Dr. H. Woodward — The Position of the Merostomata. 299 



being gill-bearing (branchiae). The hind-body has no limbs, but to 

 it is articulated a long bayonet-shaped tail-spine (or ' telson '). 



In Eurypterus the head-shield is smaller than in Limulus and the 

 body much more elongated and the segments free and movable. The 

 head-shield bears upon its upper surface both ocelli and compound, 

 eyes. On the under side is the mouth with six pairs of chelate or 

 simple maxillipeds, the first two pairs serving as tactile organs 

 (chelicerae and antennae), then three simple spinose limbs, and lastly 

 a pair of larger swimming appendages vrith well-developed basal jaws 

 and a central oval plate or metastoma. The mid- and hind-body are 

 not coalesced (as in Limulus), but the segments are distinct and 

 flexible and adapted to a swimming existence. On the under side, 

 immediately behind the head (as in Limulus), is a broadly expanded 

 plate, the operculum carrying on its inner surface the paired ovaries 

 and genital pores, followed by three or more broad plates carrying the 

 gill-packets or branchia3. The hind-body is destitute of appendages, 

 but to its last segment is articulated a long and slender telson or tail- 

 spine, which in Pterygotus is broad and spear-shaped at its extremity. 



In Scorpio the head is oblong in form and bears on its upper 

 surface ocelli in the centre and groups of eyes at the anterior angles. 

 On the under side is the buccal orifice with short cheliceree in front, 

 and a pair of large chelate pedipalps, followed by four pairs of simple 

 or clawed oral appendages, used in terrestrial locomotion in the living 

 species, and for assistance in swimming in some of the fossil aquatic 

 forms. The mid-body bears a pair of comb-like organs, and also the 

 organs of reproduction (|^nital openings). The segments following 

 carry three or more pMrs of gill-packets or tracheal openings 

 (respiratory organs) adapted for terrestrial . existence in the living 

 species, and for aquatic life in the early fossil forms. The six 

 following segments forming the hind-body- are narrow and elongated 

 without appendages ; to the last segment is articulated the ' telson ' 

 or tail-spine, which in modern Scorpions serves as a sting or 

 poison-organ. 



We know that Limulus has always been aquatic in its habit, and 

 in the earlier forms the segments of the mid- and hind-body apparently 

 were mostly free and movable, and adapted for swimming, as is the 

 case with the young stages in the development of the living king-crab.^ 

 It is the opinion of Lankester, of Pocock, of Laurie, and others that 

 the early Silurian Scorpions (^Palceophonus, Proscorpius, etc.) were 

 aquatic forms, their remains having been found in truly marine deposits. 



The correlation of Limulus with such forms as Eurypterus and 

 Pterygotus naturally suggested a close affinity with the Scorpionidae, 

 and it is also of interest to note that Scorpions have been met with 

 in America, in Scotland, and in the Baltic area in marine strata 

 associated with Eurypterus, Pterygotus, and Limulus. It is an 

 astonishing fact that two of these types, Scorpio and Limulus, should 

 have persistently survived through all the accidents and changes of 

 living things, from Silurian times to the present day, and that both 



^ See A. S. Packard, "Development of Limulus polyphemus^' (Mem. 

 Boston See. Nat. Hist., 1870, p. 154) ; Anton Dohrn, Jenaische Zeitsch., v, 

 p. 6, 1871 ; A. Agassiz, Amer. Journ. Sci., ser. Ill, xv, 75, 1878. 



