Gene expression in the developing nemertean brain indicates convergent evolution of complex brains in Spiralia

BackgroundNemertea is a clade of worm-like animals, which belongs to a larger animal group called Spiralia (together with e.g. annelids, flatworms and mollusks). Many of the nemertean species possess a complex central nervous system (CNS) with a prominent brain, and elaborated chemosensory and neuroglandular cerebral organs, which have been suggested as homologues to the annelid mushroom bodies. In order to understand the developmental and evolutionary origins of complex nemertean brain, we investigated details of neuroanatomy and gene expression in the brain and cerebral organs of the juveniles of nemertean Lineus ruber.ResultsIn the hatched juveniles the CNS is already composed of all major elements present in the adults, including the brain (with dorsal and ventral lobes), paired longitudinal lateral nerve cords and an unpaired dorsal nerve cord. The TEM investigation of the juvenile cerebral organ revealed that the structure is already composed of several distinct cell types present also in the adults. We further investigated the expression of twelve transcription factors commonly used as brain and cell type markers in bilaterian brains, including genes specific for annelid mushroom bodies. The expression of the investigated genes in the brain is region-specific and divides the entire organ into several molecularly distinct areas, partially overlapping with the morphological compartments. Additionally, we detected expression of mushroom body specific genes in the developing cerebral organs.ConclusionsAt the moment of hatching, the juveniles of L. ruber already have a similar neuroarchitecture as adult worms, which suggests that further neural development is mostly related with increase in the size but not in complexity. Comparison in the gene expression between L. ruber and the annelid Platynereis dumerilii and other spiralians, indicates that the complex brains present in those two species evolved convergently by independent expansion of non-homologues regions of the simpler brain present in their common ancestor. The similarities in gene expression in mushroom bodies and cerebral organs might be a result of the convergent recruitment of the same genes into patterning of non-homologues organs or the results of more complicated evolutionary processes, in which conserved and novel cell types contribute to the non-homologues structures.

The Posterior Lobes of the Brain of Nephtys and the Mucus Glands of the Prostomium

A pair of lobes is attached to the posterior margin of the supra-oesophageal ganglion of some species of Nephtys. They are filled with large, vacuolated mucus-cells similar in appearance and histochemical properties to those found in the anterior part of the prostomium, in the lateral walls of it, and in the parapodia. The mucus-cells of the posterior lobes, and sometimes those of the anterior prostomial group also, have long necks which run in a tract to the epidermis, where they open to the exterior. When this is so, they replace the epidermal mucus-cells found in the lateral walls of the prostomium in species lacking posterior lobes. It is suggested that there has been a centripetal migration of epidermal mucus-cells into the posterior lobes and to the anterior prostomial group, a phenomenon closely paralleling that found in the evolution of the nermertean cerebral organ. In one species, Nephtys cirrosa, the posterior lobe cells appear to have undergone a further modification, for they are much more closely integrated with the nervous system and differ in appearance from those of other species, coming to look, at least superficially, like the majority of neurosecretory cells in the brain.

On Remissions in the Course of General Paralysis

The study of the intermissions which occur in the course of general paralysis is not the least interesting chapter in the history of this disease. Is it not a matter of surprise, that an affection which occasions such grave disturbances of the cerebral organ should undergo ameliorations so pronounced as might well be taken for a real cure, did not the experience of each day teach us that it is but arrested for a longer or shorter time, and that sooner or later the paralysis resumes its fatal course. Have not even alienists asserted that they have cured demented paralytics ? The existence of intermission in the course of general paralysis does not at the present time need to be demonstrated. It is enough to glance at the treatises of MM. Calmeil and Bayle, to find the most remarkable examples of it.

On the structure of the brain in marsupial animals

The author describes a remarkable modification in the commissural apparatus, apparently provided with a view to establish communications between the cerebral hemispheres, which he has observed in the brains of marsupial animals, and which has hitherto been regarded as constituting the essential difference between the brains of oviparous and mammiferous vertebrata, but which he considers as indicating a certain relation between the greater perfection of that organ, resulting from the superior magnitude of the great commissure, or corpus callosum, and the placental mode of developement in the true mammalia. In a former paper he adduced evidence tending to show that both a small developement of the cerebral organ, and an inferiority of intelligence are the circumstances in the habits and structure of this singular tribe of animals most constantly associated with the peculiarities of their generative economy: and the repeated dissections he has since made, an account of which is given in the present paper, have afforded him the most satisfactory confirmation of this coincidence, between a brief intra-uterine existence, together with the absence of a placental connexion between the mother and the foetus, and an inferior degree of cerebral developement. Thus, on comparing the structure of the brain in the Beaver and in the Wombat, he finds that the corpus callosum, or great commissure which unites the supraventri- cular masses of the hemispheres in the former, as well as in all other placentally developed mammalia, and which exists in addition to the fornix, or hippocampal commissure, is wholly absent in the latter animal: and that a similar deficiency exists in the brain of the Great and Bush Kangaroos, of the Vulpine Phalanger, of the Ursine and Mange’s Dasyurus, and of the Virginian Opossum ; whence he infers that it is probably the characteristic feature of the structure of the marsupial division of mammalia. In this modification of the commissural apparatus, the Marsupiata present a structure of brain which is intermediate between that of the Placental Mammalia and Birds ; and hence the Marsupiata, together with the Monotremata, may be regarded as constituting a distinct and peculiar group in the former of these classes, although they include forms, which typify the different orders of the ordinary Mammalia.

On recrossed vision; being the description of a distinct tribe of ocular phenomena, supplementary to a rationale of the laws of cerebral vision, recently published

The phenomena described in this paper, and which the author designates those of recrossed vision , are cases in which objects placed between and very near the eye, such as the two sides of the nose, appear on opposite sides of the sphere of vision: the object on the right side of the nose being seen to the left by the right eye, and that which is on the left of the nose being seen to the right by the left eye. These and other phenomena illustrative of the well-known law by which we estimate the position of objects with relation to the eye to be in a line drawn from its image in the retina through the centre of the eye, are considered by the author as requiring further explanation. Not satisfied with the theory of Berkeley, that the mind is guided by the perceptions received from the sense of touch, in interpreting the signs furnished us by the sight, the author proposes to explain these phenomena by an hypothesis of his own, which he states in the following words. “Over and above the gift of two external or cranial eyes, man has been by his adorable Creator endowed with an internal cerebral organ, which performs the office of a third eye , by being the common recipient of impressions propagated either from one, or both of the external eyes; and the mind, in her chamber of percipience, steers with regard to external objects by the same principle on which the mariner steers by his compass. Thus the two cranial eyes are analogous, in principle and situation, to two magnetic compasses placed upon a ship’s deck; while the third, or cerebral eye, corresponds to another compass placed in the cabin below; and the mind, situated like the captain-mariner in his cabin, knows, from consulting the cerebral eye, on what point of direction the body is steering; although the mind no more perceives either any external object, nor yet any image in the cranial eye, than the mariner perceives (even in the vulgar sense of the word perceiving) the far-off land, or haven, towards which he is surely making his way.”