STINGING-CELLS IN CRASPEDOTR MEDUS/E. 767 



In the more distal parts of the manubrium we find nemato- 

 cysts to occur less abundantly, and their position in the 

 endoderm is very regular, the longer axes of the capsules 

 being parallel with the structureless lamella and their broader 

 ends directed towards the mouth of the medusa (PI. 42, fig. 

 3). Previous authors have shown this orientation to be 

 characteristic of migrating thread-cells, and we must come 

 to a similar conclusion ; namely, that they are making their 

 way from the base of the manubrium to the oral battery. 

 This view is confirmed by an examination of the tissues of 

 the mouth region, where one can often find thread-cells 

 actually forcing their way through the structureless lamella 

 to the oral battery. Here they take up their definitive position 

 and develop accessory structures, e.g. cnidocil and stalk, 

 from the nemutoblast. A stinging-cell occasionally turns 

 aside before reaching the oral region (PI. 42, fig. 4), and 

 passing through the lamella, forms one of the isolated 

 nematocysts to be met with in the more proximal parts of the 

 manubrial ectoderm. 



The route followed by the thread-cells of the medusa is 

 readily explained. These structures, when the nematocysts 

 are completely developed, are of considerable size, whereas 

 the ectoderm of the manubrium is very low, and, moreover, 

 forms a very definite epithelium of closely fitting cells, 

 between which the large stinging-cells could scarcely force a 

 passage. We need, therefore, not be surprised that they 

 adopt the much easier way between the large and loosely 

 packed cells of the endoderm. 



From the above account it appears, therefore, that in 

 Moerisia the nematocysts of the oral battery of the medusa 

 are developed in the endoderm at the base of the manubrium ; 

 this does not necessarily imply that the nematoblasts are 

 themselves endodermal in origin, as will be explained in the 

 section of this paper which deals with the development of 

 the medusa-bud. 



In addition to that surrounding the mouth opening, the 

 main nematocyst batteries of the medusa are situated on th^ 



