NEMATOCYSTS OF MICROSTOMA. 2"Jl 



nematocysts are transported while enclosed by cnidoblasts. In 

 the twenty nematocysts found within the endoderm my material 

 shows the stinging-threads lying free within vacuoles at the 

 bases of the endodermal cells. Martin also describes the nemato- 

 cysts that he noted within the endoderm of Microstoma as being 

 free from nematocytes or cnidoblasts. In these cases, therefore, 

 either the cnidoblasts after having carried the nematocysts 

 into the endoderm have died or the stinging-cells were not taken 

 into the endodermal tissue by cnidoblasts. Judging from what 

 has been seen I am of the opinion that these nematocysts have 

 found their way into the endodermal cells through the selective 

 action of the latter. Such selective faculties have also been 

 observed for the cnidophages of the ceras of aeolids: Foreign 

 bodies within the lumen of the cnidophore of aeolids have been 

 found "by Heckt, by Hancock and Embleton, and by Grosvenor. 

 I have found uninterpretable fragments of tissue, bodies not 

 tissue-like, and diatoms, in the liver diverticula and in the cnido- 

 phores. The ciliated canal has no power to distinguish other 

 indigestible bodies from nematocysts, but this inability to select 

 is not shared by the cnidophages, for so far as I know, these 

 ingest only nematocysts" (Boulenger, 1910, pp. 127-8). 



The cnidophages of seolids deliver their nematocysts to the 

 cnidocyst, whereas the endodermal cells of Microstoma deliver 

 their nematocysts to the mesoderm. 



In the material of this laboratory numerous nematocysts are 

 found within the mesoderm. The nematocysts in the mesoderm 

 that lie nearest the endoderm are in all cases enclosed in a vacuole. 

 These vacuoles are similar to those described and figured by 

 Martin and Vallez. About each vacuole the mesoderm crowds. 

 Two kinds of cells of the mesoderm are to be found in or near the 

 walls of these vacuoles : (a) a branched type of cells, which form 

 the frame-work of the mesoderm. These cells have nuclei 

 whose chromatin granules are conspicuous but more or less equal 

 in size. The cytoplasm of this first type of cell is not very dense 

 and is greatly branched, its branches anastomosing with those of 

 its fellows (Fig. 6, A) ; (b) cells which are much less frequent in 

 their occurrence, with denser, more compact cytoplasm than that 

 of the first type of cell, the nuclei of the second kind of cells being 



