=Fam. 16. Lichadidae.=—The body is broad, with a granular dorsal surface. The cephalic shield is small and short, with spinose genal angles. The glabella is broad, and its anterior furrows are directed backwards, limiting a convex median lobe and some lateral lobes. The facial suture extends from the posterior to the anterior margin. There are nine or ten thoracic segments with grooved pleurae, which have pointed ends. The pygidium is large and triangular, with a short axis and a toothed margin. Ordovician to Devonian. Genus: _Lichas_ (sub-genera, _Arges_, _Dicranogmus_, _Conolichas_, _Ceratolichas_). INTRODUCTION TO ARACHNIDA, AND XIPHOSURA BY A. E. SHIPLEY, M.A., F.R.S. Fellow of Christ’s College, Cambridge, and Reader in Zoology in the University CHAPTER IX ARACHNIDA—INTRODUCTION The Arachnida, together with the Crustacea, Insecta, Myriapoda, and _Peripatus_, make up the great phylum Arthropoda, a phylum which, from the point of view of numbers of species and of individuals, is the dominant one on this planet, and from the point of view of intelligence and power of co-operating in the formation of social communities is surpassed but by the Vertebrata. The Arachnida form a more diverse class than the Insecta; they differ, perhaps, as much _inter se_ as do the Crustacea, and in structure as in size and habit they cover a wide range. Lankester in his article upon the Arthropoda, in the tenth edition of the _Encyclopaedia Britannica_, dwells upon the fact that whereas the adult _Peripatus_ has but one persisting segment in front of the head, and its mouth is between the second persisting appendages, in Arachnids the mouth has receded and lies between the bases of the appendages (pedipalpi) of the third persisting segment, while two of the persisting segments, those of the eyes and chelicerae, have passed in front of the mouth. This process has continued in the Crustacea and in the Insecta; in both of these classes there are three embryonic segments in front of the adult mouth, which lies between the appendages of the fourth segment. In the larger and more complex Arachnida the number of segments is fixed and constant, and though possibly no adult member of the group, owing to the suppression of one or more segments during the ontogeny, ever shows the full number at any one time, the body can be analysed into twenty-one segments. It is interesting to note that the same number of segments occurs in Insecta and in the higher Crustacea.[204] The significance of this fact is not perhaps apparent, but it seems to indicate “a sort of general oneness, if I may be allowed to use so strong an expression,” as Mr. Curdle said when discussing the unities of the drama with Nicholas Nickleby.