358 PALAEOSTRACA. 



Of the six pairs of limbs belonging to the cephalo-thorax, the 

 most anterior (the chelicerae) shift during development in front 

 of the oral aperture, while the pair of ganglia belonging to them 

 enter into closer connection with the brain. The five pairs of limbs 

 behind these serve for locomotion and mastication. While, in 

 Limulus, the coxae of all the limbs appear enlarged and have 

 toothed masticatory ridges, in Scorpio, only the pedipalps and the 

 first two pairs of ambulatory limbs have basal blade-like structures. 

 An upper lip (rostrum, camerostome), lying in front of the mouth 

 between the chelicerae, is common to the two forms ; as also is 

 an original paired projection behind the sixth pair of limbs, which, 

 in Limulus, is represented by the chilaria, but, in Scorpio, fuses 

 to form a small pentagonal plate found in front of the operculum. 



Behind the cephalo-thorax, in Scorpiones, comes a pre-abdomen 

 consisting of seven segments, which is followed by a post-abdomen 

 of five segments, with a terminal poisonous spine. If we regard the 

 long caudal spine of Limulus as the representative of the poisonous 

 spine, we shall consider the region usually described as the abdomen 

 to be the equivalent of the pre-abdomen and the post-abdomen of 

 Scorpio. This region, in Limulus, consists of eight fused segments. 

 Taking into account certain fossil forms (Belinurus), however, we 

 may conjecture that the last of these segments, strictly speaking, 

 corresponds to several segments which have not separated. The 

 resemblance between Limulus and Scorpio finds expression in the 

 development of the abdominal limbs. In both forms, rudiments of 

 limbs appear in the embryo on the six anterior abdominal segments. 

 Of these, the most anterior pair changes, in Limulus, into the large 

 plate-like structure known as the operculum, which is also slightly 

 developed in Scorpio, and on the inner side of which lie the genital 

 apertures. The five posterior pairs of limbs, in Limulus, are leaf- 

 like, and carry gills, and thus serve for respiration. In Scorpio, the 

 most anterior pair develops into pectines, while the four other pairs 

 seem to disappear at the time when the lung-sacs develop. 



In the assumption of a near relationship between Limulus and Scorpio, an 

 important part is played by the supposed transformation of the gills of the 

 former into the lungs of the latter. In structure the two organs show remark- 

 able agreement. While, however, careful consideration shows how a transition 

 may well have taken place from the book-like gills of Limulus to the lung-book 

 of Scorpio, there are certain difficulties which Ray Lankester (Nos. 16 and 20), 

 Kincslf.y (No. 14), and MacLeod (No. 21) have sought to set aside by means 

 of various hypotheses. Ray Lankestee himself gave up his original, very 

 artificial theory, and has since derived the lung of Scorpio from the gill of 



