The transverse commissure of the right and left pallium) is in birds reduced to a narrow flat bundle of a few white fibres; it is situated immediately above and behind the much stronger anterior commissure, i.e.
Owing to the small size of the olfactory lobes the anterior arms of the latter commissure are wanting.
N 1, Commissure uniting this with ventral ganglion (not shown in fig.).
In all Chaetopods this system consists of cerebral ganglia connected by a circumoesophageal commissure with a ventral ganglionated cord.
The nervous system is embedded in the epidermis, and the pairs of ganglia are separated as in Serpula, &c.; each pair has a longish commissure between its two ganglia.
The visceral commissure, while still surrounding the digestive tract, becomes looped; its right half, with its proper ganglion, passes to the left side over the dorsal face of the alimentary canal (whence the name supra-intestinal), while the left half passes below towards the right side, thus originating the name infra-intestinal given to this half and to its ganglion.
This conclusion has shown that the Euthyneura do not represent an archaic form of Gastropoda, but are themselves derived from streptoneurous forms. The difference between the two sub-classes has been shown to be slight; certain of the more archaic Tectibranchia (Actaeon) and Pulmonata (Chilina) still have the visceral commissure long and not untwisted.
- Streptoneura In this division the torsion of the visceral mass and visceral commissure is at its maximum, the latter being twisted into a figure of eight.
The right half of the commissure with its ganglion is supra-intestinal, the left half with its ganglion infra-intestinal.
In some cases each pleural ganglion is connected with the opposite branch of the visceral commissure by anastomosis with the pallial nerve, a condition which is called dialyneury; or there may be a direct connective from the pleural ganglion to the visceral ganglion of the opposite side, which is called zygoneury.
This is shown by the labial commissure and pedal cords of the nervous system, by the opening of the gonad into the right kidney, and by other points.
A, Abdominal ganglia in the streptoneurousvisceral commissure, with supraand sub-intestine ganglion on each side.
Sub-Class II.-Euthyneura The most important general character of the Euthyneura is the absence of torsion in the visceral commissure, and the more posterior position of the anus and pallial organs.
Comparative anatomy and embryology prove that this condition is due, not as formerly supposed to a difference in the relations of the visceral commissure which prevented it from being included in the torsion of the visceral hump, but to an actual detorsion which has taken place in evolution and is repeated to a great extent in individual development.
Actaeon is prosobranchiate, the visceral commissure is twisted in Actaeon and Chilina, and even slightly still in Bulla and Scaphander; in Actaeon and Limacina the osphradium is to the left, innervated by the supra-intestinal ganglion.
The detorted visceral commissure shows a tendency to the concentration of all its elements round the oesophagus, so that except in the Bullomorpha and in Aplysia the whole nervous system is aggregated in the cephalic region, either dorsally or ventrally.
Visceral commissure fairly long, except in Runcina, Lobiger and Thecosomata.
Visceral commissure much shortened, except in Aplysia.
Visceral commissure short, tendency to concentration of all ganglia in dorsal side of oesophagus.
Visceral commissure reduced; nervous system concentrated on dorsal side of oesophagus.
Shell ovoid, with short spire, wide aperture and folded columella; inferior pallial lobe thick; visceral commissure still twisted.
This is brought about by a double commissure, of which the ventral portion is considerably thicker than the dorsal, and which, together with the brain-lobes, constitutes a ring through which both proboscis and proboscidian sheath pass.
In Carinella this separation into lobes of the anterior thickenings of the cords has not yet commenced, the ventral commissure at the same time being extremely bulky.
In certain Metanemertines the lateral stems have been noticed to unite posteriorly by a terminal commissure, situated above the anus, the whole of the central nervous system being in this way virtually situated above the intestine.
(From Burger.) i, Opening of proboscis; 2, cephalic glands running to frontal organ; 3, dorsal commissure of brain; 4, cerebral organ; 5, upper dorsal nerve; 6, under dorsal nerve; 7, rhynchocoelic blood-vessel; 8, fore-gut; 9, rhynchocoel; to, nerve to proboscis; 11, proboscis; 12, genital sac; 13, genital pore; 14, mid-gut; 15, circular nerves; 16, pore of excretory system; 17, lateral organ; 18, excretory canal; 19, lateral vessel; 20,.
Above the oesophagus is a thin commissure which passes laterally into the chief armnerve.
19, B, a) and connected in front of it by a commissure, are the representatives of the cerebral and pleural ganglia of the typical Mollusc, which are not here differentiated as they are in Gastropods.
The dorsal half of the collar is the cerebral commissure, the ventral the labial commissure.
There are usually three small ganglia on the course of this visceral commissure, namely, the right and left visceral ganglia and the abdominal.
The labial commissure supplies only the buccal mass and the oesophagus and stomach.
Separate ganglia and n6 ventral visceral commissure, may be still more primitive.
The Prorhipidoglossomorpha are distinguished by the separation of the genital coelom from the pericardium, and by the long visceral commissure passing ventral to the intestine.
The cerebral ganglion also gives off a nerve on each side to a pair of small ganglia, united by a median commissure, which have sunk into and control the muscles of the head.
The four cords are all connected anteriorly with the cerebral commissure which lies above the buccal mass anteriorly.
From the points where the cords meet the cerebral commissure, arise on each an anterior labial commissure and a stomatogastric commissure.
The labial commissure gives off a subradular commissure which also bears two ganglia, these being in close relation to a special sense-organ called the subradular organ, an epithelial projection with nerve-endings, lying in front of the radula and probably gustatory in function.
The supra-rectal commissure may be present and bear an ovoid ganglion; or may be wanting.
A small stomatogastric commissure bearing two small ganglia arises from the cerebral ganglia and surrounds the oesophagus.
According to Spengel, the pair of ganglia near the mouth, variously called labial or cerebral, represent the cerebral pair and pleural pair of a gastropod combined, and the parietosplanchnic pair correspond to the visceral ganglia, the commissure which connects them with the cerebro-pleural representing the visceral commissure.
The pleural ganglia are also united by a long visceral commissure as in Lamellibranchs, and this commissure bears two ganglia lying close beneath the epidermis in front of the anus.
It consists of ciliated epithelium, beneath which are two ganglia connected with the labial commissure by nerves.
From both birds and reptiles the class is distinguished, so far at any rate as existing forms are concerned, by the following features: the absence of a nucleus in the red corpuscles of the blood, which are nearly always circular in outline; the free suspension of the lungs in a thoracic cavity, separated from the abdominal cavity by a muscular partition, or diaphragm, which is the chief agent in inflating the lungs in respiration; the aorta, or main artery, forming but a single arch after leaving the heart, which curves over the left terminal division of the windpipe, or bronchus; the presence of more or fewer hairs on the skin and the absence of feathers; the greater development of the bridge, or commissure, connecting the two halves of the brain, which usually forms a complete corpus callosum, or displays an unusually large size of its anterior portion; the presence of a fully developed larynx at the upper end of the trachea or windpipe, accompanied by the absence of a syrinx, or expansion, near the lower end of the same; the circumstance that each half of the lower jaw (except perhaps at a very early stage of development) consists of a single piece articulating posteriorly with the squamosal element of the skull without the intervention of a separate quadrate bone; the absence of prefrontal bones in the skull; the presence of a pair of lateral knobs, or condyles (in place of a single median one), on the occipital aspect of the skull for articulation with the first vertebra; and, lastly, the very obvious character of the female being provided with milk-glands, by the secretion of which the young (produced, except in the very lowest group, alive and not by means of externally hatched eggs) are nourished for some time after birth.