II. HYDRA VIRIDIS OR FUSCA. 



(Freshwater Hydrozoa). 



A Metazoan which throughout life remains in a two layered con- 

 dition, like a gastrula. Observe with naked eye, or with pocket 

 lens, the hydras in a jar of water where they have been undis- 

 turbed for some time. Notice the general habitus of body, method 

 of obtaining food, etc. Transfer a hydra to a slide with plenty of 

 water, and observe with the dissecting microscope ; afterward cover, 

 supporting the cover glass so as not to crush the animal, and ex- 

 amine with the low power of the compound microscope. 



I. FORM. 



The body: \Yhat is the general shape? Do its length and 

 breadth vary? It is usually attached at one end, the foot, by a 

 kind of sucking disk and terminates at the other in a conical pro- 

 jection, the hypostome, with the mouth at its summit. The mouth 

 is a small aperture but it can be greatly dilated to take in food. It 

 opens into a central cavity, the enteron (digestive). The tentacles 

 are hollow processes of the body wall. How many are there? Com- 

 pare the number of tentacles in brown and in green hydras. Is there 

 more than one circle of tentacles? Observe the knob-like swellings 

 on the tentacles. Measure the length of the tentacles when expand- 

 ed ; \vhen fully contracted. For what purpose are the tentacles 

 used? Buds :- -Young hydras of various sizes and stages of develop- 

 ment may be attached to the sides of the parent. Are colonies 

 formed by budding? AYhy? 



Draw an entire animal, with all the parts named above. 



II. STRUCTURE. 



1. The body wall of the animal is composed of two layers of 

 cells, one within the other, (a) The Ectoderm is the outer layer. 

 What is its color? How much of the thickness of the body wall 

 does this layer form? (b) The Endoderm is the inner lining of 

 the body cavity (digestive cavity). In the green species (Hydra 

 viridis) it contains chlorophyll bodies; in the brown species, H. 

 fusca, it contains "sooty corpuscles". Which layer is the thicker? 

 (c) The supporting layer or Mesoglea is a thin gelatinous layer 

 between the ectoderm and the endoderm. 



2. The tentacles. Examine a tentacle with the high power. