PLANK'IONETTA ATLANTICA. 137 



hard, brilliantly retractile, and transparent, and is dissolved 

 by treatment with warm strong- acids. It exhibits over the 

 greater part of its circumference two layers, both of which 

 are homogeneous and smooth ; the inner softer, and generally 

 shrunken in preparation for sections; the outer harder, 

 splintering before the razor, and retaining its spherical shape. 

 The inner layer stains deeply with hasmatoxylin ; the outer 

 does . not stain at all, except centrally where it faces the 

 phaeodial region; in this part both layers look alike in 

 sections, and are either intimately connected or fuse in places. 

 What sometimes in sections resembles a third layer is pro- 

 bably due merely to the coagulation of the fluid contents of 

 the float. 



If I am right in thinking that the meshwork and lip spines 

 are actually continuous with the float, then the float is merely 

 a part of the general skeleton, and presumably has been 

 evolved from a skeletal meshwork between arms such as 

 occurs in many Phaeodaria (e. g. the allied Gazelletta), and 

 may prove perhaps to represent one or more pairs of arms, 

 arched over and fused at their ends, such as Borgert ^ has 

 described in species of the allied Medusetta. Wheu the float 

 has been broken away its loss could not be suspected, as its 

 connection with the shell is only through very flne spines and 

 meshwork. 



Across the narrowed neck of the shell (flgs. 8, 9) stretches 

 a circular diaphragm of fibrous character; its edge is thickened, 

 and the upper part of the thickening is inserted by processes 

 into the pits of the lowest zone of the shell-mouth below the 

 pores. Below this lie the intra-capsular protoplasm and 

 nucleus, enclosed below (aborally) by a lining membrane, 

 conspicuous in fig. 1 ; the latter becomes greatly folded and 

 shrunken in preparation for sections (fig. 8), hence my inter- 

 pretation of the structures present in this region is merely 

 provisional, in default of fresh or specially preserved material. 



We may presume that there exists something internal to 

 the shell corresponding to the characteristic central capsule 

 1 ' Noid. Tripyleen-Arlcji,' pp. 35, 3G, ligs. 42, 413. 



