CCELENTERATA 



467 



markable change of shape will be understood if we place the animal in 

 a watch-glass. with some water, and, after it has become again extended, 

 examine it under the microscope. The most careful examination will 

 reveal no trace of any supporting or protective structure (compare, e.g., 

 with mammals and insects). Hence, such a polyp could only live in 

 water, which carries it and supports it on all sides. Here, too, it finds 

 the protection against evaporation which it re- 

 quires on account of its stift and completely un- 

 protected body (see Part II., p. 253). 



D. Structure and Food. 



By bringing into the vessel containing the 

 polyps a number of minute crustaceans, such as 

 are found in every pool, we shall be able to observe 

 the manner in which our captives feed. If one 

 of the crustaceans happens to come in contact 

 with the tentacle of one of the polyps, it is sud- 

 denly arrested in its swimming movement, and 

 remains fixed to the spot as if spellbound. If the 

 polyp is hungry it contracts its tentacle ; the other 

 tentacles assist in grasping the prey, and it is soon 

 conveyed into the interior of the cylindrical body. 



(a) Since the polyp lives exclusively upon small 

 crustaceans and other minute aquatic creatures, it 

 requires prehensile organs such as these arms or 

 tentacles. (Contrast with aquatic animals which 

 sweep the food into their mouths — e.g., lamelli- 

 branchs.) 



(b) Why, however, does the crustacean remain 

 fixed, as though spellbound, to the tentacles ? If 

 a living fresh-water polyp be placed under the 

 microscope, and covered with a thin cover-glass, 

 we shall see, even under a small magnifying power, 

 a large number of clear vesicles embedded in the 

 outer cellular layer, or ectoderm, of the tentacles and body (see 

 Section D, d). If a drop of acetic acid be now admitted under the cover- 

 glass, fine threads will be seen to be ejected from the capsules, in which 

 they previously lay spirally coiled up. Owing to the action of the acid, 

 the cells explode, as it were, and hurl forth the threads like stings, the 

 thread, by a process of evagination, being turned inside out, like the finger 

 of a glove. These threads are often provided with recurved spines near 



31 



#tti. 



"Thread Cells" of 

 Fkesh-"Watbr Polyps. 

 (Very strongly magni- 

 fied ; ' somewhat • dia- 

 grammatic, and in sec- 

 .tion. ) 



I., In quiescent condition 

 (with the stinging- 

 thread rolled uj) spirally 

 in the capsule). II. , In 

 the active everted con- 

 dition (the stinging- 

 thread ejected). 



