CHAPTER II. 



MERISTIC VARIATION OF PARTS REPEATED IN LINEAR OR 



SUCCESSIVE SERIES. 



SEGMENTS OF ARTHROPODA. 



INDIVIDUAL Variation in the fundamental number of members 

 constituting a Linear Series of segments can only be recognized in 

 those forms which at some definite stage in their existence cease 

 to add to the number of the series. Hence in a large proportion 

 of the more fully segmented invertebrates this phenomenon cannot 

 be studied, for in many of these, as for instance in Chilognatha, 

 and in most of the Chsetopoda the formation of new segments is 

 not known to cease at any period of life, but seems to continue in- 

 definitely. On the other hand, while in Insecta, and in Crustacea 

 excepting the Phyllopods, the fundamental numbers are definite, 

 no case of individual Variation in them has been observed. 



Between these two extremes, there are animals in certain 

 classes, for example, Peripatus, some of the Chilopoda among 

 Myriapods, Aphroditidse among Annelids, and some of the Bran- 

 chiopoda among Crustacea, in which the number of segments does 

 not increase indefinitely during life, but is nevertheless not so 

 immutable as in the Insects and the majority of Crustacea. In 

 the forms mentioned, certain numbers of segments, though not the 

 same for the whole family, are characteristic of certain genera, as 

 in the case of the Chilopoda (excepting Geophilidre), or of certain 

 species, as in some of the Peripati. But besides this, in some of 

 the forms named, e.g., the Geophili and Peripatus edwardsii, indi- 

 vidual Variation has been recorded among members of the same 

 species. It is unfortunate that for many of the forms in which 

 Variation of this kind possibly takes place, no sufficient observa- 

 tion on the point has been made, but as examples of a phenomenon 

 which, on any hypothesis, must have played a chief part in the 

 evolution of these animals, the few available instances are of in- 

 terest. 



*1. Peripatus. The number of segments which have claw-bearing 

 ambulatory legs differs in different species of this genus. While, 



