320 Descriptive Zoology. 



A Tubularian Hydroid. The general appearance of a 

 zooid of a tubularian hydroid may be seen from Fig. 182. 

 The body is hydralike, but with a prolonged mouth. There 

 are two layers of the body, the ectoderm and the endo- 

 derm. The ectoderm of the tentacles is provided with 

 nettle cells. 



Structure of the Stem. The structure of the common 

 stem is similar to that of the zooid. The outer covering of 

 chitin is called the perisarc, while the soft tube within, con- 

 sisting of ectoderm and endoderm, is called the cenosarc. 

 The endoderm cells have flagella. 



Action of a Live Zooid. The live zooid captures small 

 animals by stinging them as does the hydra. It uses the 

 tentacles to draw them into the mouth. After the food is 

 softened and reduced to a liquid, it is circulated by the 

 vibrations of the flagella of the cells of the endoderm. The 

 nettle cells serve for protection as well as in securing food. 

 When the zooid is disturbed, it withdraws into the protect- 

 ing cup, or hydrotheca, and may thus remain for some 

 time. 



Development of a Zooid Budding. The colony in- 

 creases in extent by budding. A bud forms on the side 

 of a main stem. The bud consists of the same layers as 

 the stem, namely, the perisarc and cenosarc. The bud 

 swells into a knob, the cavity of the main stem extending 

 into it, and the nutritive liquid circulates in the bud. After 

 a time the perisarc ruptures at the end and expands into 

 the cup, the cenosarc forms a mouth at the end, tentacles 

 develop around the mouth, and a new zooid is complete. 



How a New Colony is formed Medusa Buds. By a 



continuation of the process just described the colony is 

 increased in extent, but no new colony is formed, for noi 



