114 NATURAL SCIENCE. February, 



the primitive Crustacean, and of the primitive Tracheate, which will 

 serve to show how, in company with the more speciahsed forms 

 usually associated with them, they may be distinguished from all 

 known bristle-footed worms. This is one of the reasons why, in spite 

 of my belief in the plural origin of the classes signified, the retention 

 of the term " Arthropoda '" commends itself to me as expedient. 

 Another is, that, owing largely to our ignorance of that special branch 

 of morphology which deals with the nerve-segments of the head 

 (cerebral neuromeres) and the homology of the cephalic appendages, 

 we are not yet in a position to decide as to the number of sections 

 into which the old group should be severed. 



Although the question of the classification of the Arthropoda is 

 somewhat outside the limits of the present inquiry, I should like to 

 be permitted to add that, in my opinion, the following main sections 

 may be recognised :— I. PROTOTRACHEATA or Malacopoda, for 

 Peripatus, which occupies an isolated position, though most nearly 

 related, in my opinion, to the Chilopoda of the section Opisthogoneata. 

 II. TRACHEATA, with the subdivisions: A, Progoneata (Classes : 

 Diplopoda, Pauropoda, Symphyla) ; B, Opisthogoneata (Classes : 

 Chilopoda, Hexapoda). III. BRANCHIATA or Acerata, in- 

 cluding: A, Crustacea; B, Gigantostraca (Trilobita, Eurypterida, 

 Xiphosura) ; C, Arachnida ; D, Pantopoda (Pycnogonida). 



The terms Branchiata and Acerata seem to me to be equally 

 open to objection as applied to the third assemblage ; but for the 

 present they may pass. The Pantopoda are here included, because 

 their affinities appear, on the whole, to be near the Arachnida. The 

 latter are placed near the Gigantostraca, because, in my opinion, the 

 close resemblance between the Eurypterida, not to mention LimiduSy 

 and the scorpions is inexplicable on the theory of convergence during 

 descent under totally different conditions of life from forms so remote 

 from both as the Chaetopoda ; though this is what my friend, Mr. 

 Bernard, would have us believe. As for the Linguatulina and 

 Tardigrada, the evidence that they belong to the Arthropoda seems 

 to me far from conclusive. 



R. I. PococK. 

 British Museum (Nat. Hist.). 



It may be convenient to mention that the first author to whom 

 Professor Hutton refers is, not Philipp Franz von Siebold of Japanese 

 celebrity, but Carl Theodor Ernst von Siebold, who established the 

 Arthropoda in the Text-book of Comparative Anatomy, published by 

 himself and H. Stannius 1845-1848. The second volume appeared 

 in 1845, but the first, for which Von Siebold is responsible, not 

 till 1848. It is this latter which is occupied by the invertebrate 

 animals. Among them the Arthropoda are the fifth principal group, 

 containing Crustacea, Arachnida, Insecta, the Myriopoda being 

 included under the Crustacea. Fritz Miiller's opinion that the 



