1881.] 



On Stigmata in the King Crab. 



393 



series of projecting books of (about) 150 double lamella? seen in Limulus, 

 with the similar series of sunken books of (about) 130 double lamella?, 

 seen in Scorpio. 



A careful study of the structure . of these respiratory organs in the 

 two animals, together with the explanation afforded by their external 

 position in Limulus, of the " comb-like organs " of the Scorpion, and 

 the observation of the fact that the genital operculum of Limulus is 

 represented in miniature by the genital operculum of the Scorpion, and 

 in an exactly corresponding position relatively to legs and gill-books, 

 are the factors which, above all others, are likely to contribute to 

 forming in an observer the impression that there is a very real and 

 intimate agreement between Limulus and Scorpio. This impression 

 is confirmed in a remarkable way by the agreement of details of struc- 

 ture in the two animals — such, for instance, as the form of the legs, 

 the relation of the coxa? to the mouth, and to the sternal sclerites, the 

 minute structure of the gill-plates, and their exact arrangement and im- 

 brication to form those peculiar book-like organs which have no parallel 

 in the animal kingdom. The impression is further confirmed by one of 

 the main facts on which Straus Durckheim relied for his conclusions 

 on the subject, viz., the existence in Limulus and in the Arachnida of 

 a very peculiar loose skeletal piece — the " entosternite " : further by 

 the close similarity of the digestive canal and its caeca in Limulus and 

 Scorpio ; by the close agreement of the very highly developed vascu- 

 lar system in the two animals ; by the close agreement in its general 

 disposition of the central nervous system in the two animals. In 

 addition to all this, it has recently been shown that the structure of 

 the compound eye of Limulus, though necessarily not that of any 

 living Arachnid, is also unlike that of any living Crustacean (Gre- 

 nacher). Lastly may be mentioned, as confirmatory of the existence 

 of a close relationship between Limulus and the pulmonate Arachnida, 

 on the one hand the living Arachnida, which, like Limulus, exhibit a 

 suppression of the segmentation of the hinder part of the abdomen as 

 compared with Scorpio, and on the other hand those fossil forms, 

 the Eurypterina, which correspond with Scorpio exactly in the 

 number of segments developed in the abdominal region, and very 

 closely in their general form, whilst unmistakably related in the 

 closest way to Lim,ulus (as generally admitted) by the structure of 

 their great digging leg (the hindmost pair) and of their genital oper- 

 culum. The cephalo-thoracic limbs of the Eurypterina, though but 

 five pairs in number in place of the six pairs present in Limulus and 

 Scorpio, yet, as long since pointed out by Professor Huxley, present 

 no difficulty on this account in the way of a close approximation of 

 the forms in question ; for it is obvious from the condition of the 

 anterior pair in such a Eurypterine as Slimonia, that there is a 

 characteristic tendency to the reduction and hence suppression of the 



