THE VEliTEBKATA AND AKTHROPODA. 77 



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. A striking uniformity 

 of composition is to be found in the heads of, at any rate, the 

 more highly organized members of these four classes, so that, 

 typically, the head of a Crustacean, an Arachnid, a Myriapod, 

 or an Insect is composed of six somites (or segments corre- 

 sponding 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 confirmation 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 ARTICULATA, 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 Arihropoda, 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 

 respects, Annelids differ widely from Arthropods. It may be 

 doubted whether any Annelid is devoid of cilia in some part 

 or other of its organization, and cilia constitute the most im- 

 portant organs of locomotion in the embryos of many. No 

 Annelid possesses a heart communicating by valvular aper- 

 tures with the perivisceral cavity, none have articulated limbs, 



