Schulze.) elements, more especially by nervous (ganglion) cells and musclecells derived from the epithelial layer.
A further stage in evolution is that the muscle-cells lose their connexion with the epithelium and come to lie entirely beneath it, forming a sub-epithelial contractile layer, developed chiefly in the tentacles of the polyp. The of the evolution of the ganglioncells is probably similar; an epithelial cell develops processes of nervous nature from the base, which come into connexion with the bases of the sensory cells, with the muscular cells, and with the similar processes of other nerve-cells; next the nerve-cell loses its connexion with the outer epithelium and becomes a sub-epithelial ganglion-cell which is closely connected with the muscular layer, conveying stimuli from the sensory cells to the contractile elements.
The ganglion-cells of Hydromedusae are generally very small.
The mesogloea in the hydropolyp is a thin elastic layer, in which may be lodged the muscular fibres and ganglion cells mentioned above, but which never contains any connective tissue or skeletogenous cells or any other kind of special mesogloeal corpuscles.
The nervous system of the medusa consists of sub-epithelial ganglion-cells, which form, in the first place, a diffuse plexus of nervous tissue, as in the polyp, but developed chiefly on the subumbral surface; and which are concentrated, in the second place, to form a definite central nervous system, never found in the polyp. In Hydromedusae the central nervous system forms two concentric nerverings at the margin of the umbrella, near the base of the velum.
N 1, Commissure uniting this with ventral ganglion (not shown in fig.).
In the middle of the body, where the limits of the somites can be checked by a comparison with the arrangement of the nephridia and the gonads, and where the ganglia are quite distinct and separated by long connectives, each ganglion is seen to consist of six masses of cells enclosed by capsules and to give off three nerves on each side.
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.
The right half of the commissure with its ganglion is supra-intestinal, the left half with its ganglion infra-intestinal.
Pericardium, and is therefore a typical o, Olfactory ganglion, nephridium, was not known.
9, 10) of _ the nervous systems of ` Patella and of Haliotis, e as determined by Spengel, show the identity in the origin of the nerves passing from the visceral loop to Spengel's olfactory ganglion of the fig..
Ab, Abdominal ganglion or site pe, The right pedal nerve.
A, Abdominal ganglia in the streptoneurousvisceral commissure, with supraand sub-intestine ganglion on each side.
P, Pedal ganglion with otocyst attached.
Abdominal ganglion at the extremity of the twisted visceral " loop."
P sp, Supra - intestinal visceral ganglion on the course of the right visceral cord.
Sb, S u b - intestinal ganglion on the course of the left visceral cord.
Leydig, that the otocysts of Pectinibranchia even when lying close upon the pedal ganglion (as in fig.
Beneath the ciliated groove is placed an elongated ganglion (olfactory ganglion) connected by a nerve to the supraintestinal (therefore the primitively dextral) ganglion of the long h, k, m, Stomach.
V, Pleural and pedal ganglion c, Eye.
The euthyneurous visceral loop is long, and presents only one ganglion (in Aplysia camelus, but two distinct ganglia joined to one another in Aplysia hybrida of the English coast), placed at its extreme limit, representing both the right and left visceral ganglia and the third or abdominal ganglion, which are so often separately present.
43) shows the nerve connecting this abdomino-visceral ganglion with the olfactory ganglion of Spengel.
It is also seen to be connected with a more remote ganglion - the genital.
Spengel's observation of the osphradium and its nervesupply in these forms; the nerve to that organ, which is placed somewhat anteriorly - on the dorsal surface - being given off from the hinder part (visceral) of the right compound ganglion - the fellow to that marked A in fig.
Sp, Abdominal ganglion which represents also the supra-intestinal ganglion of Streptoneura and gives off the nerve to the osphradium (olfactory organ) o, and another to an unlettered socalled " genital " ganglion.
D, Oesophageal ganglion connected with the buccal.
On account of the shortness of the visceral loop and the proximity of the right visceral ganglion to the oesophageal nerve-ring, the nerve to the osphradium and olfactory ganglion is very long.
Instead of being destral, 'the osphradium is on the left side, and receives its nerve from the left visceral ganglion, the whole series of unilateral organs being reversed.
° at this stage, so (as sp, Visceral ganglion of the left side; op observed by A.
