II. The Nervous System
(Text-figs. 6 and 7.)
On the nervous system in the Struthiolariidae we have no previous information at all with the exception of a short description by Bouvier for Pelicaria vermis. Bouvier's figure, reproduced at p. 339 of Simroth's volume in Bronn's Tierreichs, portrays the nerve ring very accurately, with the exception of the pedal ganglia which were evidently omitted from his dissection (see Simroth, (1907)).
The perioesophageal nerve ring consists of the three typical pairs of cerebral, pleural and pedal ganglia, as well as a seventh ganglion, the subintestinal, which has become secondarily incorporated in the ring. This ganglion is morphologically a part of the visceral nervous system which consists essentially of long paired trunks, connecting with the nerve ring the two sets of distal ganglia, the paired parietal ganglia and the visceral or abdominal ganglia. In the chiastoneury which followed upon torsion, the morphologically right parietal ganglion has crossed over the oesophagus to the left to form the supraintestinal ganglion, and the originally left parietal member has passed below the gut as the topographically right subintestinal ganglion.
The Perioesophageal Nerve Ring. The cerebral ganglia are approximately kidney-shaped, half a millimetre across and set close together in the midline by their convex edge. They lie on the dorsal surface of the oesophagus, and on either side a cerebropedal connective passes obliquely forward to meet the corresponding pedal ganglion at its posterior end. Each cerebral ganglion (Text-fig. 7, Ce G)
Text-Fig. 7.—Fig. 6—Lateral view of the perioesophageal nerve ring (left half removed) and the nerve supply of the proboscis and buccal mass. Fig. 7—The nerve ring in ventral view. B Mass, Buccal Mass. Bucc G, Buccal Ganglion. Ce G, Cerebral Ganglion. Cut Comm, Cu [ unclear: ] cerebral and pedal commissures. Ext Pro, External wall of the proboscis. J, Jaw C Bucc C, Left cerebro-buccal connective. L Pall N, Left pallial nerve. L Ped G, Left Pedal ganglion. L Pl G, Left pleural ganglion. L Pl 1 and L Pl 2, Left pleural nerves. L Pl-Pa C, Left pleuro-parietal connective. Oes, Oesophagus. Oes Art, Oesophageal artery. Or D, Invaginated oral disc. Or D N 1,2, Nerves to the oral disc. Oto. Otocyst. Ped A, Pedal artery Ped. N Lat. Small lateral nerves from the pedal ganglion. Pl N. B. W, Nerves from the pleural ganglion to the body wall. Pro A, Proboscideal artery. Pro N 1 2, Nerves to the wall of the proboscis. R Pall N, Right pallial nerve. R Pa-Visc C, Right parieto-visceral connective. R Pd G, Right pedal ganglion. R Pl Ped C, Right pleuropedal connective R Pl G, Right pleural ganglion Sal. G, Salivary gland Sb G. Subintestinal ganglion. Tent A. Tentacular artery. Zyg. Zygoneury.
is in close contact behind with the antero-dorsal surface of the pleural ganglion;, the cerebropleural connective in Struthiolaria, as in most Monotocardia, is so short as hardly to be separately recognisable. This short cerebropleural connective, the cerebropedal connective and the longer, horizontal pleuropedal connective (Text-fig. 7, Fig. 7, Pl Ped C) enclose on either side of the nerve ring a small 3-sided fenestra referred to by Bouvier as the “triangle laterale”.
Each cerebral ganglion gives rise to a set of buccal and proboscideal nerves, together with the following four pairs of cephalic sensory nerves:—
Cephalic tentacular nerves, arising antero-laterally from the free face of the ganglion and running outwards to the tentacles (Text-fig. 6, C Tent N).
A pair of much more slender Optic nerves (Op N), running to the optic peduncles.
Cephalic tegumentary nerves, a pair of trunks (C Teg N) of equal thickness to the tentacular nerves, arising further back and supplying the integument of the head.
Otocystic Nerves (Oto N), extremely slender trunks arising, one each side, from the ventral surfaces of the cerebral ganglia.
The pedal ganglia (Ped G) are ovoid bodies, 1.5 mm in length, united by an extremely short pedal commissure. They form the most anterior part of the nerve ring, and give rise in front to two pairs of stout pedal nerves, of which those nearer the mid-line pass beneath the pedal arteries and proceed forward without branching for a good distance along the wall of the pedal sinus. The more lateral pair (Ped N Lat) follow a deeper course and commence to branch and rebranch on reaching the side walls of the foot. In addition, four smaller pairs of pedal nerves arise further back from the anterolateral edge of each ganglion. They pass directly to the body wall at the base of the foot.
