Dissected so as to show its nervous system.
THE integument has been divided down the middle dorsal line and fastened out on either side. The entire digestive tract with the exception of the buccal cavity, most of the nephridia or excretory organs, and the septa dividing the body into compartments, have been removed. Of the reproductive organs only the spermathecae or receptacula seminis, two globular white sacs, have been left in situ on the right side. They open respectively between the ninth to tenth and tenth to eleventh somites, on a level with the dorsal row of setae. The two lobes, making up the supra-oesophageal or cerebral ganglia, are pyriform, and have their broader ends apposed to each other in the middle line. A thick nerve passes off from each of their outer or narrower ends. It bifurcates, and ends in a plexus in the prostomium, on which are situate numerous sense-bodies. A right and left oesophageal commissure surround the passage from the buccal cavity to the pharynx and connect the supra-oesophageal to the first ganglion of the ventral nerve-cord. This cord extends to the posterior extremity of the body. It takes the shape of a thick band in which ganglionic enlargements are recognizable with difficulty for a space corresponding with that occupied by the pharynx, oesophagus and reproductive organs.
Posteriorly to the fifteenth somite it becomes more slender, and the ganglia more distinct. Finally, for a length nearly equal to the posterior half of the animal, it becomes thicker and moniliform, the ganglia being plainly marked but closely apposed. The terminal ganglion is, contrary to what is seen in some Vermes and many Arthropoda, smaller than those which precede it.
The two rows of paired setae are well seen on each side in most of the somites. The enlarged inner copulatory setae of the two somites, fifth and sixth in order anteriorly to the clitellum, as well as of the clitellum itself, can be readily distinguished. In the interval between each inner row of setae and the nerve-cord in the fifteen anterior somites, a longitudinal muscular fascicle is seen passing forwards. It is inserted on the outer ends of the supra-oesophageal ganglia and on the commissures, and acts as a retractor muscle to these parts. Between the two rows of setae of the eighth to the thirteenth somite inclusive, may be seen, on the right side, the cap-sulogenous glands of D'Udekem. These structures appear to be merely enlarged setiparous sacs, and not glands. They have been supposed to secrete the albumen surrounding the ova in the cocoon. The nephridia are left in situ on either side of the nerve-cord in some of the posterior somites.
The supra-oesophageal or cerebral ganglia belong developmentally to the pro-stomial region of the first somite of the body, but generally shift in the adult backwards even as far in some instances as the 3rd or 4th somite. They retain a position in Microchaeta Rappi, etc, in the first (buccal) somite. The oesophageal commissures are composed of one or two fibrous cords. The ventral nerve-cord lies internally to the longitudinal muscular coat, except in the Lumbriculidae, where it lies next to the circular coat. The cerebral ganglia of Aeolosoma are, however, in continuity with the hypodermis, but a ventral cord is absent in this genus. In a few Chaetopoda, in Protodrilus, Polygordius, and Histriodrilus (=Histriobdella), the ventral cord as well as the cerebral ganglia are similarly continuous, and the three genera in question have been made into a separate group of Archi-annelidae (Hatschek). Saccocirrus, in which the nervous system is also hypodermic in position, is in other respects not so archaic a type, and has been classed by Foettinger apart from other Chaetopoda as Archi-chaetopoda.
The nerve-cord is composed of (1) an external layer of polygonal epithelial cells belonging to the peritoneum; (2) a coat of longitudinal muscle fibres, which does not extend up the commissures to the supra-oesophageal ganglia; (3) of a neurilemma, formed by the cord itself; and (4) the nervous matter proper, with a neuroglia or supporting connective tissue. The ganglion cells are found on the anterior surface of the supra-oesophageal ganglia, and as a layer on the ventral surface of the ventral cord, but not in the oesophageal commissures. They have no sheaths. In the first part of the cord they form a perfectly continuous layer (a fact denied by Vignal), with a right and left and two median aggregations, while in the posterior region these aggregations are separated from one another. This continuous arrangement is found only in Lumbricidae and Lumbriculidae among Oligochaeta. The number of cells does not appear to be much increased in the ganglionic enlargements, which are due chiefly to the greater amount of fibrous matter present at the origin of the chief nerves.
According to Claparede the cord is divided into a right and left half by a median connective tissue septum.
The nerve-cord has in Lumbricidae three special blood-vessels running longitudinally within the muscular sheath, one subneural and two lateral, one on either side. The three are connected by ventral loops just behind each ganglionic enlargement. The lateral vessels give off a branch which accompanies the paired nerves (infra), the median a branch which accompanies each septal nerve (infra), and all three are connected to a capillary plexus ramifying in the substance of the cord and round the ganglion cells. The presence of these vessels was supposed to be a distinctive feature of the Oligochaeta terricola, as opposed to Oligochaeta limi-cola, but the subneural vessel is absent in the terrestrial Megascolex, Perichaeta Houlleti, and Microchaeta Rappi, in the semi-marine Pontodrilus, but present in the aquatic Criodrilus.
