Prof. J. D. Dana on Cephalization. 195 



their degradatioD, approximate to Myriapods, not to Spiders. 

 In fact, Spiders stand more nearly between Insects and Crusta- 

 ceans than between Insects and Myriapods. 



There is here a cross affinity between Insecteans and Crusta- 

 ceans which is of great interest. The relation of common Spiders 

 to Brachyural Decapods or Crabs is seen (1) in the general form 

 or habit of body (some Crabs are called sea-spiders), and 

 (2) in the coalescence of the thoracic and abdominal nervous 

 ganglions into a single central thoracic ganglion. At the same 

 time, the division of Scorpions, among Spiders, is correspondingly 

 related to that of the Macrural Decapods, (1) in the body con- 

 sisting of a series of segments ; and (2) in the nervous ganglions 

 being distinct, one to each abdominal segment. Moreover the 

 maxillipeds are long and chelate, like the outer pair in some 

 inferior Macrurans. 



Again, the Myriapods are distantly related to the Tetradecapods, 

 they being similar in their annulated structure, each segment 

 having its pair of feet, and some species of the former (as those 

 of Glomeris) even resembling the latter quite closely in form, 

 articulation, and antennae, and many of them having also the 

 habit of some Oniscida (Tetradecapods) of rolling into a ball. 



Thus, the second order of Insecteans is related, as regards 

 form, to the first of Crustaceans ; and the third of Insecteans to 

 the second of Crustaceans. 



The earliest of Crustaceans, the Trilobites, one of the compre- 

 hensive types as styled by the writer, are therefore not only 

 intermediate between Entomostracans and Tetradecapods, but 

 also, in some respects, between these and the Myriapods. More- 

 over, like the latter, Trilobites are abnormal in the very large 

 number of segments of which the body is composed ; and some- 

 times also they present no distinction between the cephalothorax 

 and abdomen. 



The facts pointed out prove conclusively that Insecteans and 

 Crustaceans constitute classes of equivalent value. 



2. Megasthenes and Microsthenes. 



The two grand divisions of typical brute Mammals, the Megas- 

 thenes and Microsthenes, are not separated by any very marked 

 difference in type of structure ; and still there is a profound 

 fundamental difference between them — that to which the names 

 refer. This is in contrast with the fact among Crustaceans, the 

 Megasthenic and Microsthenic divisions of which (the Decapods 

 and Tetradecapods) stand widely apart. But in the class of Crus- 

 taceans the structure varies between remote extremes, while 

 in that of Mammals there is a remarkable fixedness or an ex- 

 tremely limited range of variation. Hence, in the distinctions 



