alimentary canal combined with the absence of any piercing of the central nervous system by the œsophagus.

So fundamentally different is the arrangement of the important organs in the two groups that it might well give rise to a feeling of despair of ever hoping to solve the problem of the Origin of Vertebrates; and, to my mind, this is the prevalent feeling among morphologists at the present time. Two attempts at solution have been made. The one is associated with the name of Geoffrey St. Hilaire, and is based on the supposition that the vertebrate has arisen from the invertebrate by turning over on its back, swimming in this position, and so gradually converting an originally dorsal surface into a ventral one, and vice versâ; at the same time, a new mouth is supposed to have been formed on the new ventral side, which opened directly into the alimentary canal, while the old mouth, which had now become dorsal, was obliterated.

The other attempt at solution is of much more recent date, and is especially associated with the name of Bateson. It supposes that bilaterally symmetrical, elongated, segmented animals were formed from the very first in two distinct ways. In the one case the digestive tube pierced the central nervous system, and was situated dorsally to its main mass. In the other case the segmented central nervous system was situated from the first dorsally to the alimentary canal, and was not pierced by it. In the first case the highest result of evolution led to the Arthropoda; in the second case to the Vertebrata.

Neither of these views is based on evidence so strong as to cause universal acceptance. The great difficulty in the way of accepting the second alternative is the complete absence of any evidence, either among animals living on the earth at the present day or among those known to have existed in the past, of any such chain of intermediate animal forms as must, on this hypothesis, have existed in order to link together the lower forms of life with the vertebrates.

Fig. 2.—Larval Balanoglossus (from the Royal Natural History).

It has been supposed that the Tunicata and the Enteropneusta (Balanoglossus) (Fig. 2) are members of this missing chain, and that in Amphioxus the vertebrate approaches in organization to these low invertebrate forms. The tunicates, indeed, are looked upon as degenerate members of an early vertebrate stock, which may give help in picturing the nature of the vertebrate ancestor but are not themselves in the direct line of descent. Balanoglossus is supposed to have arisen from the Echinodermata, or at all events to have affinities with them, so that to fill up the enormous gap between the Echinodermata and the Vertebrata on this theory there is absolutely nothing living on the earth except Balanoglossus, Rhabdopleura, and Cephalodiscus. The characteristics of the vertebrate upon which this second theory is based are the notochord, the respiratory character of the anterior part of the alimentary canal, and the tubular nature of the central nervous system; it is claimed that in Balanoglossus the beginnings of a notochord and a tubular central nervous system are to be found, while the respiratory portion of the gut is closely comparable to that of Amphioxus.

The strength of the first theory is essentially based on the comparison of the vertebrate central nervous system with that of the segmented invertebrate, annelid or arthropod. In the latter the central nervous system is composed of—

1. The supra-œsophageal ganglia, which give origin to the nerves of the eyes and antennules, i.e. to the optic and olfactory nerves, for the first pair of antennæ are olfactory in function. These are connected with the infra-œsophageal ganglia by the œsophageal commissures which encircle the œsophagus.

2. The infra-œsophageal ganglia and the two chains of ventral ganglia, which are segmentally-arranged sets of ganglia. Of these, each pair gives rise to the nerves of its own segment, and these nerves are not nerves of special sense as are the supra-œsophageal nerves, but motor and sensory to the segment; nerves by the agency of which food is taken in and masticated, respiration is effected, and the animal moves from place to place.

In the vertebrate the central nervous system consists of—

1. The brain proper, from which arise only the olfactory and optic nerves.

Fig. 3.—Vertebrate Central Nervous System compared with the Central Nervous System and Alimentary Canal of the Arthropod.

A. Vertebrate central nervous system. S. Inf. Br., supra-infundibular brain; I. Inf. Br., infra-infundibular brain and cranial segmental nerves; C.Q., corpora quadrigemina; Cb., cerebellum; C.C., crura cerebri; C.S., corpus striatum; Pn., pineal gland.

B. Invertebrate central nervous system. S. Œs. G., supra-œsophageal ganglia; I. Œs. G., infra-œsophageal ganglia; Œs. Com., œsophageal commissures.

2. The region of the mid-brain, medulla oblongata, and spinal cord; from these arises a series of nerves segmentally arranged, which, as in the invertebrate, gives origin to the nerves governing mastication, respiration, and locomotion.

Further, the vertebrate central nervous system possesses the peculiarity, found nowhere else, of being tubular, and the tube is of a striking character. In the spinal region it is a small, simple canal of uniform calibre, which at the front end dilates to form the ventricles of the region of the brain. From that part of this dilated portion, known as the third ventricle, a narrow tube passes to the ventral surface of the brain. This tube is called the infundibulum, and, extraordinary to relate, lies just anteriorly to the exits of the third cranial or oculomotor nerves; in other words, it marks the termination of the series of spinal and cranial segmental nerves. Further, on each side of this infundibular tube are lying the two thick masses of the crura cerebri, the strands of fibres which connect the higher brain-region proper with the lower region of the medulla oblongata and spinal cord. Not only, then, are the nerve-masses in the two systems exactly comparable, but in the very place where the œsophageal tube is found in the invertebrate, the infundibular tube exists in the vertebrate, so that if the words infundibular and œsophageal are taken to be interchangable, then in every respect the two central nervous systems are comparable. The brain proper of the vertebrate, with its olfactory and optic nerves, becomes the direct descendant of the supra-œsophageal ganglia; the crura cerebri become the œsophageal commissures, and the cranial and spinal segmental nerves are respectively the nerves belonging to the infra-œsophageal and ventral chain of ganglia.

This overwhelmingly strong evidence has always pointed directly to the origin of the vertebrate from some form among the segmented group of invertebrates, annelid or arthropod, in which the original œsophagus had become