40 ESSENTIALS OF ZOOLOGY 



wood, sometimes to Crustacea, but is especially common 

 on the fronds of Laminaria near low-water mark. The 

 colonies are fixed to the alga by branching roots from which 

 the stems project freely into the water. If one of the colonies, 

 or a portion of it, be removed and placed in a watch glass 

 with sea water the general features of structure will be easily 

 made out. 



The colonial structure is obtained by the fixed part growing 

 out in various directions on the substratum, and the stems 

 growing out therefrom freely and branching into a series of 

 zooids provided with mouth and tentacles. The whole struc- 

 ture is protected by a sheath secreted by the ectoderm. That 

 part of the colony which is attached to the substratum is 

 called the stolon or hydrorhiza, and the free part the stem or 

 hydrocaulus. The investing chitinous sheath is termed the 

 perisarc, and the hydroid body which it invests the coenosarc. 



The chitinous tube covers the branches of the stolon, and 

 these end blindly. On the stem the sheath is directed in a 

 succession of loops which give a characteristic zigzag appear- 

 ance to the stems. The short branches of the perisarc are 

 ringed at the base, and end in vase-like cups termed hydrothecae 

 or calycles, and each cup is widely open. 



The coenosarc of the stolon also ends blindly at the end of 

 each of the rooting branches, but in the stem, while it provides 

 a common body continuous with that of the stolon, it expands 

 in each hydrotheca into a zooid provided with mouth and 

 tentacles. These have been called the polypes or nutritive 

 zooids. The zooid is occupied at the summit by a wide mouth. 

 Between this and the ring of tentacles is the hypostome. 

 Below the ring of tentacles the cup contains a free part of 

 the body or peduncle which passes below into the coenosarc. 

 This portion is retractile, and the zooid with its tentacles may 

 be withdrawn for protection into the calycle. 



The structure is that of Hydra. The outer layer is the 

 ectoderm, the inner the endoderm, and the latter bounds a 

 gastric cavity which extends throughout the stems and roots. 

 The mesogloea which is developed between these layers is thin. 

 The ectoderm of the zooid, especially that of the tentacles, is 

 occupied by many stinging cells of a structure similar to those 



