790 CLASS V. ARACHNID A. 



interior of the cephalothorax dorsal to the ganglionic chain 

 and ventral to the alimentary canal. It is a bilaterally sym- 

 metrical cartilaginous plate, unconnected with the exoskeleton, 

 and serves as the point of origin of muscles. A similar structure 

 is found in Scorpions and Spiders (p. 778), and also among the 

 Crustacea (Apus, etc.). 



The Central Nervous System of the adult is highly concentrated. 

 The brain, which forms a compact mass obscurely divided into 

 lateral halves, together with the succeeding ganglia as far as 

 that of the first abdominal (opercular) segment, form an oval 

 ring round the oesophagus, from which a double cord extends 

 into the abdomen, with ganglia and nerves corresponding to 

 the posterior abdominal appendages. 



Its composition has been elucidated by Kishinouye, Patten and others. 

 It appears that the brain is composed of two pairs of ganglia, an anterior 

 and posterior, united by commissures, of the large ganglia of the lateral 

 eyes, and of a median ganglion supplying the median eyes. Its under 

 surface becomes markedly convoluted in the adult. Besides the eyes it 

 supplies the median olfactory area. The ganglia of the chelicerae (the 

 first postoral) do not fuse with the brain as they do in the Scorpion. 



In the embryo a pair of ganglia on the double ventral cord corresponds 

 to each pair of thoracic (including the chilaria) and abdominal appendages, 

 and a terminal pair, behind'those of the 6th abdominal, supply the hinder 

 part of the body. 



Sense Organs. Sensory cells are abundant on the masticatory 

 processes of the thoracic limbs and on their chelae, and over a 

 slightly elevated median area, the olfactory organ, lying in 

 front of the camerostome (Patten). 



The median eyes lie close together on either side of a median 

 spine. Like the median eyes of the Arachnida generally they 

 consist of two well-marked layers of cells an upper, forming 

 the " vitreous body " continuous with, and little modified from, 

 the adjacent cells of the hypodermis, and a deeper layer forming 

 the retina. Both layers are concave, in adaptation to a lenti- 

 cular thickening of the cuticle. The cells of the retina are large 

 and oval, and prolonged into nerve fibres. They are arranged 

 in groups (ommatidia) each consisting of some 7 cells enclosing 

 a rhabdome between them, and are surrounded by dark pigment. 

 It is possible that the median eyes of Limulus are partially 

 degenerated structures. 



The lateral eyes (Fig. 513) consist of a single layer of hypodermic 

 cells in contact with the transparent cuticle, which is produced 



