LARVAE OF ARBACIA PUNCTULATA 409 



cessory arm, as indicated by the outline in the drawing. The 

 oral arm-rod is lacking from the primary skeleton, and the corre- 

 sponding structm-e of the secondary functions in its stead; 

 but since the point of origin of this secondary oral arm-rod is 

 lower in the body of the pluteus than that of the primary of the 

 opposite side, the animal, in spite of a rather high degree of 

 symmetry characterizing the primary skeleton, is irregular and 

 abnormal looking. 



Figures 139 and 140 are from specimens of solution gr, and 

 are also 72 hours old^ In the latter the accessory skeleton is 

 present, and though smaller than the primary has nearly all the 

 characteristic parts, — -body-rod, anal arm-rod composed of two 

 fused single rods, greatly reduced oral arm-rod and dorso-ven- 

 tral connective. The primary and secondary skeletons are 

 joined by the ventral branches of their body-rods. In this ani- 

 mal, the oral arm-rod, though much smaller than normal and 

 united with the anal arm-rods by an exceedingly short dorso- 

 ventral connective, likewise functions instead of the correspond- 

 ing structure of the primary. Contrary to the usual compen- 

 satory regulation, the skeleton in the side possessing the acces- 

 sory structure, is more massive than in the side where these 

 parts are missing. In 139 also, one side of the skeleton is much 

 heavier than the other, and the anal arms possess three well 

 fused rods in contrast to two unfused ones in the opposite side. 



Figure 142 is from the same culture after 96 hours of devel- 

 opment. Here the accessory parts are more complicated and 

 harder to identify. Two body-rods with corresponding ventral 

 branches are evidently present, and the parts which are bent 

 over and fused to the right body-rod of the primary skele- 

 ton are possibly dorso-ventral connectives with dorsal body- 

 branches. Rudiments of anal arm-rods are present. This second- 

 ary skeleton may have originated from a single center, as in 

 some cases it is known that one side of a skeleton may cross 

 over and supply the missing parts on the opposite side; but 

 this generally extends only to some one definite element and 

 does not involve a duplication of all the structures. Hence 

 it seems probable that this skeleton must have arisen from 



