ox PARAWRIGIITIA ROBUSTA. 191 



thoy occur in the thickening of the umbrella ectoderm 

 around the opening of the umbrella (PI. XXXIII, figs. 3 

 aiul 5, n.; te.\t-fig. 6, n.). They also occur ])lentifiilly in the 

 midst of the spermatic mass (PI. XXXIV, fig. 6, n). 



It seems fairly clear that nematocysts can have no function 

 imbedded in the spermatic mass, or even in the general 

 cocnosarc, since they are shut off from the exterior by a thick 

 ])erisarc. It would appear that the ectoderm, being endowed 

 with the power of forming nematocysts, produces these 

 structures whether or not they can become functionally active. 

 It is conceivable that the presence of nematocysts throughout 

 the entire substance of the ectoderm Avould render the 

 hydroid distasteful to be eaten as a whole ; but I do not 

 consider that this is the correct view. The abundance of 

 nematocysts around what would be the margin of the 

 umbrella is an interesting case of the persistence of a 

 structure after the loss of its function. 'J'he fact that the 

 nematocysts are somewhat larger in the gonophore thnn in 

 the hydranth is also of some interest. 



p]ndoderm. — The endoderm of the hypostome consists of 

 narrow columnar cells, with small deeply-staining nuclei 

 (PI. XXXIII, fig. 4, en. A.). The small vacuolated cells, which 

 stain intensely, and are frequeutly seen in the endoderm of 

 the hypostome of various hydroids, do not appear to occur in 

 this species. In transverse section the endoderm of the 

 hypostome is seen to be raised into six to eight prominent 

 ridges. 



At the base of the hy])ostome the endoderm abruptly 

 changes its character, and the cells are large, vacuolated, 

 and richly granular. These endoderm cells frequently 

 contain globules of yolk (PI. XXXIII, fig. 3, y. gl.), more 

 especially towards the base of the polj^p. Special glandular 

 cells {gl. c.) Avith large granules occur in the upper portion of 

 the digestive cavity of the hydi-anth, and they are also found 

 irregularly scattered in the endoderm of the stem and stolon. 



(3) The development of the hydranth. — A lateral 

 branch of the stolon, a hydranth, or a gonophore, originate 



VOL. 1, PART 2- 14 



