ON THE DEVELOPMENT OF LINGULA ANATINA. 79 



in length, attaining at the 7-9 p. c. stage the form of a twisted 

 sausage (PL VI., Fig. 84). As far as the external appearance 

 of this organ is concerned, Brooks' description leaves little to be 

 desired : so I shall discuss in the following pages only the inner 

 structure. In passing, however, I might add that the peduncle 

 is colorless in early stages but gradually assumes a light green 

 tint (PI. VI., Fig. 84). In all the larva?, of this stage examined, 

 the peduncle (PI. VI., Fig. 84) has its attachment a little to the 

 right of the occlusor posler ior ; thence it is bent forward to the 

 right, then it turns dorsal ly, is twisted again to the left, takes 

 an almost horizontal course and finally terminates with a dilated 

 end. This agrees with Brooks' description and figures : in his 

 figure 11, however, the peduncle is shows as turning ventrally. 



As the peduncle is nothing more than an outgrowth of the 

 posterior end of the body cavity, it consists of two layers : the 

 outer ectoblast and the inner peritoneum. As is seen in sections 

 (PI. VIL, Figs. 93, 94) the outer layer is an epithelium com- 

 posed of high columnar cells, whose nuclei are of a long spindle 

 shape and have a strong affinity for stains. At the base of 

 the peduncle this epithelium passes over to, and forms, the 

 inner lamella of the posterior mantle. The inner peritoneal 

 layer, on the contrary, is made up of loosely placed cells, with 

 attennated bases and enlarged tips, in the latter of which a round 

 compact nucleus is present. In some places, however, the layer 

 becomes somewhat epithelium-like. This layer produces externally 

 a layer of longitudinal (slightly oblique) muscle, which at this 

 stage is only one fibre thick. The muscle layer must be con- 

 sidered as of the same category as the parietal muscles. Brooks 

 has given an entirely different interpretation : " between this 

 [outer layer of of the peduncle] and the ciliated epithelium is a 



