NERVOUS ACTIONS IN ANNULOSA. 331 



and the movements exhibit no volitional character. In not even the highest 

 Molluscs can we imagine that memory, emotion, or intelligence exist. 



The pedal ganglia, usually forming only a single pair, but in the Cephalo- 

 poda much subdivided and scattered, are probably excito-motor nervous cen- 

 tres, and purely reflex ; they govern many of the locomotive acts, and repre- 

 sent one of the segments of the spinal cord of the Vertebrata. The surface 

 of the so-called foot may be stimulated through impressions on afferent 

 nerve-fibres, and these may excite the reflex motorial impulse through the 

 pedal ganglion and its efferent fibres. Like the spinal acts in the Vertebrate 

 animals, these reflex locomotive movements in the Mollusca, are, however, 

 subjected to the control of the cephalic ganglionic centres, which, through the 

 longitudinal commissural fibres, exercise a consensual, if not a weak volitional 

 influence over them, as in the spontaneous search after food. The locomotive 

 acts of these creatures are all sluggish, but more or less concatenated. It is 

 remarkable, that the auditory organs of the Mollusca, where they exist, are 

 usually attached, by their nerves, to the pedal ganglia ; but the nerve-fibres 

 probably run on, past these ganglia, to the cephalic sensorium. 



Lastly, the parieto-splanchnic ganglia usually forming a single pair, but 

 sometimes more numerous, supply not only the sides of the body and mantle, 

 but also the respiratory organs (usually branchiae), and the heart, as well as 

 the digestive viscera ; it is by these that the movements of deglutition and 

 respiration are governed, and that the action of the heart is regulated or in- 

 fluenced. But these ganglia are also placed under the control of the cephalic 

 ganglia, especially by commissural bands, which join the cords running on 

 the sides of the esophagus, from the supra- to the suboesophageal ganglionic 

 masses ; hence, these cords, with the parieto-splanchnic ganglia, are said to 

 represent functionally, the tracts of the medulla oblongata. These ganglia 

 probably serve as centres for any sympathetic nerve-fibres which these animals 

 may possess. 



Mottuscoida* 



These are simplified Molluscs, and the single ganglionic mass which consti- 

 tutes their chief nervous centre is probably at once feebly sensory, sensori- 

 motor, and reflex. It represents the three kinds of ganglia in the Mollusca ; 

 it sends nerves to a ciliated sac, believed to be a sensory organ, and some- 

 times has a pigment mass or supposed eye-spot upon it ; it also supplies 

 branches to the tentacles in the Polyzoa, and others to the body and viscera. 

 In position and connections, however, it rather resembles the pedal ganglion 

 of the Mollusca, and, like it, its office is essentially excito-motor or reflex. 

 The locomotive acts of these animals are extremely limited, most of them 

 being fixed, or merely borne about in the sea ; the most active motions which 

 they present are those of the sides of the body, intended to aid in the drawing 

 in and expulsion of water for the purpose of respiration. 



Annulosa. 



The nervous system of these animals might be compared with that of the 

 typical Mollusc, by supposing the pedal ganglia of the latter to be multiplied 

 by the addition of numerous other pedal ganglia behind them, according to 

 the number of segments in the Annulose animal ; or, in other words, the 

 Molluscous nervous system is like that comprehended in the cephalic and sec- 

 ond pair of ganglia of the Annulosa. But the sympathetic system here re- 

 ceives a peculiarly diffused development. 



The functions of the cephalic or supra- and sub-oesophageal ganglia in the An- 

 nulosa are also precisely similar to those of the Mollusca, being sensory, sen- 

 sori-motor, and, in the higher or social insects, perhaps feebly perceptive and 

 volitional. With these are connected the optic nerves, which are, for the 

 most part, very large in the Insecta and Crustacea, in correspondence with 

 the highly developed eyes of these animals ; also the smaller nerves from the 

 antennae or organs of touch, and from the antennules, supposed to be the seat 

 of the sense of smell ; the nerves of the auditory organs, where these exist, 

 and however distant they may be from the head ; and lastly, the nerves of 



