LARVAE OF ARBACIA PUNCTULATA 



407 



body rod, and transverse rod. It is connected with and apparently- 

 fused to the primary skeleton at the base of the arm-rods. Fig- 

 ures 137 and 138 are from/. Although the latter at first glance 

 would seem to be rather symmetrical, closer inspection reveals 

 that in many details the two sides are unUke. In the right, 



Fig. 139 20g, 72 hrs. General form of the pluteus highly symmetrical, but 

 right side heavier than the left and four well fused rods present in the corre- 

 sponding anal arm. Left anal arm possesses only two rods which are just be- 

 ginning to unite in two places. 



Fig. 140 20g, 72 hrs. In this also, the right side of the skeleton is much 

 heavier than the left, and possesses an accessory skeleton. Right oral arm is 

 lacking in the primary structure, and its place is supplied by that of the sec- 

 ondary. This oral arm is small and gives the pluteus an abnormal form. 



Fig. 141 20/, 96 hrs. Primary skeleton highly symmetrical. Accessory 

 spicule on the left side and secondary skeleton on the right, having a well 

 developed body-rod with ventral branches. The other portions cannot be posi- 

 tively identified. 



Fig. 142 20^, 96 hrs. Primary skeleton less regular. Secondary skeleton 

 on right side, consisting of two body-rods with ventral branches, and dorso- 

 ventral connectives (?) with dorsal body-branches. The secondary skeleton 

 probably originated from two centers both of which subsequently shifted to the 

 right side. 



