the AirniKopouA. 75 



chain of ganglia, united by longitudinal commissures, and the 

 gullet passes between two of these commissures. No one of the 

 members of these four classes is known to possess vibratile cilia. 

 The great majority of these animals have a distinct heart, pro- 

 vided with valvular apertures, which are in communication with 

 a perivisceral cavity containing corpusculated blood. But the 

 Cirripedia and the Ostracoda among Crustaceans, and many of 

 the Mites among Arachnida, have as yet yielded no trace of 

 distinct circulatory organs, so that the nature of these organs 

 cannot be taken as a universal character of the larger group we 

 are seeking ; still less can such a character be found in the 

 respiratory organs, which vary widely in character, and are 

 often totally absent as distinct structures. Some years ago I 

 endeavoured to show* that a striking uniformity of compo- 

 sition is to be found in the heads of, at any rate, the more highly 

 organized members of these four classes, and that, typically, the 

 head of a Crustacean, an Arachnid, a Myriapod, or an Insect is 

 composed of six somites (or segments corresponding with those 

 of the body) and their appendages, the latter being modified so 

 as to serve the purpose of sensory and manducatory organs. I 

 believe this doctrine to be substantially correct ; and that, 

 leaving all hypothetical suppositions aside, the head of any 

 animal belonging to these classes may be demonstrated to 

 contain never fewer than four, and never more than six somites 

 with their appendages ; but, until this view has received confir- 

 mation from other workers, I shall not venture to put forward 

 any statement based upon it as part of the definition of the 

 large group or " province ' containing the four classes above 

 mentioned, which has received from some naturalists the name 

 of Akticulata, from others that of Arthropoda, the latter 

 being perhaps the more distinctive and better appellation. 



The members of the class Annelida present marked differ- 

 ences from all the Arthropoda, but resemble them in at least 

 one important particular ; and that is, the arrangement of the 

 nervous system, which constitutes a ganglionated double chain, 

 traversed at one point by the oesophagus. In almost all other 



* "On the Agamic Reproduction and Morphology of Aphis" Transactions of 

 the L/iiiui-an Society, vol. xxii. 



