366 SYNOPTIC COLLECTION. 



The spines on the sides of the abdomen differ from 

 those on other parts of the body by being movable. The 

 abdomen is provided with a so called operculum which 

 consists of a pair of appendages soldered together and of 

 five pairs of thin plate like swimmerets (No. 900, C) 

 chiefly used as respiratory organs. On the under side of 

 these plates are hundreds of leaves which are supplied 

 with blood vessels. 



The tubular heart with the larger blood vessels is 

 seen in the preparation (No. 901). The upper portion 

 of the exoskeleton has been cut away and the circulatory 

 organs distended with wax. In No. 902 the arteries 

 which arise from the anterior end of the heart are ex- 

 posed. These unite to form the sternal artery. The 

 latter is seen in the preparation No. 903 with the 

 branches that run to the limbs. 



Allied to the Merostoma on the one hand and to the 

 Arachnida on the other are the Eurypterids, of which 

 Pterygotus bilobus Salt. (No. 904) is a representative. 

 The body is long and distinctly segmented, excepting the 

 forward part which is covered by the small, short cara- 

 pace. 



The first pair of appendages are in front of the mouth, 

 as in the trilobites, and are provided with claws. These 

 are followed by several pairs which were used for catch- 

 ing food and for locomotion, while the largest pair, which 

 extend outward on either side like wings, were powerful 

 swimming organs. 



ARACHNIDA. 



The resemblances between Limulus and the scorpion of 

 the Arachnida are shown in the preparations (Nos. 905, 

 906). The cephalothorax of both animals is probably 

 made of six segments, while there are twelve in the 'abdo- 

 men. 



