As to the origin of the Enteropneusta, opinions are somewhat divided. Their blood system and their development would seem to suggest a descent from the ringed worms. On the other hand, their possession of a snout, and their very slight indication of division into segments, would seem to separate them from the group mentioned and to connect them rather with the Nemertines. The latter view is perhaps the more probable. CHAPTER IV THE EARLY VERTEBRATES AND THE FISHES The lowest of the vertebrates--if indeed it can be called a vertebrate at all, seeing that it has no vertebræ--is the lancelet, Amphioxus. The common species of this animal (there are some eight in all) occurs in the sea off our own coasts, and is usually to be found half buried in the sand or mud of the sea floor. It is some two inches in length and has the shape of a laterally flattened cigar, and one of its very obvious features is the arrangement of the muscles in regular layers from front to back, in the same manner as those of a fish. To describe some of its features in detail, the alimentary canal bears a somewhat striking similarity to that of Balanoglossus. There is a round, simple mouth, unprovided with jaws, and surrounded by a number of projecting bristles. This leads into a large pharynx, through the walls of which, on either side, pass a large number of gill slits. The pharynx is not divided into an upper and a lower canal, but there is a shallow groove along the bottom which serves the same purpose as the food canal in Balanoglossus. The remaining, digestive, part of the gut is practically a simple tube, with a blind sac attached, representing the liver. The gill slits do not open directly to the exterior, but into the so-called peribranchial chamber, formed by the junction below the body of two flap-like outgrowths, one from the upper part of either side. This chamber opens to the outside by a single pore.