It receives the ventral root of the perioesophageal commissure; and gives rise to the two pairs of stomatogastric nerves and tone nerves in the lips. The hindbrain is much smaller, having only a small fraction of the volume of even the smallest (posterior) division of the midbrain. The equivalent in Amphinomidae of the forebrain must also include the part which receives the ventral root of the perioesophageal ligament.
4.-(After Heider's fig. 15) Dorsal view of brain and anterior parts of central nervous system and visceral nervous system of Eunicepunctata. Badly suggested by the fact that "both Kleinenberg (1886) and Meyer (1901) have shown that the ganglia of the hindbrain are marked. The anterior and largest pair, midbrain III, innervate the anterolateral antennae (the "palps" of authors) and receive the dorsal branch of the perioesophageal ligaments.
NO, 8 THE POLYCHAET — RAW 9 and hind-brain — into four divisions, grouped above in two categories
THE STOMODEUM IN EUNICE AND OTHER POLYCHAETS At the outset in this paper the Eunicimorpha have been claimed as
THE STOMATOGASTRIC NERVOUS SYSTEM IN POLYCHAETS A visceral nervous system has been known since the time of.
THE STOMATOGASTRIC NERVOUS SYSTEM IN POLYCHAETS A visceral nervous system has been known since the time of
INTERRELATIONS OF THE STOMODEUM, THE VISCERAL NERVOUS SYSTEM, AND THE BRAIN IN THE
ANCESTRY OF EUNICE
NO. 8 THE POLYCHAET — RAW II
It contains the "lower jaw" plates, here interpreted as representing a few more appendages added to the mouth-armature; and it is bounded by a new, the present, lower lip. Each of these pharyngeal invaginations produced a double sac, each sac partially subdivided by the pair of cushions, which may be compared to parapodia, on which the pair or pairs of teeth are situated. The visceral nervous system and its distribution, as suggested above, are explainable as direct consequences of the invaginations, which involved not only ordinary ectoderm and pairs of appendages, but also the "central" nervous system, which was still continuous with the ectoderm.
It may not be possible to assign to each intussusception its exact contribution to the visceral nervous system; in turn, this is clearly subdivided into the section of the esophagus and pharynx. One effect of this is to be seen in the fact that the visceral nerve cords of the esophagus from the forebrain are quite free from the hypodermis until, at the supra-esophageal visceral ganglion, they reach the esophagus. The open nerve cords, after any vaginism involving them, appear to have been closed by the progress of the greater part of the intact ganglia of the brain.
NO, 8 THE POLYCHAET — RAW 13 able, for nature in such an unimaginable time would simplify a
NO. 8 THE POLYCHAET — RAW I5
Judging from the structure of the brain, this is now the posterior part of the midbrain t^midbrain 1: its small size (quite out of proportion with. The second pharyngeal invagination, the mandible s;ic. appears to have only added the anterior part of the nerves of the pharynx, as explained below, p. 18. In plan these are irregularly square, with concave sides and horn-like angles, and they lie above the muscular posterior bases of the jaw-pads, on either side of the dividing fissure, and with their convexities directed towards each other.
8 THK I'OLYCIIAKT KAW IJ JJcidcr, after satisfyinginj^himself as t*; the nerve character of the body suggests that they are motor centers for the jaw muscles of the jaw pads; and although he could not trace connections, he suggests that their posterior horns may connect with the nerves of the terminal visceral oesfj[;hagwd ganglion ()., and that th(Mr. anterior ventral horns connect with thetw(j visceral pharyngealcords. Such a connection might be caused by the sharp folding of the hypodermis which the stomodeum exhibits, or might arise when the lip between the two invaginations became bifid Each of the three parts of the midbrain carried to the head a pair of cirri, and the first two also a pair eyes; for each segment in Eunice generally has a pigment spot with the structure of an eye just above each appendage.
The cirri survive as the antennae of the prostomium, but usually in Eunice only one pair of eyes, belonging to midbrain II, survives. The union of the pair of cirri belonging to this last in the median antenna can. According to our theory, the ganglia of the mandibular segment were the last of the ganglia of the central nerve cords to the brain, forming midbrain III, the largest as well as the last of these additions.
Like their predecessors, they attended to and came after the front parts of the brain, for they alone receive from the middle divisions of the brain the connective tissue of the oesophagus; and they too pushed their predecessors into the background. Again, they brought to the forebrain not only the central nerve cords, which in this case persist as esophageal ligaments with their two roots, but also the ends of a new loop of the visceral nervous system, which persist as pharyngeal nerves and reach the brain with the ventral roots of the esophageal ligaments.
NO, 8 THE POLYCHAET — RAW I9
COMPLETION OF THE BRAIN
SUMMARY OF THE AUTHOR'S THEORY
As discussed below, for the primitive situation we can assume the presence of three pairs of longitudinal nerve cords shown. Their positions in the primitive annelid, if symmetrical, would be dorsolateral, lateral, and ventrolateral, respectively. In the primitive annular body (Table 2, stage i), the original brain, represented by the present forebrain, must have already formed the dominant nerve center of the body; retained its dominance until the brain was completed.
The loops were closed by the progression of the front pair of uninvaginated gangliatthebrain (forebrain) to form midbrainI, which brought with it the ventral sides of the esophageal loops and the ventral chains (table 2, stage 3). The nerves were brought to the forebrain and appropriated, while the midbrain I presided over a few.
