The composition of the plexus varies much, not only in different species, but even individually.
Nephridia always paired and without plexus of blood capillaries.
In the polyp the nervous tissue is always in the form of a scattered plexus, never concentrated to form a definite nervous system as in the medusa.
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.
The Crural Plexus is divided into a crural, ischiadic and pubic portion.
The last nerve which contributes to the ischiadic plexus leaves the spinal column in most birds either between the two primary sacral vertebrae, or just below the hindmost of them, and sends a branch to the pubic portion which is composed of post-ischiadic nerves, partly imbedded in the kidneys, and innervates the ventral muscles between the tail and pubis, together with those of the cloaca and copulatory organs.
In the pelvic region, from about the level of the posterior end of the ischiadic plexus, the strand of each side becomes single again, passing ventrally over the transverse processes.
Concerning the spinal nerves and their plexus: H.
In the case of many Oligochaeta where there is no vascular network surrounding the nephridium, this function must be the chief one of those glands, the more elaborate process of excretion taking place in the case of nephridia surrounded by a rich plexus of blood capillaries.
In Carinella, where the longitudinal nerve-stems are situated exteriorly to the muscular layers, this plexus, although present, is much less dense, and can more fitly be compared to a network with wide meshes.
It stretches forward as far as the brain, and in Carinella is again continued in front of it, whereas in the Heteronemertines the innervation of the anterior extremity of the head, in front of the brain, takes the form of more definite and less numerous branching stems. The presence of this plexus in connexion with the central stems, sending out nervous filaments amongst the muscles, explains the absence, in Pro-, Mesoand Heteronemertines, of separate and distinct peripheral nerve stems springing from the central stems innervating the different organs and body-regions, the only exceptions being the L.N.
A definite plexus can here no longer be traced.
The plexus of nerve-fibrils which underlie the ectoderm and are in places gathered up into nerves, and the great development of connective tissue, are worthy of notice.
From these tracts a plexus of nerve-fibres is developed in connexion with the musculature and cuticle.
It is enclosed in a fibrous capsule from which it is separated by the prostatic plexus of veins anteriorly.
As in other animals there is a minute but extensive nervous plexus, which permeates the whole body and takes its origin from the chief ganglia.
The nervous system consists as in Hydromedusae of a diffuse plexus beneath the ectoderm, concentrated in certain places to form a central nervous system.
A general nerve-plexus probably exists over considerable parts of the skin, and there are special nervous concentrations in the region of the epistome and along a double crescent (N) which follows the parietal attachment of the coelomic septum.
The separate nervecentres are, as a rule, placed in communication only by the general nerve-plexus, but in Charybdaea there is a zigzag marginal nerve connecting them up.