58 RIOHAUD EVANS. 



it is almost impossible to improve upou it. However^ 

 the failure of Kennel's attempt to explain the mysterious 

 plieuomena in P. edwardsii hardly accounts for the whole 

 difference between the two descriptions. But before proceed- 

 ing any further a few words must be said regarding the 

 terms which Mr. Sedgwick used in his description^ and those 

 which will be adopted in the present account. On page 493 

 of the third part of his account (5) Mr. Sedgwick uses the 

 terms " dorsal " and " ventral," by which he means the dorsal 

 and ventral portions of the almost unmodified somite, with its 

 fully developed coelom. On the following page he speaks of 

 an " outgrowth " from the posterior part of the somite into 

 the rudimentary limb, and describes it as the "lateral or 

 appendicular portion" of the coelom, in contrast with the 

 " median portion." On the same page he labels the "median 

 portion" as the "dorsal," and on page 496 the "lateral" or 

 " appendicular" becomes, in Mr. Sedgwick's nomenclature, the 

 "ventral division." In the passages referred to, each of the 

 terms dorsal and ventral has been used in two different 

 senses, for if I understand him rightly, Mr. Sedgwick does 

 not apply the expressions dorsal and ventral on pages 494 

 and 496 as he does on page 493 (5). In the present account 

 the expressions dorsal and ventral will be used in the 

 same sense as was given them on page 493 of Mr. Sedg- 

 wick's account, — that is, to refer to the upper and lower 

 portions respectively of the unmodified somite. The ex- 

 pressions median and appendicular, alone, will be applied 

 to the portions of the coelom resulting from its division into 

 a portion situated in the body, and a portion placed in the 

 appendage. 



In treating of a mesodermal somite and its coelomic cavity, 

 it will conduce to clearness if I describe the development of 

 a somite situated somewhere about the middle of the body 

 before proceeding to consider the special modifications oc- 

 curring at either end of the animal. 



The first appearance of the somites and of the cavities 

 situated in them has been considered in describing the second 



