CLASS II ARACHNIDA 769 



Cretaceous and Tertiary deposits). Sculda Miinster (Eeckur Miinst. ; Buria 

 Giebel) (Fig. 1495), also from the Solenhofen beds, differs considerably from 

 Recent forms. It is of interest to note that larvae of Stomatopods belonging 

 to what is known as the Erichtlms type have been recognised in the Cretaceous 

 of the Lebanon. 



[With the underuoted exceptions this revision of the Eucrustacea has been prepared for the 

 present treatise by Dr. \V. T. Caiman, of the British Museum of Natural History. The 

 systematic account of the Branchiopoda and Ostracoda has been revised by Dr. R. S. Bassler, 

 of the United States National iMuseum, and that of the Phyllocarida by Dr. John M. Clarke, 

 State Geologist and Director of tlie New York State Museum at Albany. — Editoe.] 



Class 2. ARACHNIDA. 



Arthropods in which the branchial folds function as gills or as lungs, or heroine 

 metamorphosed into air-tuhes (tracheae) penetrating the body. The body is divided 

 into two regions, cephalothorax and abdomen, the line between the two passing behind 

 the sixth pair of appendages. Cephalothoracic segments usually coalesced, those of 

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

 lacking ; genital openings upon the first abdominal somite ; midgut long ; spermatozoa 

 motile ; development without nauplius or zoea stages. 



The affinities of the Recent Limulus and its extinct Xiphosurous allies 

 with the group represented by Scorpions, Spiders, etc., was pointed out by 

 Straus-Diirckheim as long ago as 1829, and additional reasons for removing 

 the Merostoraes from association with Crustacea were brought forward at a 

 later period by various writers, among whom may be mentioned Henri and 

 Alphonse Milne -Edwards, Dohrn, Lankester, van Beneden, Kingsley, Laurie, 

 Clarke and Ruedemann. Kingsley, in discussing the relations between 

 Limulus and the Crustacea on the one hand, and the Arachnida on the other, 

 has indicated the following points of agreement: (1) a branchial respii'ation ; 

 (2) absence of malpighian tubes ; (3) absence of salivary glands ; (4) absence 

 of embryonic envelopes ; and (5) presence of compound eyes. He has also 

 shown 28 points in which Limulus and the Arachnids agree, and in which 

 both differ from the other " Tracheates " (Myriapoda and Insecta). 



The following points of likeness are considered as of special importance 

 for justifying the association of Merostomata with the Arachnida : 



(1) The numerical homologies of segments and appendages; (2) the exact 

 homologies existing in the respiratory organs ; (3) the fact that the cephalo- 

 thoracic appendages are pediform, the basal joints serving as jaws ; (4) the 

 presence of true nephridia opening in the base of the 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 entosternite ; (8) inclusion of the ventral nerve 

 cord and its nerves in the external artery and its branches ; (9) the close 

 similarities in the central nervous system. 



The Arachnida form a more diverse class than the Insecta, and display 

 nearly as much differentiation among themselves in structure, size and habit 

 as do the Crustacea. The larger and more complex forms have a fixed and 

 constant number of segments, and in all Arachnida, as in Insecta and the 

 higher Crustacea, it is possible to analyse the body into twenty-one 

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