chap. II.] SEGMENTS OF ARTHROPODA. 95 



contemporary instances of the way in which such a change at its 

 origin may be brought about. Though there are several things to 

 be gained by study of these instances, one feature of them calls for 

 attention now, namely, the definiteness of the variations recorded. 

 The change from a form with one number to a form with another 

 number here shews itself not as an infinitesimal addition or sub- 

 traction, but as a definite, discontinuous and integral change, pro- 

 ducing it may be, as in Peripatus edwardsti, a variation amounting 

 to several pairs of legs, properly formed, at one step of Descent. 

 This will not be seen always to be the case, but it is none the less 

 to be noted that it is so here. 



Among Insects I know no case of such individual variation in 

 the fundamental number of segments composing the body. Among 

 Crustacea two somewhat remarkable examples must be mentioned, 

 though it will be seen that both of them belong to categories very 

 different from that with which we are now concerned. But in- 

 asmuch as they relate to the general subject of Meristic Variation 

 they should not be omitted. 



3. Carcinus maenas. The abdomen of these crabs consists normally of seven 

 segments, including the last or telson. In the female the divisions between all 

 these seven are very distinct. The abdomen of the normal male is much narrower 

 than that of the female, and in it the divisions between the 3rd, 4th and 5th 

 segments are obliterated. Males, however, which are inhabited by the Rhizo- 

 cephalous parasite Sacculina do not acquire these sexual characters, and in them 

 there are distinct divisions between the 3rd, -4th and 5th segments. (Fig. '.• c. I 



Fig. 9. A. Abdomen of Carcinus mamas, female, normal. 



B. Abdomen of male, normal. 



C. Abdomen of male infested by Sacculina. After Giabb and 



Bonnier. 



In male Carcinus mamas inhabited by the Entoniscian parasite, Fori union, a similar 

 deformity may occur, but is often very much less in extent, sometimes being only 

 apparent in a slight alteration in the contour of the sixth abdominal Bomite. In 

 specimens of Portunus, Platyonyckus, Pilummu and Xantho inhabited by Knto- 

 niscians, no change was observed. Giahd and Bonne b comment on the remarkable 

 fact that the change in the sexual characters effected by Sacculina is greater than 

 that resulting from the presence of Entoniscian- ; for since the latter are more 

 internal parasites, preventing the growth of and actually replacing generative 

 entirely or in part, it might have been expected that the conseqnenoes of their 

 presence would be more profound. <im:i>. A., and Bonnibb, J., Contrib. a L'e'tnde 

 des Bopyriens, Travaux de Vinst. zool. de Lille et <lu laboraioire tool. </<■ Wimereux, 

 1887, torn. V. p. 184. 



