MEBOSTOMATA. 785 



Sub-class II. MEROSTOMATA.* 



Aquatic Arachnida with a fused cephalothorax (prosoma), a 

 more or less completely segmented abdomen, and broad abdominal 

 appendages, in some and probably in all cases provided with 

 foliaceous branchiae. The appendages of the pregenital segment 

 appear to persist, though the segment itself is evanescent. 



The name Merostomata (Dana) is here used as adopted by 

 H. Woodward, and in place of Gigantostraca, which has been 

 employed in antithesis to the crustacean divisions Entomostraca 

 and Malacostraca. The subdivision Eurypterida is used in the 

 sense adopted here by Huxley (Lectures on Natural History). 



The Merostomata f are characterized by the modification of 

 the coxal segments of the second to the sixth cephalothoracic 

 limbs to subserve mastication. The first pair of apparent limbs 

 (chelicerae) are chelate, and do not take part in limiting the mouth 

 space. The last pair (seventh) are represented in the Xiphosura 

 by a pair of unsegmented appendages (the chilaria) , to which the 

 more or less deeply divided median plate, the metastoma, appar- 

 ently corresponds in the Eurypterida. The abdomen in the 

 latter group is completely segmented into twelve segments, 

 as in the Scorpions ; of these the anterior six only are indicated 

 in the recent Xiphosura, although intermediate conditions are 

 presented by fossil forms. The anterior five or six segments 

 of the abdomen bear broad plate-like limbs. In the Xiphosura 

 the second to the sixth of these are furnished with foliaceous 

 branchiae, and there are indications of similar structures in the 



* Woodward, " Monograph of the Brit, fossil Crustacea belonging to 

 the order of Merostomata." P. I., & II., Palceont. Soc. of London, 1866- 

 1869. Woodward, " On some points in the structure of the Xiphosura, 

 having reference to their relationship with the Eurypteridae," Quarterly 

 Journ. Geol. Soc. of London, 1867 and 1871. Dohrn A. Zur Embryologie 

 u. Morphologie von Limulus polyphemus Jen. Zeits., vi, 1871. Milne 

 Edwards, A., Recherches sur 1'anat. des Limules, Annales d. Sciences Nat., 

 Ser. v, T. xvii, 1873. Packard, A. S., The anatomy, histology and em- 

 bryologie of Limulus polyphemus. Mem. Boston Soc. Nat. Hist., Boston, 

 1880. Lankester, E. R., Limulus an Arachnid, Quart. Journ. Mic. Sc., 

 xxi, 1881. Schmidt, Fr., Crustaceen fauna der Eurypteren schichten 

 von Rootzikull auf Oesel Miscell. silurica, iii, Mem. Acad. Imp. de Sc. de St. 

 Petersbourg, Ser. 7, T. xxxi, No. 5, 1883. Kingsley, J. S., The embryology 

 of Limulus, Journ. Morph, vii and viii, 1892-3. Kishinouye, On the 

 development of Limulus longispina, Journ. Coll. Sc., Japan, 1882. Patten, 

 W. and Redenbaugh, W. A., Studies on Limulus, Journ. Morph., xvi, 1890. 

 Holm, G., Ueber die Organisation der Eurypterus fischeri, Mem. Acad. 

 Imp. d. Sc. de St. Petersbourg, Ser. 8, Cl. Phys. Math., vol. viii, No. 2 (1897). 



f fj.rip6s a thigh. 



Z III 3 E 