NO. 8 THE POLYCHAET — RAW 21
The increased ganglia, following the established rule, addressed themselves to the previous brain, their ancestors, which they forcefully turn back. They traced back the abdominal cords and extending into the forebrain caused the double roots of the esophageal connectors. Importantly, these arise from the ventral roots of the theoesophageal junctions—the two joined and at the time of midbrain III addition.
LIMITS AND SIGNIFICANCE OF THE PROSTOMIUM OF THE POLYCHAET
One more change and the brain's basic plan was completed: . midbrain I had now been pushed so far back that it was approaching the ganglia and these also added themselves in stage 10 to form the hindbrain. Soderstrom, who espoused this latter view, actually tried to change its definition to the unsegmented anterior part of the body; and he despised anyone who did not have the same 'prostomium idea'. However, such a definition is completely unjustified. The current theory, formulated in complete ignorance of Lameere's, also reverses the order, but for an entirely different reason; furthermore, while according to Lameere the order of the three 'pairs' had already been developed in the coelenterate ancestor, according to current theory the three pairs of ganglia that innervated them (and also like the antennae themselves) were developed during the evolution of the polychaete by the nature selected at random from a long series of postcephalic segments, and transferred to the head.
Prior to Soderstrom's studies of the Spionidae, the prostomium was always considered to also include the entire compact mass of nervous matter that constitutes the brain. Ill postcephalic segments of Spionidae, so that the antennae have theirs in the dorsal cirri of the body segments, and the eyes have theirs in the postcephaHceye spots. The principle that Soderstrom would rule out the mastermind is invalid and reducible to an absurdity.
Furthermore, the reason given above for including the hindbrain at the same time as the midbrain III was given. Gustafson, who doubts whether to accept Hanstrom's theory that the stomatogastric lobes (or anterior part of the brain) are added from the ventral chain, therefore doubts whether the oral lips they animate should be regarded as part of the prostomium. Many may be surprised at the inclusion of the hindbrain after Soderstrom's significant and important body of work; but it will be clear that the same principle which would exclude it would also exclude the whole of their midbrain and all the cephalic tentacles and eyes.
They are not the representative in modern forms of the head of the primitive annelid, but the result of the long development from this of the head of the polychaete. Further, this large aggregation of the brain and the cephalicsense organs, which built up the polychaete prostomium, is here argued as a consequence of the development of the stomodeum, which is again referred to changes in the mode of feeding.
MORPHOLOGICAL SIGNIFICANCE OF THE HIND-BRAIN The nuchal organs are quite dorsal in position. This is seen very
It is not simply due to the cephalization of further anterior segments of the body, as many have believed, but is largely due to the advancement to the brain of three pairs of segmental body ganglia at three widely separated times, and at the same time also the advancement to the head of the body. three-segmented pair of cirri. Incidentally, this aggregation caused the brain to also incorporate the hindbrain, which innervates the neck organs, which is therefore only a large part of the prostomium, which are the other cephalic sense organs and the midbrain.
MORPHOLOGICAL SIGNIFICANCE AND RELATIONSHIPS OF THE PERISTOME
In his view the peristome did not differ materially from a normal segment (1896, p. 154); the head of the ancestor became the head of the polecat; the parts were already there, including the primitive elements of the cerebral nervous system - the "aire palpaire aire sincipitale" and "aire nucale." Only further development of these was necessary: that of the sense organs on them to palps, antennae, eyes and neck organs; that of the areas themselves in the forebrain, midbrain and hindbrain. On the contrary, according to the theory presented here, the ancestor was already equipped with eyes and parapodia throughout the body, and possibly also some form of appendages and eyes on the head, which already had a brain and boreales, an early stage of the lips. But what sense organs the primitive annelid had on its head, except for the lips, these organs were replaced in the evolution of the polychaete by formerly postcephalic eyes and antennae; while his brain had to be extensively supplemented and partially replaced.
A fundamental difference between the two views is that in developing the polychaete brain, Racovitza thought he was developing that of the primitive annelid, while according to the theory offered here, no primitive annelid survives.
APPLICATION OF THE AUTHOR'S THEORY TO POLYCHAETS IN GENERAL
Moreover, these comparisons are of which there can be no doubt, but that they all spring from the same stock; and if the basic plan of the brain above detailed be accepted, it will be agreed that the nearest approach to this stock is to be found in the Eunicimorpha. The evolution that has been traced, if essentially true, is therefore the ancestral polychaete. If the above given conclusions as to the nature and origin of the original polychaete brain be accepted, its further development can be depicted with some certainty.
The forebrain, hitherto dominant, has lost its obsolescence even in the Eunicidae; and in other families is often greatly reduced. And whereas, as here assumed, the palpal function in the primitive polychaet was in the lips and operated by the forebrain, this is now only the case in Eunicidae and Amphinomidae. In the Amphinomorpha additional pairs of cervical ganglia have joined the hindbrain, and this has here become by far the most voluminous division of the brain.
In contrast, in Aphrodite it is compact, folded between the front and back and very highly developed.
NO. 8 THE POLYCHAET — RAW 29 errant families and perhaps in all the sedentaries it has degenerated
NO. 8 THE POLYCHAET — RAW 33
CONCLUSION
