TIJBULABIA SOLITARIA. 8o 



to a quite variable extent. At least half of the. stem (fig. 3) 

 is usually below the surface. At the base of the imbedded 

 hydrocaulus is the hydrorhiza, and from it may be given out 

 branches of two kinds : (1) thin rootlets which penetrate into 

 the substance of the sponge and fix the hydroid in its bed 

 (fig. 4, r) ; (2) thickened tleshy structures (fig. 4, t) in which 

 apparently nutritive substances are stored in the endoderm 

 in a manner recalling the storage of cai'bohydrates, etc., in 

 the tubers of a plant. These thickened branches are of a 

 characteristic yellow coloiir in preserved specimens. In some 

 individuals no special " tubers " are formed, but the hydro- 

 rhiza as a whole expands and takes on the function (fig. 3 ; 2 

 and 4). 



The apices of the rootlets are swollen into a sort of cap 

 (fig. 3, a) ; this appearance, recalling the root-cap of a plant, 

 is due to a marked thickening in the ectoderm at the apex. 



(2) Histology. — The ectoderm of the hydroid is of typical 

 structure ; but the endoderm shows much differentiation and 

 is highly specialised. 



Both large and small nematocysts occur in the ectoderm, 

 and the endoderm is richly supplied with the large variety 

 (figs. 8 and 10, /. n.). The large nematocysts are nearly 

 spherical in shape; their average size in spirit specimens 

 is 12* 1 /x in length and 11*6 /.t in breadth. The small nemato- 

 C3''sts are probably of several kinds, as they tend to vary con- 

 siderably in size and shape ; their average size is 6'24 fx in 

 length and 4'6 fi in breadth. 



Fig. 5 is a vertical median section of the hydranth and 

 upper part of hydrocaulus. The oral or distal tentacles (o. t.) 

 adhere to the sides of the hypostome, and the ectoderm is, so 

 to speak, squeezed out between the endoderm of the tentacle 

 and that of the hypostome, and only a thin layer of mesoglea 

 remains (fig. 8, mes.). The endoderm of the oral tentacles is not 

 regularly septate in character. The endoderm of the hypostome 

 is markedly ridged, so that in transverse section (fig. 11, e. r.) 

 eight to ten prominent projections may be seen. These 

 ridges hang down into the dio-estive cavitv of the hvdranth as 



