REPORT ON ZOOLOGY. 185 



vertebrata to be arranged on two lines, one occupied by the 

 Crustacea, Arachnida, and Insecta, the other by the Mollusca 

 and Zoophyta: he then supposes a lateral branch from the 

 Mollusca to the Crustacea, passing successively through the 

 Cirripeda, Annelida, and Entozoa, the connecting link at this 

 end of the ramification being found in the LerncBce of Linnaeus. 

 That this arrangement is, however, not quite correct, is rendered 

 probable by discoveries connected with the Cirripeda to be here- 

 after spoken of, and by the indisputable affinity between the 

 Annelida and Cyclostornous Fishes, which affinity points to the 

 former group as being necessarily at the head of one series, and 

 therefore not forming part of any lateral ramification *. 



The following are the classes considered by Cuvier as belong- 

 ing to the Annulose type : Annelida, Crustacea, Arachnida, 

 and Insecta. 



Mr, MacLeay adopts five classes t independently of the-^wwe- 

 lida, which he regards as an osculant group connecting the ver- 

 tebrate and annulose animals. Two of these are the Crustacea 

 and Arachnida of Cuvier. Two others are formed out of the 

 old class Insecta, and are the same as Clairville's groups of 

 Mandibulata and Haustellata. The fifth, to which the name 

 of Ametabola is given, includes the Myriapoda and Thysanura 



* Latreille seems to consider, as he had done in his original memoir on this 

 subject, that the Crustacea are the most perfect of the articulated animals, and 

 that therefore they necessarily approach nearest to the Vertebrata. Mr. 

 MacLeay has controverted both these points. He maintains that Insects are 

 more highly organized than Crustacea. Furthermore he observes, that so far 

 from its being by the most perfect, it is by the least perfect group in the series 

 that we might naturally expect to find a passage to the Vertebrata. Every 

 vertebrate animal would seem to " have been constructed with reference to one 

 type, and every annulose with reference to another ; and as the former is more 

 imperfect in its organization according as it approaches the annulose structure, 

 so the latter is more imperfect in proportion as it possesses a distinct system of 

 circulation and other characteristics of the Vertebrata. It thus follows that the 

 animals which connect them ought to be extremely imperfect in their organi- 

 zation." Such animals are the Cyclostornous Fishes on the one hand, and the 

 Annelida on the other, the striking affinity between which groups has been 

 noticed both by Lamarck and Cuvier. See Hor. Ent., p. 292, &c., for a further 

 development of Mr. MacLeay's reasoning. It cannot be doubted that his views 

 on this last point are correct. They likewise fall in with those of Straus-Durck- 

 heim, who has touched on the same subject in the Introduction to his valuable 

 work on the structure of the Articulated Animals {^Consid. Gen. sur I'Anat. 

 Comp. des An. Artie, pp. 13, 15, 20.) published in 1828. Geoffroy, however, 

 agrees with Latreille in thinking that the Crustacea should follow immediately 

 after the Fish. See Mem. du Mia., tom. xvi. p. 2 ; also Cours de VHist. Nat. des 

 Mammif., Lee. 3, p. 18. Robineau Desvoidy entertains the same opinion. 

 Recherches, ^-c, p. 78. 



i Hor. Ent., pp. 288 and 390. 



