METAZOA VERMES. 273 



and the pedicle valve becomes circular in form, the shell 

 passing through a stage similar to that which we have 

 already described as the Discinoid stage of the Neotrem- 

 ata. This is shown in Leptaena rhomboidalis Wilck. (PI. 

 665, fig. i). As growth continues, the neanic stage (fig. 

 2) is marked by radiating lines. The peripheral layers 

 are resorbed, so that the pedicle opening approaches the 

 edge (fig. 2) and in the ephebic stage reaches it (fig. 3; 

 see also No. 666). In this stage the cardinal areas of 

 both the convex pedicle valve (No. 666) and the concave 

 brachial valve are long, flat, and narrow with a triangular 

 fissure for the pedicle which is partly filled by a deltidium. 

 Finally the pedicle disappears altogether, and Leptaena 

 is free as in its young condition. In old age the anterior 

 and lateral portions of the valves bend at nearly right 

 angles (No. 666) to the plane of the younger shell, and 

 the layers of growth are crowded closely together (No. 

 667). 



The arms or brachia in Leptaena and in the Protremata 

 generally are not supported by a limy skeleton. 



The only living representative of the Protremata is the 

 genus Thecidium ( = Lacazella). The cephalula stage 

 (as it is called) is represented in PI. 668, fig. i, a dorso- 

 ventral longitudinal section of T. mediterraneum Risso. 

 The mantle lobes (fig. i, d and v) are unequal. The 

 limy dorsal valve (ds) and the shell-plate (del) begin to 

 form ; the latter on the dorsal side of the body. The 

 larva transforms by the mantle lobes bending upward, 

 which causes the shell-secreting surfaces (indicated by the 

 heavy black lines) to come on the exterior (fig. 2). The 

 shell-plate (del) is below the hinge line (fig. 2, hi). The 

 adult (fig. 3) shows one valve and the deltidium (del), 

 and the side view (fig. 4) gives both valves marked by 

 concentric lines, the hinge line (hi) and the deltidium 

 (del). 



These figures show that the deltidium is formed on the 

 brachial side of the body, as already stated, and on that 



