432 ZOOLOGY. 



major group of which both are equally members. Professor Lankester 

 is therefore fully justified in the postulate that "there is not only a gen- 

 eral resemblance of the Eurypterine body to that of the Scorpion, but 

 that in many of the most important points the Eurypterine body and 

 appendages agree precisely with those of the Scorpion, and not in a 

 merely general way. The Eurypterina, in fact, confirm the validity of 

 the comparisons between Limulus and Scorpio. 



Finally, Professor Lankester has summed up the points of agreement 

 of the Horseshoe Crab and Eurypterids with the Arachnids, and their 

 differences from the Crustaceans, in the following terms: 



1. Limulus and the Eurypterines (the one supplementing the other) 

 agree precisely with the Scorpion in the existence of eighteen segments 

 expressed in the structure of their bodies, and in the distribution of 

 these segments into three groups of six each, viz: a leg-bearing cephalo- 

 thoracic region ; an anterior abdominal region, in which each SQ^meut 

 carries lamellate appendages ; and a jiosterior abdominal region devoid 

 of appendages, ending with the anus and a postanal spine. No Crus- 

 tacean presents this number and groupiug of its constituent somites. 



2. Liuiulus and the Eury])terines agree with the Scorpion precisely in 

 the position of the genital aperture beneath an opercular plate formed 

 by the coalescence of the seventh pair (in Euryj)terines the actual sixth 

 pair of appendages). Xo Crustacean has the generative orifice so far 

 forward, and in none is there a genital operculum of the kind having 

 such relations of position to the general apertures. 



3. They agree with the Scorpion in the character and position of the 

 mouth and upper lii^. 



4. They agree with the Scorpion in possessing a metathoracic steruite, 

 in the possession of a fibro-cartilagiuous entosternite, and in the precise 

 form and relations of that organ. No Crustacean possesses an ento- 

 sternite or any structure resembling it. 



5. They agree with the Scorpion in the disposition of central (single) 

 and lateral (grouped) eyes on the cephalothorax. No Crustacean has 

 an identical arrangement of single and groui^ed eyes. 



6. Limulus agrees with the Scorpion in the form of tlie alimentary 

 canal and its lateral outgrowths (liver), which are more than one pair. 

 In Crustacea it is very exceptional to find more than one pair of 

 such diverticula, though a single pair may carry numerous secondary 

 branches. 



7. It agrees with the Scorpion in i)ossessing a supra- or circum- 

 medullary (spinal) artery, which arises from the dorsal aorta by two 

 arches embracing the oesophagus. No Crustacean has such a supra- 

 spinal arter}- so originating. 



8. It agrees with the Scorpion in the form of the generative glands. 

 No Crustacean has its generative glands in the form of an anastomos- 

 ing network. 



