The absence of distinct ganglia in the neighborhood of the mouth in star-fishes is doubtless due, in the main, to the form of these animals, and their low type of organization. Each arm or ray presents its own nervous system, and the ring or band round the mouth seems to be little more than a commissure connecting these otherwise distinct parts of the common system.
In the larger parasitic nematoids the nervous system is more concentrated. The œsophageal ring and immediately adjacent parts constitute almost all that is known of the nervous system in these organisms, and it contains, or is in relation with, a larger number of ganglion-cells than the similar part in star-fishes. Thus, in addition to the cells intermixed with the fibres of the ring itself, there are five or six groups adjacent to and in connection with it, which receive fibres from certain large papillae surrounding the mouth and having a rudimentary tactile function. These papillae are, in all probability, the nematoids' principal sensory organs. By means of the connecting nerve-fibres and ganglion-cells they are brought into relation with the nervous ring, and from this other outgoing fibres are, doubtless, given off to the four great longitudinal muscular bands by which the movements of the animal are effected. The distribution of these latter or motor nerve-fibres, however, has not been distinctly traced. The absence of ganglionic swellings on, or in connection with, the œsophageal ring of nematoids is probably dependent upon the comparative simplicity and limited number of impressions capable of being received through these cephalic papillæ.
We turn now to the nervous system, and to those parts of it, more especially, which answer to the brain of higher animals as it occurs in the three sub-kingdoms of the Invertebrata, containing its higher types of life. These sub-kingdoms are Vermes, Arthropoda, and Mollusca.
Among representatives of the sub-kingdom Vermes, the nervous system varies a good deal in minor details, in accordance with the degree of organization, and with the diversity of the sensory and locomotor endowments of the several organisms. The broad features of the nervous system, however, are very similar in all.
The Nemertidæ, a class of marine worms, possess a nervous system of very simple type. They have soft and highly-contractile bodies, covered with cilia, but are otherwise wholly devoid of external appendages or traces of segmentation. On the anterior extremity of the body, a little posterior to the mouth, two, four, or more specks of pigment are met with, which are conjectured to serve the purpose of rudimentary ocelli, and while the animal is moving from place to place this anterior part of its body doubtless acts as its principal tactile surface. Nerve-fibres proceed from these regions, and converge so as to form three or four nerve-trunks on each side, which enter a comparatively large ganglionic mass lying on the lateral aspect
