222 BRITISH FOSSIL CRUSTACEA. 



1. "Bearing in mind that all Crustacea have two pairs of antennae receiving nerves 

 from the supracesophageal ganglion, thus distinguishing them from the Spiders, 

 Myriopoda, and Insects, it is (says Dr. Dohrn) impossible to ignore that the presence of 

 only one pair of extremities (the antennulcs) in Limulus so supplied with nerves separates 

 it from the Crustacea. And, further, to which of the two pairs of antennas, and 

 therefore to which of the two anterior pairs of extremities, in Nauplius does this first 

 pair in Limulus correspond ?" " This question," he adds, " we must leave entirely un- 

 answered, as we have at least hitherto been unable to detect any trace of a rudimentary 

 second pair either before or behind this first pair.'' 



Dr. Dohrn therefore concludes that Limulus cannot be retained with the Crus- 

 tacea, but must be classed with Spiders and Insects which have only one pair of 

 antennas. 



2. With regard to the under lip (metastoma) Dr. Dohrn says that in all the Crustacea 

 it is a fold of the germinal membrane behind the mouth. It is found on a level with the 

 mandibles, or sometimes even in front of them. In Limulus the under lip is behind the 

 sixth pair of appendages, and is divided into two parts. 



This led Dr. Dohrn to suspect that it might prove to be really a seventh pair of 

 appendages ; but this point he has been unable to clear up ; for, owing to the fact of his 

 specimens having been preserved in spirit, he could not satisfactorily make out whether 

 or not it has a separate nerve-ganglion. He, however, concludes that " these two 

 peculiarities make it improbable that Limulus belongs to the Crustacea." 



Dr. Dohrn remarks that " what Savigny has hinted at, and what Strauss-Diirckheim 

 has one-sidedly expressed, reappears now under the light of the theory of evolution. 

 The connection of the Arachnida with the Crustacea is probably through Limulus and 

 the Eurypterida," as indicated by myself in 1866 (see ' Quart. Journ. Geol. Soc./ vol. 

 xxiii, p. 35). 



In comparing the Limuli and Eurypteri with the Arachnida one naturally turns to 

 the Scorpionidae as the group most suitable for that purpose. 



Both in Pterygotus and Scorpio we find the antennary system modified in the same 

 manner, not only in form but in function also ; the larval ocelli are seen in both ; the 

 locomotory appendages are all cephalic in both ; and the elongated body, with its peculiar 

 trapezoidal head-shield, its enlarged thoracic segments (the first in both) bearing the 

 reproductive organs on the underside, and the more slender abdominal series followed by 

 a broad apiculate tail-joint, naturally seems to invite a comparison of the two groups. 

 But the respiration in the former is performed aerially by pulmonary sacs, arranged in 

 pairs along the ventral borders of the thoracic segments ; whereas in Limulus and 

 Pterygotus the respiration is performed by branchiae borne on the same series of segments 

 (the thoracic) in Limulus, and by one or two only of the same in Pterygotus. But, on 

 the other hand, the great cordiform under lip of Pterygotus and the two pieces homo- 

 logous therewith in Limulus, the broad thoracic plate or operculum in both Limulus 



