CLASS D ACERATA .;<;: 



Oyclosphaeroma, Woodw., in the Great Oolih- and l'url,<-rk ; I',,!,,,,,,,, \\'.A\\: (Fig. 



1409), in the brackisli water marls 



(Oligocene) of Paris and tin- Isle of 



Wight. Eosphaerwnn , Wmnlw. (Fig. 



1410), and several Oniscids are found 



in the Miocene of Oeningen, Baden, 



and in amber. 



[The different orders of Malacostraca, 

 with the exception of the Phyllocarids, 

 have been revised by Professor J. S. 

 Kingsley, of Tufts College, Massachusetts. 

 He has also drawn up the diagnoses of 

 the larger groups of Arthropods, and tin- 

 entire MS. for this phylum has received 

 the benefit of his expert criticism. 

 TBANS.] 



Class 2. AOERATA. 

 Kingsley. 



Primitively branchiate Arthropods 

 in which the branchial folds function as 

 gills or as lungs, or become metamor- FIG. 1409. 



phosed into air-tubes (tracheae) penetrat- 

 ing the body. The body is divided into 

 two regions, cephalothorax and abdomen, 



Fin. 1410. 



A, Eosphaeroma Brongninrti, 

 M.-Edw. Middle Oligocene; 

 Butte de Chaumont, m 

 3/j (after Woodward). IS, Frag- 

 ment of matrix, !/i (after 

 Quenstedt). 



Palaega scrobiculata, v. 

 Aminon. Lower Oligo- 

 cene ; Haring, Tyrol, an, 

 Antennae ; o, Eyes ; yfi, 

 Hindennost swimming- 

 t/ie Line between the two passing behind foot; I VI I, Thoracic 



the sixth pair of appendages. Cephalo- 3^ e \^' At " i '"" i ' 



thoracic segments usually coalesced, those 



of the abdomen either free or fused. Frequently a post-anal spine is present. Antennae 



lacking; genital openings upon the first abdominal somite; midgut lnn<i : -<j nnatozoa 



motile ; development without nauplius or zoea stages. 



The sole living representative of this group, Limulus, has long been regarded as occupying 

 a peculiarly isolated position among Crustacea. Its Arachnidian aflinitirs were long ago 

 pointed out by Straus-Durckheim, and additional reasons for removing the Merostomes from 

 association with the Crustacea have been brought forward by later writers, among whom may be 

 mentioned H. and A. Milne-Edwards, Dohrn, Lankester, van Beneden, Kingsley, and Laurie. 



The points most relied upon for the association of Merostomata and Aru<-l,,ii<l<i are: (1) 

 the numerical honiologies of the segments and appendages ; (2) the exact homologies existing 

 in the respiratory organs ; (3) the fact that the cephalothoracic appendages aiv pedifbm. tin- 

 basal joints serving as jaws ; (4) the presence of true nephridia opening in the base <>f tin- 

 third or fifth pair of appendages, or in both ; (5) genital openings in the seventh (or more 

 probably eighth) segment of the body ; (6) extreme length of the midgut ; (7) presence of an 

 internal structure, the cntostcrnite ; (8) inclusion of the ventral nerve cord and it< nrm-> in 

 the sternal artery and its branches ; (9) the close similarities in the central nervous system 

 (Kingsley). 



The Acerata are divided into two groups Merostomata and Arachnuln. 



Sub-Class 1. MEROSTOMATA. (Dana.) Woodward. 1 



Six pairs of ambulatory limbs about the mouth, the first of which in some, if i,t in nil, 

 cases terminate in chelicerae. The rest serve as organs of locomotion, <nl ////. 



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

 be found in the following memoirs : Woodioard, If., A Monograph of the British F<>s>il Crustacea of 

 the Order Merostomata (Palaeont. Soc. pp. 21-30), 1866-78. I'urkuril, A. S., On the Carboniferous 

 Xiphosurous Fauna of North America (Mem. Nat. Acad. Sci. III. j.p. 153-156), 1885. 



