an operation in the thalamus opticus begins to move spontaneously again if the optic lobes and the pars commissuralis of the medulla are removed.

In Limulus an anterior and a posterior nerve originate from every ganglion of the ventral chain. It was interesting to determine whether these nerves have functional differences like those of the anterior and posterior roots of the spinal cord of Vertebrates. It has been maintained that Arthropods are Vertebrates that walk on their backs. Faivre has stated that there is not only a separation of the motor and sensory roots in Arthropods, corresponding to Bell's law, but that also in Arthropods, in contrast with Vertebrates, the ventral side of the ganglia is sensory, the dorsal motor. Now this is not true of the nerveroots which start from the ganglia in Limulus. If the posterior nerve be severed and its peripheral stump stimulated, we get inspiratory movements of the half of the gills to which this nerve goes. All the other gills are unaffected. Hence this nerve contains motor fibres. If the ventral stump be stimulated, the whole animal becomes much excited. From this we see that the posterior nerve also contains sensory fibres. If the anterior nerve be severed, stimulation of the peripheral stump has no effect. Stimulation of the central stump excites the entire animal. Hence the anterior nerve is purely sensory. Limulus is better adapted for deciding this question than the smaller Arthropods. The conditions in the latter are probably the same as in the former, for Vulpian (4)

and latterly Bethe (5) energetically reject the idea that dorso-ventrally the ganglion-chain of Arthropods is the reverse of the spinal cord of Vertebrates.

4. We will now turn our attention to the crayfish as the next representative of the Arthropods whose brain-physiology has been carefully investigated. Fig. 32 gives a diagram of the central nervous system of the lobster, which is almost identical with that of the crayfish. o is the supraœsophageal ganglion with the nerves for the eyes and antennæ. In addition it gives off the sympathetic nervous system which goes to the intestine. Both œsophageal commissures, c, go backwards to the suboesophageal ganglion, u. The latter is seemingly one ganglion, but it supplies six pairs of segmental organs, namely, the mouth-appendages. The microscopical examination shows that this subœsophageal ganglion in reality consists of six separate ganglia. We often meet with a fusion of ganglia, and consequently an apparent lack of clearness in the segmental arrangement. It is due to this fact that in the brain-physiology of Vertebrates the segmental arrangement of the central nervous system has been left entirely out of consideration. Next after the subœsophageal ganglion come the five thoracic ganglia (I-V T, Fig. 32) belonging to the segments of the forceps and the four pairs of locomotor appendages. In addition to these, there are the five ganglia of the abdomen (I-V Abd., Fig. 32) that innervate the swimmerets, and the tail, which serves as a swimming-organ. The best experiments

o, supracesophageal ganglion (brain); c, commissure; ", subœsophageal ganglion; I-VT, five thoracic ganglia; I-V Abd., first five abdominal ganglia.

on the central nervous system of these animals have unquestionably been made by Bethe, and we shall in the main follow his presentation. Many of the facts which Bethe describes from the animals used in his experiments are familiar to me from personal observation, and I am convinced that the picture he gives is correct.

If in a crayfish both the commissures (c, Fig. 32) which connect the supraœsophageal ganglion o with the rest of the brain be severed, the behaviour of the animal is no longer controlled by the brain o. It does not make spontaneous progressive movements. When stimulated it begins to move, but after having gone about 20 cm. it stops. This lack of spontaneous progressive movements agrees with the description given by Flourens of the Vertebrate from which the cerebral hemispheres had been removed. Flourens's representation was wrong, however, for a dog operated upon in this way shows increased spontaneity in its progressive movements.

Annelids and Arthropods are closely related as regards the central nervous system. However, Nereis shows an excess of progressive movements after removal of the supracesophageal ganglion, while Astacus no longer moves spontaneously. I believe that the difference depends only upon circumstances of minor importance. Ward has already found-and Bethe has confirmed the fact that in brainless crayfish the legs are unceasingly active, either cleaning each other or performing pendulum - movements.

I

They, however, make no progressive movements. believe this is due possibly to a secondary effect of the extirpation of the supracesophageal ganglion. The legs of such an animal have an abnormal position, being more strongly flexed at the joints nearest the body than they are normally. The tension of the extensors probably suffered severely from the operation. Such mechanical disturbances might easily cause difficulty in locomotion, while simple pendulummovements of the legs, which require practically no labor, could still be performed. The fact that after removal of the brain of crayfish the tension of the flexors predominates in certain joints is of interest, as we meet with the same phenomenon in dogs that have lost the anterior region of the cerebral hemispheres, and as it also comes to our attention in man after apoplexies which result in the paralysis of an arm.

Bethe concludes from these pendulum-movements that the brain is an organ of inhibition. As regards this, the remarks hold good that have already been made in this connection on annelids (see p. 94).

The weakening of the muscles in the crayfish whose brain has been extirpated shows itself also in the fact that the forceps no longer pinch as hard as those of normal animals.

After what has been said concerning the segmental character of the central nervous system, it is to be expected in the crayfish that, since the segmental ganglia of the organs of mastication are located in the subœsophageal ganglion, extirpation of the supra