This is the hepatic caecum (Fig. 2, l), which base of the myotomes (Fig. 2). upon position of posterior neurenteric canal. is quite median at its first origin, but, as it grows in length, At the breeding season the comes to lie against the right wall of the pharynx. Although walls of the pouches burst and the sexual elements pass into the within the atrial cavity, it is separated from the latter by a atrium, whence they are discharged through the atriopore into the narrow coelomic space, bounded towards the atrium by coelomic and water, where fertilization takes place. atrial epithelium. No food passes into the hepatic caecum, which Development.—The development of Amphioxus possesses many