XXII.] CLASSIFICATION. 285 



Those classes of animals whereof the body is composed 

 of a succession of rings from head to tail are called the 

 Annulosa. The annulose type is a very well-marked Annulosa. 

 natural type. As in other groups, the lower limit of the Their 

 Annulosa is somewhat difficult to trace, in consequence of jg Ynde™^ 

 the affinities of its lower classes being less definite than nite, their 

 those of the higher ones ; but in its higher classes, the groups 

 annulose type is as well-marked a one as the vertebrate. ^^J^ .^ 



"^ ^ definite 



The annulose structure is best seen in the Annelids, or character, 

 true worms, in which the division into ring-like segments Annelids, 

 is distinctly visible to the eye. Above the Annelids, and, Arthro- 

 as I believe, descended from them, are the four classes of ^gsg^ejj(je^ 

 the Arthropoda, which are distinguished from the Annelids from 

 by the possession of well-developed jointed legs. The 

 classes of Arthropods are as follow : — 



1. Myriapods (millepedes and centipedes) ; 2. Crus- Number of 

 taceans ; 3. True or hexapod Insects ; 4. Arachnids j^^^^ ^ 

 (spiders and mites). In these four classes, according to and body- 

 Professor Huxley, the head generally consists of six seg- pods, 

 ments consolidated together. In the Myriapods the 

 number of the segments of the body differs greatly as 

 between species, and, in the lower genera of the class, 

 probably even as between individuals of the same species; 

 but in the Crustacea, Insects, and Arachnids, most species, 

 according to Professor Huxley, have the whole body, in- 

 cluding the head, consisting of twenty segments. Such 

 characters as these cannot be adaptive; they must be 

 hereditary, and due to community of origin. These homo- 

 logies are similar to the fact mentioned in a former chapter, 

 that nearly all the ]\Iammalia, whether their necks are 

 long or short, have seven neck- vertebrae. But this number 

 is subject to variations •,^ and if there are similar variations 

 in the number of segments in the Arthropoda, this in no 



the Acarus described agrees with Sarcoptes, but has an extraordinary 

 maggot-like appearance. The discovery of an external parasite inside an 

 animal, in such numbers as Mr. Robertson records, is very remarkable. 

 Colonel Montagu found sucli Acari in the Gannet, and Mr. Robertson has 

 since found them in the Pelican." (Quarterly Joui-nal of Science, January 

 1867.) ^ P- 247. 



