CLASS II ARACHNIDA 771 



they are then commonly called), the metasoma together with the mesosoma 

 make up the abdomen. The abdominal segments, although usually distinct, 

 are sometimes coalesced or fused. 



Arachnids have the sexes distinct, and do not reproduce asexually or, 

 so far as known, parthenogenetically. As a rule there is little external 

 difference between male and female, except for a very frequent disparity 

 in size and an occasional modification of some of the appendages. In several 

 genera of Eurypterids two forms of opercular appendages of sexual significance 

 have been recognised, and, by analogy with Lmiulus, the more primitive of 

 these is assigned to the male, the more elaborate to the female. From this 

 it appears that, in Eurijpterus at least, the adult males are smaller than the 

 females, as is true of Limulus also. Mites, Scorpions and Pedipalps are 

 viviparous, but all other members of the class lay eggs. 



Primitive Arachnids appear to have been altogether marine, and to have 

 breathed by gill-books borne on appendages. During or after Silurian times, 

 when their descendants acquired a terrestrial habitat and changed from 

 water-breathing to air-breathing, the gill-books sank into the body and 

 became lung-books or were replaced by tracheae. Eeference may be had to 

 the recently published works by Gaskell on The Origin of the Vertebrates (1908), 

 and Patten on The Evolution of the Vertehrates and their Kin (1912), for an 

 extended discussion of the so-called Arachnid theory of the origin of 

 vertebrates. 



The Arachnida are divided into two subclasses, Merostomata and Enibolo- 

 branchiata. The chief distinguishing character of the former of these groups 

 is that the gills are patent and exposed, and (in living representatives) 

 malpighian tubules are absent. It is to be noted that both these features 

 are associated with aquatic life. 



Subclass A. MEROSTOMATA Dana (emend. Woodward).^ 



(Syn. Gigantostraca Haeckel ; Delobranchiata Lankester.) 



Six pairs of ambulator ij limbs about the mouth, the foremost of which terminates 

 in chelicerae. The rest serve as organs of locomotion, and their coxal joints for 

 prehension and mastication. Behind the mouth is a single or paired metastoma. 

 Frosoma {" cephalothorax") depressed, with usually a pair each of median ocelli and 

 laterally placed kidney-shajyed comptound eyes. Respiration by means of lamellar 

 branchiae borne on the apipendages of all, or all but one of the first six post-cephalic 

 segments, which collectively form the mesosoma. In Limulus there are no salivary 

 glands, no malpighian tubules, and no embryonic membranes {^^ amnion") are found 

 in development. 



Concerning the origin of the subclass, it is to be noted that the early 

 appearance and later atrophy of the abdominal appendages is clearly a 

 feature that points to a common ancestor for the Scorpion and Merostomes 



^ The best bibliographies of Merostomata, including also historical reviews of the group, are to 

 be found in the following memoirs : — Woodward, IL, A Monograph of the British Fossil Crustacea 

 of the Order Merostomata. Palaeont. Soc, 1866-78, pp. 21-30. — Packard, A. S., On the Car- 

 boniferous Xiphosurous Fauna of North America. Mem. Nat. Acad. Sci., 1885, vol. iii. pp. 153-6. 

 — Clarke, J. M., and Ruedemann, R., The Eurypterida of New York. Mem. 14, N.Y. State 

 Museum, 1912, p. 438. 



