246 



HIGH SCHOOL ZOOLOGY. 



One end of the tube is closed, and serves for attachment, the 

 other opens by the mouth, which is surrounded by tubular 

 tentacles, capable of extraordinary elongation, and employed 

 for seizing the minute animals on which they feed. Both layers 

 of the body-wall (ectoderm and endoderm) take part in the 

 formation of the tentacles. Hydra owes its generic name to its 

 extraordinary power of recovery after injury, any fragment of 

 an individual being capable of reproducing the rest. The 

 marine Hydroids form colonies, often arborescent (Fig. 178), the 

 connecting stems of which are always, and the individuals 

 frequently, protected by a horny exoskeleton — the perisarc 

 (Fig. 179). Eggs are formed in peculiarly- shaped individuals, 



Fig. 179.— Diagram of Hydroid 

 colony (after Allnian) b, root or 

 hydrorhiza: a, coenosarc; «!, peris- 

 arc; h, modified iiKiividual with 

 reproductive buds (/) about to as- 

 sume tiie form of locomotive 

 medus* hkak; c, nutritive individ- 

 ual; e, mouth; </, tentacles. 



Fig. 178 — Hydroid colony, (flhclaria gelatinosa). 