The pleural ganglia (Text-Fig. 7, Fig. 6, R Pl G, Fig. 7, L Pl G) are roughly spherical in shape, and are much smaller than the pedals They are widely separated from each other; a pleural commissure is never present in Gastropoda and the ventral moiety of the nerve ring is thus, in its primitive condition, open posteriorly. In Struthiolaria it becomes closed by the short right pleuroparietal connective, by which the subintestinal ganglion (Sb G) is drawn into the nerve ring, and by the zygoneury on the right side (Zyg) which secondarily connects the subintestinal and the right pleural ganglia. The pleural ganglia give rise to pleuroparietal connectives and as well to several important pleural nerves supplying the body wall in the trunk region. From the left pleural ganglion springs also the large left pallial nerve (L Pall N) passing obliquely backwards to supply the left side of the mantle, which—with the exception of the ctenidium and osphradium—is entirely innervated from this source. A much smaller nerve (L Pl I) supplies the musculature of the body wall on the left side, while a third pleural nerve (L Pl 2) arises mesially to the pallial nerve and runs back for some distance along the floor of the haemocoele before entering the musculature.
From its dorsal surface, the right pleural ganglion gives off the prominent left pleuroparietal connective (Text-fig. 6, L Pl-Pa C), which runs to the supraintestinal ganglion, and forms the anterior section of the visceral loop on the left side. The right pleural ganglion has only one important nerve to the body wall, which corresponds in position with the left pallial nerve. This ganglion is linked with the subintestinal by the (right) zygoneury, a short curved con-
nective passing directly beneath the oesophagus. It should be noted that in Struthiolaria, as well as in Buccinum (Dakin, 1912) and other prosobranchs in which the subintestinal ganglion has moved forward, the right side of the mantle is innervated not from the right pleural ganglion, but by a right pallial nerve arising from the subintestinal ganglion and corresponding to the ctenidial nerve of the left side. In addition to the above nerves, the pleural ganglia also supply part of the wall of the trunk by a pair of nerves arising not from the ganglia themselves but from the pleuropedal connectives about half-way along. These nerves branch within the haemocoele, sending one branch directly to the body wall, the other backwards for some distance along the roof of the haemocoele.
The Nerves of the Buccal Mass and Proboscis (Text-fig. 7, Fig. 6). Two systems of nerves supply the organs lying within the proboscis, the first derived from the buccal ganglia; the second, to the oral disc and the wall of the proboscis, proceeding directly from the cerebral ganglia. Along the anterior edge of each cerebral ganglion arises a series of six nerve trunks, which pass forward through the haemocoele of the proboscis, lying close together on either side of the oesophagus. Beginning near the midline, the first and second nerves in this series are respectively the dorsal and ventral oral tegumentary nerves, (Text-fig. 7, Fig. 6, Or D N 1, 2) which run close together as far as the buccal bulb, diverging there to break up into long, fine branches over the integument of the oral disc. The third nerve is distributed anteriorly on the side wall of the proboscis. The fourth is the stout cerebrobuccal connective (C Bucc C), which runs first alongside and finally below the oesophagus, taking a rather sinuous course forward to the buccal ganglion. At the side of this connective arises two tegumentary nerves of the proboscis (Pro N 1, 2), which break up at various levels along the proboscis to supply its muscular wall.
The buccal ganglia (Bucc G) are small, spherical bodies located at the sides of the radular caecum, and united immediately behind the radula by a stout, transverse buccal commissure. From the anterior surface of each ganglion, four nerves run to the wall of the buccal bulb. The two nearest the mid-line are the ventral pharyngeal nerves; the third, the lateral pharyngeal nerve, branches over the sides of the buccal mass, while the dorsal pharyngeal nerve runs on to the roof and is usually concealed by the overlying salivary gland. Near the junction of the buccal commissure an odontophoral nerve runs inwards, at the side of the radular caecum, near the base of the odontophore. From the posterior face of each buccal ganglion a second series of nerves arises; these pass outward to the wall of the proboscis, where they break up into fine branches and mingle with the ultimate branches from the cerebral nerve supply. On the lower surface of the anterior part of the oesophagus, there are two pairs of extremely slender ventral oesophageal nerves, following a direct course backwards along the gut. The outer pair seem to spring from the cerebrobuccal connectives just before the junction of these with the buccal ganglia; but their origin is no doubt in the ganglia themselves. The inner pair are from the cerebral ganglia and appear to correspond with nerves from the original stomatogastric ganglia, which in Struthiolaria and other Monotocardia are quite suppressed as separate centres.
