A STUDY OF METAMERISM. 399 



both anteriorly and posteriorly in free ends. The two meta- 

 meres involved take two complete turns around the body. 



Variations of this (like those of category v) readily suggest 

 themselves. 



Category vii, fig. VIT, a, b, c— This spiral form has been 

 already described as a type of the spiral. 



Category viii,fig. VIII, a, b, c. — On the upper side are two 

 half-compound segments opening in opposite directions (a). 

 On the lower side three normal segments are present (b). 

 The reconstruction (c) shows that the posterior arm of the 

 first half-compound segment takes a complete turn around the 

 body, and then unites with the second half-compound segment 

 as its anterior arm. The spiral involves three segments. 

 The middle of the three, instead of forming its proper union 

 above, unites above on the right side with the segment 

 anterior to it, and on the left side and above with the segment 

 posterior to it. 



The variations of this form consist in having the union 

 above or below, on the right and on the left, or on the left 

 and on the right side. 



Category ix, fig. IX, a, b, c. — This form is merely a longer 

 spiral than that of category vii. The spiral turns one and a 

 half times around the body, beginning and ending as in vii. 

 Three segments on one side of the body correspond to four on 

 the opposite. 



This process may theoretically be continued through an 

 indefinite number of segments, but generally a few turns 

 suffice to bring the spiral to a close. The most extreme case 

 that I have found involved twelve and a half turns to the 

 spiral (PL 41, fig. 26). 



Category x, fig. X, a, b, c. — This spiral is an extension of 

 that of category viii. It involves a greater number of meta- 

 meres, and in consequence the spiral is longer. Two turns 

 are taken around the body. 



In both categories ix and x all the variations found possible 

 for VIII and ix are here applicable. 



