332 



APPLIED BIOLOGY 



hydroids. One who does not examine with a band-lens would 

 probably mistake a group of hydroid colonies on a rock or 

 other object for a mass of moss-like plants (Figs. 100, 101, 102). 



(D) Various species of hydroids can be obtained from dealers 

 in biological supplies. They are best preserved in formalin. Single 

 colonies can be picked out from a mass 

 and placed in small glass vials for ex- 

 amination with a hand-lens. Some of 

 the individual hydroids (polyps or hy- 

 dranths) should be mounted in water or 

 glycerin for examination with low power. 

 Some stained permanent preparations of 

 entire hydranths and sections will be 

 useful. 



Observe : (1) general form of colony; 

 (2) position of individuals ; (3) the sup- 

 porting stem or stalk (which has the same 

 layers as in Hydra plus a transparent 

 outer sheath of a hardened material which 

 is protective and at the same time affords 

 an elastic support to the colony) ; (4) 

 hydra-like form of the individuals (ten- 

 tacles, mouth, nettle-cells, digestive cav- 

 ity, two layers of cells, digestive cavity 

 continued into the hollow stalk). In 

 some species of hydroids the transparent 

 sheath of the stalk stops at the base of 

 each individual polyp, but in others it expands to form a transparent 

 cup for each polyp. 



FIG. 101. A colony of Hy- 

 dractinia hydroids with 

 differentiated nutritive 

 (ri), defensive (d), and re- 

 productive (r) individu- 

 als which co-operate. The 

 digestive cavity of n con- 

 nects with that of d and r 

 which are unable to take 

 food. (FromMcMurrich.) 



288. Medusae. Some species of hydroid colonies have 

 certain polyps of a bell-shaped form (Fig. 100, 6, c), and in 

 other species there are special polyps which are elongated 

 and without tentacles. Those of the bell-form break away 

 from the hydroid stem and swim freely as medusae or jelly- 

 fishes. Those of the elongated type form a number of buds, 

 and each bud develops into a bell-shaped medusa. All the 

 medusae are very small when first freed, but they have 

 mouth and tentacles and are able to catch food, as a hydra 



