CHAPTER VIII EXPERIMENTS ON MOLLUSKS The literature on the functions of the central nervous system of Mollusks is extremely meagre. It is nevertheless valuable, as it furnishes us with further proofs of the theory that the simple and rhythmical spontaneity, as well as reflex processes, do not depend upon the brain or specific peculiarities of the ganglia. A Gastropod whose brain (g, Fig. 33) P g FIG. 33. SCHEMATIC REPRESENTATION OF SNAIL (PALUDINA VIVIPARA). g, brain; P, pedal ganglion. (Modified after Leydig.) swimming organ. organ. Neither one- nor two-sided destruction of the supraœsophageal ganglion has the slightest influence upon the character and the quantity of the spontaneous progressive movements. Destruction of the pedal ganglion, on the other hand, puts an end to all locomotion. Steiner concludes, therefore, that "the pedal ganglion alone Cg. Bg Spg 你 FIG. 34. BRAIN OF SEPIA. has control of the entire locomotion of the animal." This anthropomorphic conclusion goes too far. The only conclusion we are justified in drawing from this observation is, that the protoplasmic connecting fibres between the skin and the footmuscle of the animal pass through the ganglion. Steiner further attempted to see if he could produce circus-motions by means of a one-sided division of the œsophageal commissure in other Mollusks, Pleurobranchia and Aplysia. He succeeded no better than in Pterotrachea. One-sided destruction of the pedal ganglion in Cymbulia, however, caused paralysis of one-half of the locomotor organ. The animal naturally moved in a circle, for only one wing served as an oar. Cg, cerebral ganglion; Spg, supraœsophageal ganglion; Bg, buccal ganglion; Tg, ganglia of the tentacles. (After Claus.) The Cephalopods have an extremely complicated brain (Fig. 34). It consists of a dorsal and a ventral mass, each of which is composed of several ganglia. The dorsal and ventral ganglia are connected by commissures. In addition, they possess a series of peripheral ganglia, the tentacle-ganglia (Tg, Fig. 34), for instance. It is of significance for the segmental theory that the tentacle-ganglia suffice to produce tentacle-reflexes, as v. Uexküll has shown in Eledone (2). It has been inferred from experiments on Vertebrates that peripheral ganglia cannot transmit reflexes. Now, as regards experiments on the brain of Cephalopods, Steiner reports as follows concerning Octopus vulgaris: "If the dorsal ganglion on one side be removed, or both commissures of one side be severed, not the slightest change is visible in the lifeprocesses of the animal, for it moves spontaneously as before, attacks its prey (Carcinus mænas) cleverly, and devours it. But the picture changes if the dorsal ganglion be entirely removed. To be sure the two forms of locomotion are preserved, for the animal creeps with the aid of its arms, or shoots like an arrow through the waves, when water is forced out of the mantle-cavity rhythmically. These movements are, however, no longer spontaneous, for they occur only when the animal is stimulated, neither does it take its food spontaneously. The normal octopus, which is endowed with marked intelligence [?], is wont to observe its surroundings most attentively, but now it sits indifferent to its surroundings, as though idiotic, and only its regular breathing gives evidence that it still lives. Vision is unimpaired, for it draws back when a stick is brought toward its eye." V. Uexküll's When anyone ap article on Eledone is more exhaustive than Steiner's. One of his observations, describing the extraordinarily excited condition of an animal whose cerebral ganglion had been removed, is worthy of mention. "All the reflexes seemed increased. proached the basin the Eledone that had undergone this operation swam off, while the normal animals remained quiet. There was an incessant play of colors. During the second night, in spite of the protecting net, it escaped and died on the floor of the laboratory." V. Uexküll concludes from this that there are inhibitory centres in the cerebral ganglion. We have seen that Bethe arrived at a similar conclusion in regard to the supraœsophageal ganglion of the Arthropods. We have discussed this possibility in connection with Maxwell's experiments on Nereis. sures. The arm-nerves originate in the pedal ganglion. But the latter is connected with the supracesophageal ganglion directly by means of the anterior commissures and indirectly by means of the posterior commisNow it is of interest to know that the influence which the anterior part of the supracesophageal ganglion exerts on the arm-movements when stimulated is exactly the opposite of that exerted by the posterior part; if the entire supracesophageal mass between both pairs of commissures be separated by a frontal incision and both stumps be stimulated down deep, where the central ganglia are located, according to v. Uexküll, we obtain the following results: Stimulation of the anterior stump causes the cup-like suckers to take hold strongly; stimulation of the posterior stump causes the suckers to let go and the arms to be withdrawn. Thus the antagonistic activities of the arms depend upon two different parts of the central nervous system. "An animal whose supraœsophageal mass has been divided in the vicinity of the first central ganglion behaves like an animal that is only able to take hold of objects. It grasps every object firmly and liberates itself again only with difficulty. It usually retains its hold and sits with extended arms, or crawls forward with the greatest difficulty. Such an animal placed on the back of a torpedo seizes it firmly with the arms, and no shocks of the electric organs are of avail to rid the fish of its burdensome rider. On the other hand, it is evident that the Eledone only participates in the ride involuntarily from the fact that it becomes dark brown and throws ink. If a normal Octopus by mistake grasps after a torpedo, it never remains in so dangerous a neighborhood more than a few seconds [I have observed this in Octopus, never in Eledone]." It seems to me that the conclusion to be drawn from these facts is, that the anterior and posterior parts of the supraœsophageal ganglion are connected with antagonistic muscle-groups. This relation is of interest in view of galvanotropic experiments, which we shall discuss later on. It is furthermore probable from v. Uexküll's experiments that the act of eating depends upon the integrity of the first central ganglion, while the second and third central ganglia are necessary for all |