330 SPECIAL PHYSIOLOGY. 



conditions, as well as a uniform perfection, quite irrespective of previous trial, 

 experience, or education. These instinctive acts are performed through the 

 agency of a sensori-motor nervous apparatus, duly stimulated, and physio- 

 logically homologous, if not anatomically so, with the sensori-motor ganglionic 

 nervous centres and accessory nerves, found in the Vertebrata. The cephalic 

 ganglia of the non-vertebrate animals constitute the sensorial centres, and rep- 

 resent functionally, therefore, the sensory parts of the vertebrate cerebro- 

 spinal axis ; whilst exci to-motor ganglia, few or many in number, as the case 

 may be, together with the commissural cords connecting them with the cepha- 

 lic ganglia, represent functionally the medulla oblongata and the spinal cord. 

 The anatomical homologies of these parts are not yet accurately determined. 

 It has, however, been argued that not only are the cephalic ganglia of the non- 

 vertebrate animals, th sensorial centres, the seats of conscious sensation, and 

 therefore of the instinctive sensori-motor impulses, but that, to a certain ex- 

 tent, in the highest forms, they must exercise not only the faculty of percep- 

 tion, in the recognition of the relations between the images produced in the 

 sensorium, and the external objects which cause them, but also an imperfect 

 form of volition ; otherwise the 'lives of these beings must be passed, without 

 their experiencing anything more than mere bodily pleasure and pain, and 

 they could not exhibit that feeble manifestation of will, which they display in 

 the selection of materials for building purposes, in their search after food, and 

 for the companionship of their own species. Besides the sensori-motor acts 

 and apparatus, there exist also in these animals, in very great perfection, 

 excito-motor parts of the nervous system, by means of which many of their 

 movements, particularly those of locomotion, complicated as they are in many 

 species, especially in the case of Insects and Myriapods, are essentially gov- ' 

 erned, not only without volition, but often even without conscious sensation, 

 as a guide, as is proved by experiments to be presently mentioned ; such move- 

 ments are performed, in the higher examples, through the intervention of 

 those ganglia and nerves, which correspond functionally with the medulla and 

 spinal cord, although, as just stated, the anatomical homology between them 

 is not so evident. Lastly, certain portions of the nervous centres and nerves 

 are undoubtedly concerned in the regulation of the nutritive or vegetative 

 functions, sometimes, perhaps, acting indirectly through the vascular appa- 

 ratus, but sometimes directly on the nutritive processes themselves. Corre- 

 sponding in function with the Sympathetic system of the Vertebrata, these parts 

 in the lower animals are but slightly developed, and are only seldom, as in 

 Insects, distinguishable from the rest of the nervous system, and even then, 

 cannot be compared anatomically with the vertebrate sympathetic nervous 

 system. 



We may now briefly consider the functions of the different parts of the ner- 

 vous system in the several non-vertebrate subkingdoms. 



Mollusca. 



The cephalic ganglia, consisting, in the typical forms, of the supra-o2sopha- 

 : geal and sub-oesophageal ganglia, sometimes also including a distinct buccal 

 ganglion, and, in the Cephalopods, other ganglionic masses connected with the 

 olfactory and optic nerves, receive all the nerves of special sense, which may 

 be present in any one case, and probably also those of common sensation, and 

 thus constitute the central sensorium, both special and common ; they are 

 analogous to the sensorial ganglia at the base of the vertebrate cerebrum ; 

 they are sometimes named cerebral, and are the only parts which can be so 

 regarded. It is believed, however, that the cerebral hemispheres are not here 

 .represented ; but, at the same time, any perceptive powers or will, which the 

 higher Mollusca exhibit, must be manifested by virtue of these cerebral gan- 

 glia. They receive branches from all the other ganglia, including the pedal 

 and parieto-splanchnic, un arrangement which probably enables them to re- 

 ceive impressions calculated to excite their sensori-motor channels of action, 

 and to regulate the movements of all parts of the body. In the Lamellibran- 

 chiate Molluscs, which are acephalous, there are no cephalic organs of special 

 ;sense, the chief ganglia are quite simple, small, and placed near the mouth, 



