NATURE OF THE INVERTEBRATE BRAIN. 709 



wood-lice (such as talitrus and oniscus), in which the body is elon- 

 gated and composed of many almost similar segments, the nervous 

 system is not very different from that of the more highly-organized 

 worms. 



In slightly higher forms of Crustacea, however, the two divisions 

 of the originally double ventral cord approximate and become fused to- 

 gether, while, at the same time, the equality of its ganglia diminishes. 

 Thus, in such forms as the lobster and the crayfish, the ganglia of the 

 thorax which supply nerves to the limbs are distinctly larger than 

 those of the abdominal segments, though these are also of good size, 

 since the tail-segments are actively called into play during locomotion. 



In the prawn a further development and concentration of the 

 nervous system is seen. The thoracic ganglia are fused into a single 

 elliptical mass, though those of the abdominal segments still remain 

 separate. 



But in the ordinary edible crab and its allies (Fig. 4), a still more 

 remarkable concentration of the nervous system is met with. All the 

 thoracic and all the abdominal ganglia are here fused into one large 

 perforated mass of nervous matter (c), situated near the middle of the 

 ventral region of the body. 1 From this large compound ganglionic 

 mass nerves are given off to the limbs, to the abortive tail, and to 

 other parts. 



The brain of the crab (a) is represented by a rather small bilobed 

 ganglion. It receives nerves from the pedunculated compound eyes, 

 from the two pairs of antennse, and from the palpi-bearing mandibles. 

 The posterior antennse (or antennules, as they are sometimes termed) 

 contain in their basal joint a body which is supposed to represent an 

 olfactory organ, though others have regarded it (on very insufficient 

 grounds) as an organ of hearing. The rather small bilobed brain is, 

 indeed, regarded by many naturalists as essentially composed of three 

 pairs of ganglia, completely fused into one another, but in relation 

 with the three pairs of sensory organs the eyes, the tactile antenna?, 

 and the supposed olfactory antennules. It is connected, by means of 

 a long cord (b, b), on each side of the oesophagus, with the anterior 

 extremity of the great ventral ganglion. These cords are long be- 

 cause of the absence of any separate sub-cesophageal ganglia, and 

 because of the comparative distance of the great ventral nervous 

 mass into the composition of which these ganglia enter. The great 

 length of the oesophageal cords is one of the most notable character- 

 istics of the nervous system of the higher Crustacea. 



The " stomato-gastric " system of Crustacea is closely similar to 

 that which exists in centipedes. One part of it is given off from the 

 oesophageal cord on each side, while another median branch proceeds 

 from the posterior part of the united cephalic ganglia, as in lulus 



(Fig. 3,/). 



1 An artery passes through the perforation in this ganglion. 



