1 8 MORPHOLOGY OF INVERTEBRATE TYPES 



hydranth and the same is true for every branch. The reason 

 for it lies in the monopodial method of branching. The original 

 or founder-polyp becomes elongated at its base, producing the 

 first or main stem. As the main stem grows in length the first 

 bud appears near its base. This bud becomes the end polyp 

 of the first or oldest branch. The main stem continues to grow 

 and produces a second bud between the first branch and the 

 top polyp, then a third bud and so on. The same process ap- 

 plies to the branches. The main stem with its root and branches 

 is hollow and this cavity is naturally in direct continuation with 

 the cavity of every hydranth. This cavity is the caslenteron or 

 gastro-vascular cavity. The hydranth consists of a short stem 

 or peduncle and a flask-shaped head. The mouth is at the end 

 of a conical hyposto'me. At the base of the hypostome are two 

 or three verticels, each composed of from 5 to 7 short oral ten- 

 tacles. These tentacles terminate in a knob of nematocysts. Near 

 the base of the head is a circle of from 12 to 16 long basal ten- 

 tacles with nematocysts arranged in little groups along each 

 tentacle. Each group of nematocysts appears as a little swell- 

 ing of the tentacle. All tentacles are solid (not hollow). The 

 medusa buds appear on the head, between the oral and basal 

 tentacles. They become either free-swimming medusae or re- 

 main rudimentary and sessile. 



Microscopic structure. The walls of the hydrocaulus or 

 stem, of the branches, and of the hydrorhyza are known under 

 the name of ccenosarc and are composed of three concentric 

 layers. The outer layer secreting the perisarc is the ectoderm. It 

 is composed of "indifferent" epithelial cells. Some of these cells 

 in the hydrorhyza are modified into adhesive cells. The middle 

 layer has the appearance of a thin membrane. It is a structure- 

 less, non-cellular mesoglosa. The inner layer is the endoderm and 

 is composed of so-called circulatory endodermal cells. The walls 

 of the hydranth are composed of the same three layers and here, 

 too, the mesoglcea remains structureless. But the elements 

 which enter into the formation of the ecto and endoderm are 



