NATURE OF THE INVERTEBRATE BRAIN. 7 q 3 



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 ani- 

 mals, 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 dis- 

 tinct parts of the common system. 



In the larger parasitic nematoids the nervous system is more con- 

 centrated. The oesophageal 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 connect- 

 ing 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 

 oesophageal ring of nematoids is probably dependent upon the com- 

 parative simj^licity and limited number of impressions capable of 

 being received through these cephalic papilla?. 



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 Tnvertebrata, containing its higher 

 types of life. These sub-kingdoms are Vermes, Arthropoda, and 



MOLLUSCA. 



Among representatives of the sub-kingdom Vermes, the ner- 

 vous 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 JVemertidce, a class of marine worms, possess a nervous sys- 

 tem of very simple type. They have soft and highly-contractile 

 bodies, covered with cilia, but are otherwise wholly devoid of exter- 

 nal appendages or traces of segmentation. On the anterior extrem- 

 ity 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 prin- 

 cipal 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 



