ANATOMY OF OBELIA 243 



sends off at frequent intervals little branches some 10- 

 20 mm. in height. Each branch consists of a somewhat 

 zig-zag axis, the hydrocaulus, which at every bend gives off a 

 very short ringed branch ending in a little terminal polype 

 which, from its resemblance to a flower, is called a hydra-flower 

 or Hydranth. The hydrocaulus and its branches consist of a 

 delicate tube with a triple wall of ectoderm, mesoglcea, and 

 endoderm protected externally by a relatively stout but 

 transparent chitinous investment, the perisarc. The soft 

 tissues do not fill the cavity of the perisarc, but are bound 

 to it at intervals by processes of the ectoderm cells. At every 

 joint the perisarc is ringed, as shown in the figure, and the 

 little branchlets which bear the hydranths are made up of 

 three or four such rings. At the base of each hydranth the 

 perisarc is expanded to form a pretty vase-like cup, known as 

 the hydra-case or hydrotheca, into which the hydranth can be 

 withdrawn. 



An individual hydranth does not differ very much in 

 structure from a Hydra, but its cell-elements are smaller and 

 in some particulars simpler. The tentacles are numerous, 

 twenty-eight to thirty in number, arranged in a single circle, 

 and they differ from those of Hydra in being solid instead 

 of hollow, the axis of each being occupied by a single row 

 of large discoid endoderm cells with stout walls of cartilaginoid 

 consistency. The interior of each of these cells is greatly 

 vacuolated, the protoplasm being reduced to a little column 

 surrounding the nucleus placed in the centre of the cell, and 

 a few radiating strands. The ectoderm is thin, but loaded 

 with nematocysts at the extremity of each tentacle. 



The hypostome is enormous, forming a sort of ante-chamber 

 to the gastrovascular cavity, and the endoderm lining it is 

 richly supplied with gland-cells. Further details may be 

 learned from an inspection of fig. 52. The proximal part 

 of the hydranth is flat and rests upon a sort of shelf near 

 the bottom of the hydrotheca. The centre of this shelf is per- 

 forated by the neck of the hydranth or hydrocope, very narrow 

 at first, but soon swelling up to pass into the hydrocaulus. 

 A glance at the figures shows that the cavities of all the 

 hydranths communicate with the cavity of the hydrocaulus, 

 and so with one another. 



The hydranths are solely nutritive persons. Their function 



