276 



VARIATION AND MONSTROSITIES. 



pouches or ingrowths occur, but in these cases they have become perforate at their 

 inner ends, and open into the cephalic diverticula of the alimentary canal. 



II. ASYMMETRY. 



It has been shown that in typically segmented animals the organs are arranged 

 symmetrically, in checkerboard fashion, on either side of a median line. (Fig. 

 157.) Asymmetry occurs when any of these paired organs differs in form from 



FIG. 184. Limulus embryos of different ages, showing various forms of asymmetry, due either to the absence 

 of organs usually present, or to the presence of extra organs on one side of the median line. A, The right side of 

 the abdomen and the right half of the last three thoracic metameres, absent. X,io B, Left half of abdomen 

 and left chelicera absent; posterior thoracic appendages abnormally small; cam. Xso. C, Right half, except 

 the sixth thoracic appendage, absent; cam. X 16 1/2. D, Stage L. M. (about ready to hatch). Right half of 

 nerve cord is present but all the other structures of the right side are absent, except traces of third(?) and sixth 

 thoracic appendages, and the margin of the thoracic and abdominal shield; cam. Xso. E, Stage G. H. 

 There are two lateral eyes and four cheliceras on the left. The apex of the third thoracic appendage on the right, 

 is invaginated; cam. Xso. 



its mate on the opposite side. If this local variation occurs, all the other 

 parts of the body respond to it by a change in position or form. A common form 

 of asymmetry is the reduction in size, or the absence of certain organs on one 

 side of the median line. In such cases the opposite half of the body tends to form 

 a spiral or circle, with the defective area as its center. 



It is generally assumed that the morphological unit of segmented animals 

 is the metamere or a complete transverse row of organs. But it appears that in 

 Limulus, and no doubt it is true of other animals also, the right and left halves of a 

 metamere attain a higher stage of organic unity, are more variable than the meta- 

 meres themselves, and should be regarded as the true morphological units. 



