1902.] NATURAL SCIENCES OF PHILADELPHIA. 785 



Both cell-layers are seen to be concerned in the formation of i he 

 new tentacle. The endoderm (End.) is pushed out from the 

 region of the circular canal, and has the shape of a solid plug of 

 tissue composed of a few cells arranged radially about a central 

 axis (T.R.). The nuclei are at the inner ends of the cells. Out- 

 side of this endodermal core is the ectoderm (Ed. ) which is, in the 

 region of the bell-margin, of the character of gelatinous tissue, 

 containing large numbers of rudimentary nematocysts. These 

 inclusions and the nuclei of the cells are more numerous at the 

 point where the tentacle is to appear than elsewhere. In the 

 medusa, as in the polyp, the greatest activity in the formation of 

 a new tentacle is manifested by the endoderm. According to 

 Allman,'" in some hydroids ( Campanidaria Johnstoni, for example) 

 the first indicaton of tentacle formation is the thickening of the 

 ectoderm at the point where the tentacle is to appear. This is con- 

 trary to the condition which we have in Gonionema. 



But to continue our description : along with the growth of the 

 endodermal process, which is to be the core of the tentacle, the 

 ectoderm also increases rapidly and constitutes an investment which 

 contains within it numerous nematocysts and concretions which were 

 scattered throughout the ectoderm at the margin of the umbrella. 

 After the tentacle has grown out for a little distance beyond the 

 bell -margin the cells on the upper or aboral surface become modi- 

 fied to form an adhesive organ (PI. XXXIII, figs. 20 and 21). 

 The cells over a disk-shaped area become elongated until they have 

 the form of a thick pad (PI. XXXIII, fig. 21). The tissue im- 

 mediately around the pad grows out in a flange so that the organ 

 becomes a vacuum-cup strongly muscular around the edge. After 

 the tentacle has grown out to a length of six to eight millimeters 

 and has increased in diameter connderably, the cavity of the circu- 

 lar canal is drawn out .into it. The endodermal cells, arranged 

 radially about the central axis, thicken until they are forced away 

 from the centre, and a tubular cavity is left (PI. XXXIII, fig. 

 21). As this process takes place first at the proximal eud of the 

 tentacle, within the tissue of the bell-margin, the cavity of the 

 circular canal is carried out along the axis of the tentacle toward 

 the tip. In this way the tentacle, which was originally imperforate 

 as in the polyp, becomes hollow. 



" AiiLMAN, Monograph on the Tuhularian Hydroids. 

 50 