The Left Pallial Nervous System. The pallial innervation is asymmetrical and on the left side is derived from two sources, the left pallial nerve and the ctenidial nerve. The pallial nerve breaks into two chief branches supplying the dorsal and the ventral portions of the pallial skirt respectively. The dorsal branch,
running with the pallial vein and artery, gives off a rather large branch to the siphonal lappet, and continues around the mantle, deep to the osphradium, giving off frequent branches to the pallial margin. At the mid-dorsal line the left pallial nerve breaks into a series of fine branches which ultimately anastomose with similar branches of the right pallial trunk. There is a similar anastomosis in the ventral mid-line of the pallial skirt.
The ctenidial nerve (Text-fig. 6, Ct N) is a slender trunk running from the supraintestinal ganglion, and running out parallel to and behind the left pallial nerve. It crosses below the osphradium at the point where that organ diverges at an angle from the gill. Here it gives off a small osphradial nerve which joins the axis of the overlying osphradial ganglion, a long, slender cord running the whole length of the osphradium.
A short distance beyond the wall of the trunk, the left pallial and the ctenidial nerves are placed in communication with each other by a short cross connective, the left dialyneury (Text-fig. 6, Fig. 5, L Dial). Such a connection forms, in the prosobranch nervous system, a peripheral link between nerves derived from the left pleural and the topographically left parietal ganglion. As a result of chiastoneury, the left pallial and the ctenidial nerves are centrally connected with opposite pleural members. The pleural ganglia themselves have no direct connection with each other—hence the functional importance of a peripheral connection by the dialyneury on the left, and by the analogous zygoneury on the right.
The ctenidial nerve gives rise posteriorly to numerous slender branches, which run outwards obliquely, cross beneath the osphradium, and finally—just before reaching the axis of the gill, break up into very numerous, parallel filamentar nerves (Text-Fig. 6, Fig. 5, Fil N) running along the individual gill filaments of the gill. As they run towards the gill these filamentar nerves branch and anastomose with each other in the mantle wall.
The Visceral Loop. The forward migration of the sub-intestinal ganglion has obscured the figure-of-eight disposition of the visceral loop in Struthiolaria. The right (or inferior) pleuroparietal connective is virtually eliminated and the point of chiastoneury thus shifts forward to lie within the nerve ring itself. The right half of the visceral loop now consists essentially of a long right parietovisceral connective and the left half incorporates both the left pleuroparietal connective (L Pl-Pa C) and the left parietovisceral connective (Text-fig. 6, Fig. 5, L Pa-Visc C).
The left parietal or supraintestinal ganglion (Text-fig. 6, Fig. 5, S Int G) is a large, triangular body, located just inside the wall of the left side of the trunk at the point of departure of the pallial artery. In addition to the ctenidial nerve, it has three much smaller branches running backwards in the body wall. The left parietovisceral connective is much more slender than the pleuroparietal and passes backwards along the left compartment of the cephalopedal venous sinus. It presently buries itself in the muscular floor of the haemocoele, through which it continues back to the left visceral ganglion. The right parietovisceral connective (Text-Fig. 7, Fig. 7, R Pa-Visc C) is an equally slender trunk, lying in the right compartment of the cephalopedal sinus and, further back, in the superficial muscles of the floor of the haemocoele.
There are two visceral or “abdominal” ganglia, lying at the extreme posterior end of the trunk on the broad upper surface of the columellar muscle, within the subrenal blood sinus. The right visceral ganglion (Text-fig. 6, Fig. 5, R Visc G) is considerably the larger, lying just mesially to the albumen gland in the female and alongside the renal vas deferens in the male. This ganglion has a very characteristic shape; it consists of a pale yellowish body, to which is attached an elongate whitish lobe which replaces the transverse visceral commissure for about one-third of the distance between the two ganglia. The left visceral ganglion is much smaller, lying on the extreme left side, between the oesophagus and the cephalic aorta. The nerves from these ganglia supply directly the whole of the visceral mass, including the stomach, the digestive gland, the renal organ, the heart and the pericardium as well as the posterior part of the intestine and the genital ducts. The long visceral nerve (Text-fig. 6, Fig. 5, Visc N) arises posteriorly from the right visceral ganglion and runs back over the concave surface of the digestive gland to the tip of the spire. To the right, the same ganglion gives off a stout rectal nerve (Text-fig. 6, Fig. 5, Rect N), as well as two much finer nerves to the genital duct. Towards the mid-line arises a renal nerve, breaking up over the floor of the renal sac. The elongate lobe of this ganglion gives off two slender nerves towards the middle line, supplying the integument nearby. From the left visceral ganglion a small nerve branches over the ventral surface of the cephalic aorta, while two larger nerves pass backwards, the first following the aorta back to the ventricle and the second passing back over the oesophagus to its final distribution upon the wall of the stomach.