Marine Chordata, with a body pointed at each end, and provided with a continuous dorsal, anal, and caudal cuticular fin. There are no paired limbs: no skull, vertebral arches and centra: no jaw-arches: no differentiated brain, sympatltetic nervous system, or auditory organ: no heart, spleen, kidneys, or sexual ducts. There is but one genus contained in the group - Amphioxus - with species found near the coasts in various parts of the world. The animal, when adult, lives buried in sand 7vith the oral aperture just exposed.
The epidermis consists of a single layer of columnar cells, ciliated in the larva, with interspersed sense-cells furnished each with a stiff projecting hair, and continuous basally with a nerve-fibre. The cutis is transparent and non-nucleated. The sub-cutaneous tissue contains a remarkable system of anastomosing (lymph?) tubes, lined by an endothelium, and connected with two ventrally-placed tubes running longitudinally one in each metapleure. The notochord extends nearly from one end of the body to the other, is pointed at each extremity, and consists of a series of transverse discs with a dorsal furrow containing a retiform tissue, the whole enveloped in two sheaths. The outer sheath sends out dorsal laminae, which inclose the spinal cord; and ventral laminae, which extend into the walls of the body and the epipleures (infra). These laminae are continuous with the sheaths (myocommata) of the myomeres both of the epipleures and of the body behind the branchial region. The myomeres, 62 in number in A. lanceolatus, are arranged on each side in a continuous series. The dorsal and ventral sections of each myocomma which are not separated organically from one another, meet at an angle pointing forwards. There is also a small and remarkable set of transverse ventral muscles.
The muscles are striated rhombic plates devoid of sarcolemma. The nervous system consists of a spinal cord with a central canal, both of which swell out anteriorly. The anterior enlargement probably corresponds to the hind-and mid-brain with that part of the fore-brain which extends dorsally as pineal gland (?), a structure represented by the so-called olfactory nerve, which is really a tube opening externally in a ciliated depression. The tube represents an aperture left when the neural plate folds over in development to form the neural tube or canalis centralis. The ciliated depression is at first median and dorsal, and afterwards shifts to the left side. Three pairs of nerves arise from the anterior enlargement. The spinal nerves possess dorsal and ventral roots which are not connected outside the cord (Rohon). The ventral roots are purely motor, and arise as short columns. The dorsal are the principal and, according to most authorities, the only, spinal nerves. Their roots possess no ganglia, and the right and left series alternate more or less one with another. The grey matter consists only of ganglion cells. A sac, containing sense-cells with refractile hairs, opens on the dorsal wall of the oral cavity in the centre of a disc of ciliated cells.
It is probably an organ of smell or taste, and is derived from the left anterior entero-coelic pouch of the larva (infra). It is doubtful whether the pig-ment speck at the anterior end of the brain can be considered as an eye.
The mouth is a somewhat oblique ventral slit surrounded by twelve cirrhi, which are supported by rods borne upon a ring of twelve pieces, and covered by papillae rich in sense-cells. The oral cavity, produced by a forward growth of the epipleures, is separated from the pharynx by a free fold or velum with fringed edges and many sense-cells. The velum can be constricted by a muscle. Its aperture represents the original larval mouth. The pharynx extends for nearly half the length of the body, and is traversed from end to end by a ventral hypopharyngeal groove, the homologue of the endostyle in Urochorda, and, like it, furnished with ciliated cells. Its walls are pierced by oblique branchial slits, more than 100 in number, in fully grown individuals. The slits open externally into an atrial or peribranchial cavity. The bars that separate them are alternately complete and incomplete, and in the latter case free at their ventral ends. The slits are consequently U-shaped. The bars are supported by elastic rods, all connected at their dorsal ends, and their inner or pharyngeal surface is covered by a ciliated epithelium.
