In the Chordata the alimentary tract has very different relations from those found in the Anne lida and Arthropoda. In the latter groups the alimentary tract lies dorsal to the maia nerve and ventral to the heart ; in the ohordates the tract is dorsal to the heart and ventral to the spinal cord. The question how the vertebrate condition is derived from the invertebrate con dition is a difficult one to answer. It has even led some to deny that vertebrates are related to Annelida or Arthropoda, as it is impossible to think of an animal adapted to traveling on one surface turning over and traveling on its back and transmitting this tendency to its de scendants. It is more likely that the intermedi ate form was one that, like many of the lowest Chordata—the tunicates—was sessile in a vent ral position at some time of life, and conse quently had neither dorsal nor ventral surface.
KMBRYOLOGICkL HISTORY. The history of the alimentary tract in vertebrates is as follows: Part of the outer layer of the germ becomes infolded as a pocket to form the lining of the arehenteron or primitive gut. (See EMBRYOLOGY.) By the continued growth of the mesoderm and body cavity the archenteron comes to lie as a canal, closed at the anterior end and communi cating posteriorly through the "neurenteric canal" with the neural tube. Later, an infolding of ectoderm occurs on the ventral surface of the embryo to form the proctodeum and anus. The neurenterie canal closes and the post-anal gut degenerates. Finally the ectoderm is inpocketed at the anterior end of the archenteron, forming the stomodeum, and the two cavities become eon fluent by the breaking away of the opposed walls. Thus, the completed alimentary tract is composed of an ectodermal anterior and posterior end and of an entodermal middle portion. It is enveloped by a mesodermal layer.
Phylogenetically, two quite distinct parts in the alimentary tract of vertebrates can be dis tinguished, and these do not coincide with the embryological divisions. In Amphioxus more than the first half of the length of the alimentary tract is devoted to the purpose of respiration, since its walls are provided with gill-slits. This may be called the respiratory part of the ali mentary tract in contradistinction to the remain ing hinder portion—the digestive part. The first is also known as the prosenteron. The hinder part is often divided into two—its ento dermal part (mesenteron) and its proctodeal portion (metenteron). It will be convenient to treat of the alimentary tract under the three heads of prosenteron, mesenteron, and meten teron. Before going on to this analytical treat ment of the tract and its appendages, a few words may be said concerning the general his tology of the entire tract. As already stated,
two germ-layers are involved: entoderm (or ectoderm) and mesoderm, and to these must be added mesenehyme. The entoderm is always a single layer and forms the so-called mucosa; it gives rise to the digestive and glandular epi thelium. Next outside lies the mesenehymatous mass, with its blood-vessels and nerves—the sub 'mucosa. Outside of this is the muscular layer derived from mesoderm and containing within circular muscle fibres and without longitudinal ones. Lastly, outside of all and continuous with the lining of the body cavity in all its parts, is the layer of fiat epithelial cells, eonstitnting part of the peritoneal membrane.
The Prosenteron.—This region is character ized, in the lower forms, by gills. In Balano glossus and in Tunicata such a gill-bearing region is well developed, and in the Tunieata becomes extremely complex, in adaptation to their sessile habit, which requires large respiratory surface, since a change of water cannot be got by travel ing. In Amphioxus the gill-slits are simple but very numerous—a hundred or more. The varia bility in number arises from the fact that the slits continue to increase in number as the animal grows older, new ones being formed at the posterior end of the series. Thus the pros enteron grows at the expense of the mesenteron. In embryologic history a single row of fourteen slits first arises as ventro-dextral organs: next, a second row of nine slits is formed at the right of the first, which, as it grows larger, pushes the first row to the left side, where it lies perma nently. The cause of the development of the left series on the right side is that the large mouth at first occupies the left side, and its movement ventrally is accompanied by profound changes in the surrounding parts. It has been suggested that the mouth of the ancestors of vertebrates was placed in the mid-do•sal line in front of the notochord; but that, the support of the noto chord being needed for the snout in the animal's journeys through the sand, it pushed forward and thrust the mouth to one side. The mouth is dorsal, or, better, neural, in young tunieates and in adult annelids; and the embryonic changes in the position of the mouth in Amphi oxus apparently recapitulate the phylogenetie changes. The gill-slits of the young Amphioxus open from the gut cavity directly to the exterior, but later they open into a common atrium on the ventral side, which functions somewhat as an opereulum. The details of the gill system of higher vertebrates will be discussed under RES PIRATORY SYSTEM. It is here merely necessary to say that the number of gill-slits becomes much reduced, usually to five or six pairs of slits.