128 PHYLUM CCELENTERATA 



Detach a polyp by means of a pipette, place it in a watch glass 

 of water, and study it under a microscope. 



Exercise 1. Draw several outlines of the animal on a large scale, 

 showing different shapes and positions it can assume. Notice the 

 mouth in the midst of the tentacles. 



Place a polyp on a slide in water and cover it with a cover 

 glass, but support the corners of the latter with wax to avoid 

 crushing the animal. Study its structure under the microscope. 

 We see that the type of structure is radial and not bilaterally 

 symmetrical ; that the animal is tubular in shape, and that its 

 internal cavity opens to the outside through the mouth; that 

 the mouth is a small opening at the end of the conical, terminal 

 portion of the body, called the hypostome, at the base of which 

 are the tentacles. The internal cavity is called the gastrovascu- 

 lar space and is the common digestive and circulatory cavity of 

 the animal. The end by which the animal is attached is called 

 the foot. It is an adhesive disk containing gland cells which pro- 

 duce a sticky secretion. 



Examine the finer structure under a high power of the micro- 

 scope. The body wall is made up of two layers of cells, the outer 

 ectoderm and the inner entoderm. The cells of the latter are 

 much longer and more irregular than those of the ectoderm ; their 

 inner surfaces are amoeboid and also often flagellate. Embedded 

 in the entoderm cells are the chromatophores, the bodies which 

 give color to the animals. In the green hydra these are chloro- 

 phyll bodies ; in the brown hydra they consist of a substance 

 similar to chlorophyll. In both cases- they are probably single- 

 celled algae living symbiotically with the polyp. Between these 

 two cell layers is a thin noncellular one called the supporting 

 layer. The tentacle is a hollow projection of the body wall and 

 has the same structure. 



Among the ectoderm cells of the distal portion of the body, and 

 especially of the tentacles, notice the oval, highly refractive sting- 

 ing organs, or nematocysts. Each one consists of a spiral, thread- 

 like tube, with several barbs at its base, which lies coiled within 

 the cavity of a cell called a cnidoblast. The cavity is filled with a 