The peribranchial cavity originates at an early period by the growth of a right and left fold, or epipleure, from the dorsal region, which meet and fuse ventrally, leaving a ventrally-placed abdominal pore some little distance in front of the anus. The epipleures form two prominent longitudinal folds - the metapleures - one on either side the median ventral line. These folds, therefore, border a ventral furrow; but at the time, when the genital products are ripe, the consequent distension of the epipleures smooths them away. The peribranchial cavity on each side extends behind the abdominal pore for a short distance, and the two cavities are continuous beneath the pharynx from side to side. The epipleures contain the genital glands on their inner surface in the slight extension of the coelome which they inclose. Their outer walls are muscular, and, as before stated, the muscles are divided into myomeres. The water used in respiration and the genital products escape by the abdominal pore, but the latter not invariably. The pharynx is constricted posteriorly into an 'oesophagus,' followed by a widening, the stomach. The intestine runs straight to the anus, which lies asymmetrically on the left side. The anal aperture has a sphincter muscle.
A liver caecum opens into the stomach, generally oh the right, more rarely on the left, side, and projects into the peribranchial cavity. Its lining cells, as well as those of the stomach, contain a green pigment. The vascular system consists of a dorsal aorta, double in the region of the pharynx, single behind it. The sub-intestinal veins unite to form a vena porta, which supplies the walls of the liver-caecum. The vein bringing away the blood from the caecum runs ventrally beneath the pharynx, and sends up to the dorsal aortae, but only along the complete branchial bars, branchial vessels, which are provided with small pulsatile dilatations at their ventral ends. Anteriorly to the pharynx the ventral vessel is dilated and sinuous. It gives off a right and left branch to the velum and a wide vessel on the right side, which enters the right aortic trunk. The corresponding vessel on the left side is seemingly rudimentary. The whole system is tubular, and the portal vein and ventral pharyngeal vessel are said to be contractile. The blood corpuscles are for the most part amoeboid leucocytes. Rohon states that there are a few oval red haematids.
A tubular ciliated saccule, extending on the left side from the margin of the mouth to close behind the velum, where it opens into the pharynx, is supposed by Hatschek to represent a kidney, and to be homologous with the infra-neural gland of Urochorda. It is said to be mesodermic in origin. Ray Lankester has pointed out the existence of two tubes, one on each side, opening into the posterior region of the peribranchial cavity at one end, and at the other into the coelome (?). They are possibly renal. The coelome in the region of the body behind the peribranchial cavity is capacious, and contains a coagulable lymph. Its extensions forward above the pharynx and into the epipleures are small.
The ovaries and testes are segmentally arranged, and lie about the level of the union between the lateral trunk and ventral transverse muscles. They consist at first of masses of cells lying beneath the inner wall of the epipleures, from which they are probably derived. A central cavity appears in each mass, and the cells, which are now placed peripherally, develope into ova and spermatozoa respectively. The ripe products appear to be set free by rupture of the inner epipleural wall into the peribranchial cavity, whence they may escape by the abdominal pore; sometimes, however, through the branchial slits into the pharynx, and thence by the mouth. The ovum undergoes total, and at first fairly regular segmentation. There is an invaginate gastrula, and the coelome is an entero-coele, which is divided from before backwards into a series of paired sacs, the primitive somites; see p. 334. It may be added to what is stated there that the two anterior paired sacs grow forward into the head on either side of the notochord, and that an anterior sac, at first single, then dividing into a right and left half, originates from the fore-end of the archenteron.
The left sac remains small, acquires an opening to the exterior, and becomes the organ of taste or smell mentioned above p. 438. The right sac increases in size, and takes up a position anterior to the mouth and below the notochord. Hatschek does not state its ultimate fate, but it gives origin presumably to mesoblastic structures. A somewhat similar anterior entero-coelic pouch is seen in the larva of Balanoglossus: see Enteropneusta among Vermes.
Ray Lankester, Q. J. M. xv. 1875; Langerhans, A. M. A. xii, 1876; Rolph, M. J. ii. 1876; Schneider, Beitrage zur Vergleich. Anat. Berlin, 1879; Balfour, Q. J. M. xx. 1880; Rice, American Naturalist, xiv. 1880; Rohon, Dk. Akad. Wien, 45. 1882; Hatschek, Arb, Zool. Inst. Wien, iv. 1882; Id. Z. A. vii. 1884.
Oviposition, Milnes Marshall, Journal of Anat. and Physiol, x. 1876.