Krohn posite to it is the visceral ganglion of in Marsenia =Echino- the right side, which gives off the long spira) there may be a nerve to the olfactory ganglion and break at a later stage, osphradium o.
Farther up, within the velar area, the rudiments of the cerebral nerve-ganglion ng are seen separating from the ectoderm.
Oe, Gullet; op, optic nerve; sb, sub-oesophageal ganglion; mn, mx, mx', nerves to jaws; t, tentorium.
The continuous layer of cells from which the nervous system is developed undergoes a segmentation analogous with that we have described as occurring in the ventral plate; there is thus formed a pair of contiguous ganglia for each segment of the body, but there is no ganglion for the telson.
The nervous system of the embryonic head exhibits three ganglionic masses, anterior to the thoracic ganglionic masses; these three masses subsequently amalgamate and form the sub-oesophageal ganglion, which supplies the trophal segments.
In front of the three masses that will form the sub-oesophageal ganglion the mass of cells that is to form the nervous system is very large, and projects on each side; this anterior or " brain " mass consists of three lobes (the prot-, deut-, and tritencephalon of Viallanes and others), each of which might be thought to represent a segmental ganglion.
These three divisions subsequently form the supra-oesophageal ganglion or brain proper.
Lateral nerve; 21, oesophageal nerve; 22, mouth; 23, ventral ganglion of brain; 24, dorsal ganglion of brain; 25, rhynchodaeum.
The nervous system is represented by an oesophageal collar and a suboesophageal ganglion, whence paired nerves pass outwards to innervate the anterior extremity and backwards towards its posterior end.
The central ganglion of the nervous system lies in the proboscissheath or -septum.
In Discinisca and Lingula, however, the sub-oesophageal ganglion is not drawn out, but lies medianly; it gives off two posteriorly directed nerves to the stalk, which in Lingula unite and form a substantial nerve.
This latter includes in its course numerous ganglion cells, and forms, according to F.
Medianly, it has its origin in the sub-oesophageal ganglion, which, like the supra-oesophageal, is drawn out laterally, though not to the same extent.
In the middle line the sub-oesophageal nerve mass is small; the ganglion is in fact drawn out into two halves placed on either side of the body.
The wide divarication of the lateral cords in the prosoma and their connexion by transverse commissures, together with the " attraction " of ganglia to the prosomatic ganglion group which properly belong to hinder segments, are very nearly identical in the two animals.
The form and disposition of the ganglion cells are also peculiar and closely similar in the two.
23) each ommatidium has a peculiar ganglion cell developed in a central position, whilst the ommatidium of the lateral eyelets of Scorpio shows!
A very remarkable feature in Limulus, first described by Owen, is the close accompaniment of the prosomatic nerve centres and nerves by arteries, so close indeed that the great ganglion mass and its out-running nerves are actually sunk in or invested by ch.
A, Fasciola hepatica, from the ventral surface (X 2); the alimentary and nervous systems only shown on the left side of the figure, the excretory only on the right; a, right main branch of the intestine; c, a diverticulum; g, lateral ganglion; n, lateral nerve; o, mouth; p, pharynx; s, ventral sucker; cs, cirrus sac; d, left anterior dorsal excretory vessel; m, main vessel; v, left anterior ventral trunk; x, excretory pore.
From these ganglia, nerve-tracts provided with ganglion-cells are given off.
The nervous system is represented by a ganglion situated between the mouth and the anus.
The nervous system consists of a ganglion or brain, which lies dorsally about the level of the junction of the pharynx and the stomach, a nerve ring and a segmented neutral cord.
Here, and in the narrow neck of land between the Appomattox and the James, was the ganglion of the Confederacy, and the struggle for its possession was perhaps the greatest of modern history.
- There is a large ganglion lying in close contact with the pharynx, proximal to the crop and on its antero-dorsal side; in Bdelloidaceae at least it is united by short connectives with a smaller postero-ventral ganglion to form a nerve collar.
The nerve ganglion is formed by an ingrowth of epiblast, and so are the pedal glands.
F nr x s.n ganglion cells below invertebrate sense-organs.
In the nervous system of Aplysia the great ganglion-pairs are well developed and distinct.
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.