20 George Thomas Hargitt 



in which the tentacles are fully formed. The presence of. more 

 than one cell in sections of the tentacles as shown in some of the 

 figures is due to the tentacle being cut somewhat obliquely in- 

 stead of transversely. 



In the mature hydranth the tentacles are sometimes com- 

 posed of two series arranged in a single whorl. Those of one 

 series are somewhat drooping in habit, the others more erect. 

 The tentacles of the two series alternate in position and there 

 is also an alternate elevation and depression of their bases. 

 This condition .of the tentacles is shown in the process of de- 

 velopment in fig. 51. The irregular mass of polygonal cells 

 shown in this figure is due to the section being made across the 

 mass of cells between the enteric cavity of the hydranth and the 

 hypostome (cf. figs. 58, 60). 



The hypostome develops as a hollow sphere at the extreme 

 distal end of the hydranth, beginning as an outgrowth of the 

 layers of the regenerating hydranth. At a very early stage, 

 however, all connection between the enteric cavity of the hy- 

 dranth and the cavity of the hypostome is blocked up by a mass 

 of entoderm cells, probably due to a multiplication of cells in 

 this region. The further development is for the most part an 

 increase in size. The shape of the cells change, the ectoderm 

 being finally composed of very much flattened cells, the result 

 of the increase in size of the hypostome without great multipli- 

 cation of cells of the ectoderm. The entoderm is made up of a 

 large number of columnar cells very much elongated radially 

 and very narrow. Figs. 58, 60, show the relation of the twa 

 layers and also the mass of entoderm cells blocking the passage 

 between the enteric cavity and the hypostome. This mass of 

 cells is at first without regular arrangement, being crowded 

 together into a dense mass. Later in the development, these 

 crowded cells take on a more regular arrangement as repre- 

 sented in fig. 58. There is already present in this stage a sort 

 of dividing line between the cells of the two sides, and by the 

 separation of the layers of the two sides along the dividing line, 

 the opening between the enteric cavity and the hypostome is 

 formed (fig. 59). At the angle formed by the junction of the 



294 



