362 LOUIS MAX HICKERNELL. 



tissues is well marked. The " blasendarm " contains none of 

 the reserve food granules and its plasma has only occasional nuclei 

 scattered through it. Zelinka does not show a section through 

 this region of Discopus but there is probably no great difference 

 here between the two animals. 



Fig. 12, Plate III., is a transverse section through the second 

 foot segment. It shows the pedal gland (/. g.) whose cytoplasm 

 consists of fine alveoles and whose cell-walls are distinct. A 

 single nucleus is present in each cell. The form of this nucleus 

 is subject to little variation in the glands of the undried animal. 

 It consists of a karyosome surrounded by a clear space and a 

 well defined nuclear membrane. 



The foot-glands of Discopus differ but slightly from those just 

 described. The individual cells are perhaps more nearly round 

 than those of Philodina and the chromatic part of the nucleus 

 more nearly spherical. The general arrangement and appearance 

 of these cells is, however, not much different from those of 

 Philodina. 



Ventral to the pedal glands and separated from them by a 

 narrow space, lies the cloacal cavity. This section throws some 

 light upon the nature of the contractile bladder itself. It has 

 been assumed by Huxley, Claus, Vogt and Yung, Hudson and 

 Gosse, that the contractile bladder is merely the enlarged ending 

 of the lateral canals and must therefore be considered as a part 

 of the excretory system in a strict sense. Semper thought that 

 the contractile bladder merely forced the excretory fluid into 

 the end-gut and that other contractions of the cloacal wall were 

 necessary before the fluid reached the exterior. 



In Fig. 12 the cloacal chamber is shown to be composed 

 dorsally of a thin membrane such as one would expect in a struc- 

 ture as distensible as the contractile bladder is known to be. 

 Ventrally the wall is thicker and in all respects similar in its 

 texture to that of the "blasendarm" (Fig. n). On one side 

 and at the junction of the thin dorsal and thick ventral walls 

 there is a break which I interpret as the entrance of the excretory 

 canal. The large cell lying beside the entrance of this canal is 

 part of the sphincter which prevents the fluid from re-entering 

 the lateral canals at the time of the contractions of the bladder. 