The nerves originating from the cerebral ganglia usually break up into a plexus, in which ganglion cells are interpolated. The network is especially well-developed and visible in Tubifex, and in Limnodrilus and Anachaeta its ganglion cells are aggregated into special ganglia.
The first ganglion of the ventral chain generally gives off many nerves; the following ganglia a pair of nerves on either side. A single septal nerve arises also on each side, between successive ganglionic enlargements, and is distributed to a septum.
A pharyngeal plexus or 'Vagus' system is derived in most, probably in all, Oligo-chaeta from the oesophageal commissures. In some of the lower Oligochaeta there are distinct pharyngeal ganglia. In the Earthworm the system consists usually of a more or less elongated mass lying on either side of the pharynx, continuous with a rich plexus, the fibres of which are ultimately lost among the muscular structures. Leydig and Vignal state that ganglion cells are found in all parts of this plexus; Claparede, on the contrary, that none occur in the plexus so far as it is visible to the naked eye, but that they are present on the finer branches among the muscular bundles.
A cord of ganglion cells in continuity anteriorly with the cerebral ganglia runs down each side of the body in most Oligochaeta. It is especially easy to see in Nats, and is contained in the hypodermis. It is said to supply the muscles of the head, sacs of the setae, and the nephridial apertures; and is, perhaps, in connection with the ganglion cells, which have been found in the walls of the digestive tract in some instances. A zone of ganglion cells encircles each somite in the Naid Slavina appendiculata, is connected to the lateral cords, and supplies the tactile eminences (infra). In Lumbricidae the lateral cords are to be detected clearly in the young posterior somites, but they are resolved in the older anterior somites into scattered cells.
As to organs of special sense. Oligochaeta never possess otocysts. Eyes are found only in some Naidomorpha; supposed gustatory organs in the pharynx of Enchytraeidae and Limnodrilus; and olfactory (?) organs as a couple of ciliated pits on the head in Aeolosoma, Ctenodrilus, Parthenope. The last-named structures occur also in some Polychaeta. Tactile organs, however, are commonly distributed, and in various forms; as (1) a hypodermis cell furnished with an external tactile seta, and continued basally into a nerve fibril, in its turn often connected to a ganglion cell, - found in numbers on the prostomium of Aeolosoma, Chaetogastridae, and Naidomorpha; (2) tactile papillae, in which the hypodermis cell is protrusible, and furnished with short setae (Chaetogastridae); (3) tactile eminences, apparently composed of aggregations of tactile hypodermis cells, arranged fifteen to twenty in number in a zone on each somite of Slavina appendiculata; and (4) goblet bodies, or aggregations of very delicate hypodermis cells provided with sense-hairs, found most plentifully on the prostomium and buccal somite of Lumbricidae, more sparingly on the anterior somites of the body, but especially round the setae.
A single goblet body is found also on each side of the somites of the Lumbriculidae, seated on the lateral cord of ganglion cells. They become enlarged on the clitellum of the Lumbriculid Rhynchelmis, where gland cells occur among the sense cells. See on the subject Vejdovsky, op. cit. pp. 96-100.
On the dorsal aspect of the nerve-cord there are to be found three remarkable giant tubular fibres. They are found in all Oligochaeta except in Aeolosoma, Phrae-oryctes, and Branchiobdella, and the number given is usually that found in the Earthworm. They occur also in many Polychaeta, where their number varies from one as in Eunice, to six as in Glycera. These fibres do not extend into the oesophageal commissures, and they taper anteriorly and posteriorly. Each fibre is composed of a doubly contoured sheath with clear contents. No connection between the giant fibres and nerve fibres has ever been demonstrated. On the contrary the giant fibres are separated from the nerve-cord by the inner neurilemma, and they are imbedded in a connective tissue sheath containing reticulate cells. They appear to have a purely supporting function, and the apparatus is hence termed 'Neurochord' by Vejdovsky (op. cit. pp. 86-87), who compares it physiologically with the noto-chord of Chordata.
Nervous system. Claparede, op. cit. ante, p. 585. Leydig, Vom Bau des Thierrischen Korpers, i. Tubingen, 1864, pp. 139, 168. Id. Tafeln zur Vergleich. Anatomie, Tubingen, 1864, Taf. i. fig. 5; Taf. iii. fig. 8; Taf. iv. figs. 7 and 8; Taf. v. figs. 1 and 2. Vignal, A. Z. Expt. (2), i. 1883, p. 373. Of Oligochaeta in general. Vejdovsky, op. cit. pp. 79-96; and on Neurochord, p. 87.
Organs of special sense in Oligochaeta. Vejdovsky, op. cit. pp. 96-100.