254 MANUAL OF ZOOLOGY. 



i 



yet known, in the Permian period, and has survived up to the 

 present day. Its first appearance, therefore, was just at the 

 close of the Palaeozoic epoch. The two remaining genera, 

 which constitute with Limulus this sub-order (viz., Belinurus 

 and Prestwichia), are Palaeozoic, and are not known to occur 

 out of the Carboniferous Rocks. The genus Neolimulus is 

 Upper Silurian. 



7. Isopoda. The earliest known Isopod is the Prcearcturus 

 of the Devonian Rocks. 



8. Stomapoda. This order is represented in the Carboni- 

 ferous Rocks, by the genus Gampsonyx. 



9. Decapoda. The Macrurous Decapods commence their 

 existence in the Carboniferous period, with a few Prawn-like 

 forms. The Decapoda are, however, well represented, in all 

 their three tribes, in the Secondary and Tertiary epochs, 

 attaining their maximum at the present day. The London 

 Clay (Eocene) is especially rich in the remains of Macrura and 

 Brachyura. 



CHAPTER XXXVI. 

 ARACHNIDA. 



CLASS II. ARACHNIDA. The Arachnida including the Spi- 

 ders, Scorpions, Mites, &c. possess almost all the essential 

 characters of the Crustacea, to which they are very closely 

 allied. Thus, the body is divided into a variable number of 

 somites, some of which are always provided with articulated 

 appendages. A pair of ganglia is primitively developed in 

 each somite, and the neural system is placed ventrally. The 

 heart, when present, is always situated on the opposite side of 

 the alimentary canal to the chain of ganglia. The respiratory 

 organs, however, whenever these are differentiated, are never 

 in the form of branchiae as in the Crustacea, but are in the form 

 either of pulmonary vesicles or sacs, or of ramified tubes, 

 formed by an involution of the integument, and fitted for 

 breathing air directly. Further, there are never " more than 

 four pairs of locomotive limbs, and the somites of the abdomen, 

 even when these are well developed, are never provided with 

 limbs ; " the reverse being the case amongst the Crustacea. 

 Lastly, " in the higher Arachnida, as in the higher Crustacea, 

 the body is composed of twenty somites, six of which are 

 allotted to the head ; but in the former class, one of the two 



