THE ANNALS 



AND 



MAGAZINE OF NATURAL HISTOKY. 



[SIXTH SERIES.] 



" perlitora spargite museum. 



Naiades, et circiim vitreos considite fontes : 

 Pollice virgineo teneros h'lc earpite flores : 

 Floribus et pietiuii, divae, replete canistrum. 

 At vos, o Nymphae Craterides, ite sub undas ; 

 Ite, recui'Tato variata corallia trunco 

 Vellite muscoais e rupibus, et mihi conchas 

 Ferte, Deas pelagi, et pingui conehylia succo." 



y. Parthenii. Giannettasii Bel, 1 . 



No. 61. JANUARY 1893. 



I. — On some Points in the Morpliohgy of the Arachnicla 

 {s. s.)j with Notes on the Classification of the Group. By 

 R. 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 pay-ihus, 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 this maxim, the latter animal 

 with its gangliated 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 



