NERVOUS SYSTEM.—In tracing the develop ment of the Nervous System through the Heterogangliate or Molluscous type of Orga nization, we find in the Gasteropodous genera which approach nearest to the Cephalopodous or highest division, that the ganglions which are concentrated about the head, are arranged in three groups : one, which is supracesopha geal, supplies the sentient organs, as the eyes and feelers; a second, which is subcesophageal and anterior, supplies the buccal apparatus ; a third, which is subcesophageal and pos terior, is the centre from which the sensitive, motive, and plastic nerves of the trunk ori ginate. The anterior or buccal ganglions are united together, and to the cerebral ganglions, forming a nervous collar around the cesophagus ; a similar collar is formed by the corresponding intercommunicating chords of the posterior subcesophageal ganglia.
In the Cephalopods the nervous system is disposed on the same general plan, but the nervous substance is accumulated in a greater degree at the different centres of radiation, according to the superior development of the parts that are to be supplied therefrom.
In the Tetrabranchiate Order the principal parts superadded to the structure which we observe in the Gasteropodous Mollusk are those locomotive and prehensile organs which sur round the buccal apparatus ; and the chief modification of the nervous system is therefore seen in the enlargement of the oral ganglia and collar, and their close approximation to the cerebral ganglion. This part is compara tively little advanced, since the organs of sense which it immediately supplies, retain 'the same simple structure as in the inferior class of Mollusks, and are only augmented in bulk. The brain therefore is represented by a thick round tranversely extended chord (1, fig. 231), communicating at its extremi ties with the anterior and posterior cesopha geal collars (3, 4), and with the small optic ganglions (2, 2), which supply the sim ple pedunculated eyes. Four small pairs of nerves (5) also pass from the supracesophageal band to the fleshy mass supporting the man dibles. The cranial cartilage seems in the Nautilus to be principally developed with re ference to the strong muscular masses to which it affords a fixed point of attachment, and is not extended upwards so as to inclose the brain : this part is defended by a strong mem brane which loosely surrounds it; but the ex tremities of the transverse band, the optic gan glions, and the anterior cesophageal collars rest in grooves of the cranial cartilage.
The nerves which arise from the anterior collar are very numerous : the larger branches (6, 6) enter respectively the roots of the ten tacles which are lodged in the digital pro cesses : the ophthalmic tentacles are also sup plied from this source (5s); no lateral con necting filaments are found between these nerves, corresponding to those which associate the corresponding nerves of the Poulp for the simultaneous action of the parts they supply. Below the digital nerves small nerves are given off (12), which enter the external labial processes, and penetrate in a similar manner the roots of the tentacles which are there lodged. The internal labial processes are, however, supplied in a different manner : a larger nerve (7, 7) comes off on each side near the ventral extremity of the ganglion, and after a course of half an inch swells out into a flattened ganglionle (8, 8), from which nu merous filaments (9, 9) extend into the sub stance of the process, and are continued into the tentacles as in the preceding case ; a larger twig (10) inclines inwards and distributes fila ments to the olfactory laminw. The infundi bular nerves (11) come off near the lower part of the anterior collar.
From the ganglions composing the posterior collar (4, 4) arise numerous nerves of a flat tened form, (13, 13,) which pass in a radiated manner to the inner sides of the shell-muscles which they perforate, but there are no columns prolonged backwards from the lateral parts of the brain to form pallial ganglia as in the higher Cephalopods ; the structure and func tions of the cloak to which these ganglia are subservient, not being enjoyed by the shell clad Nautilus. The nerves corresponding to the large visceral nerves of the Dibmnchiates are, however, proportionally developed; for in the organs of plastic life the Nautilus is upon an equality with its naked congeners. These nerves, which combine the functions of the sympathetic and par vagum, consist of a larpt.,.e pair derived from the lower part of the pos terior cesophageal collar, and extending back wards on each side of the vena cava ; and of smaller tvvigs (17) coming off between the origins of the preceding nerves, and forming a plexus upon the parietes of the vein: The larger chords swell into ganglions at the termi nation of the vena cava, (16, 16,) and send off ramifications to the branchi2e, (15, 15,) the contents of the pericardium, and the viscera of digestion and generation.