THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTORY. 



[SIXTH SERIES.] 



" perlitora spargite museum. 



Naiades, et circum vitreos considite fontes : 

 Pollice virgineo teneros hie carpite floras : 

 Floribus et pietum, divae, replete canistrum. 

 At Tos, o Xvmphae Craterides, ite sub undas ; 

 Ite, recurvato variata corallia trunco 

 Vellite muscosis e rupibus, et mihi conchas 

 Ferte, Deae pelagi, et pingui conchylia succo." 



JV. ParthenuGiannettasii Eel. 1 . 



No. 61. JANUAKY 1893. 



I. — On some Points in the MorpJiology of the Arachnida 

 {s. s.), with Notes on the Glassijication of the Group. By 

 E. I. POCOCK, of the British Museum (Natural History). 



[Plates I. & n.] 



Since it is generally admitted that the ancestor or ancestors 

 of the Arthropoda must be sought for in animals resembling 

 the Annelidan worms in the complete segmentation of the 

 body, it seems clear that a species in which the metamerism 

 is highly developed is, cceteris ijarihus^ more primitive than an 

 allied form in which it is obscurely manifested. The two 

 common decapod crustaceans, the crab and the lobster, furnish 

 a good example of the truth of tiiis maxim, the latter animal 

 with itsgangliated nerve-chord, its long, segmented, and limb- 

 bearing abdomen, being unquestionably more nearly related to 

 the primitive form or ancestor of the Decapoda than the crab. 

 Consequently in tracing the phylogeny of this group of crus- 

 taceans we should conclude that the Brachyura are the descen- 

 dants of the Macrura, and that as such they should occupy a 

 higher branch of a genealogical tree. 



The truth of this, however, is so very obvious that the only 

 excuse to be offered for its restatement is the circumstance 

 that not all authors have borne it in mind in dealing with the 



Ann, & Mag. N. Hist. Ser. 6. Vol xi. 1 



