An Arachnid. 121 



The poison-glands, which are probably modified salivary 

 glands, will be observed in the anterior portion of the cephalo- 

 thoracic cavity, immediately over the chelicerae. Trace 

 their ducts to the terminal openings already noted. 



Make an imaginary drawing of the viscera, thus far studied, 

 as they would appear in side view. 



Carefully remove the digestive tract and note, lying imme- 

 diately below it in the cephalothorax, the " diaphragm," a 

 thin sheet of tissue extending horizontally almost entirely 

 across the cavity. Recall the sternal plate of Limulus. 



The Respiratory System. The external openings of 

 the lung-sacs have already been noted. Each lung-sac con- 

 tains a cavity into which a series of lamelke, like the leaves 

 of a book, project. In these thin-walled lamellae the blood 

 circulates and is purified. Recall the gill-books of the king 

 crab. 



The spiracular opening of the tracheal system of air-tubes 

 has been already noted. The arrangement of the trachea 

 or air-tubes is much as in insects. It is through them that 

 the air is conveyed to many of the tissues. 



The silk glands. In the abdominal cavity, immediately 

 over the spinnerets, the silk glands will be observed. Each 

 gland is composed of a brush of tubular organs of which the 

 more anterior are the larger. 



The Nervous System. - - The brain or supra-cesopha- 

 geal ganglion occupies a position in front of the oesophagus 

 and innervates the ocelli and the chelicerse. 1 The cesopha- 



1 The arrangement of the ocelli is different in different species. In 

 the present type two pairs of median ocelli form a quadrant, while a pair 

 of lateral ocelli will be found on elevations just above the base of each 

 chelicera. 



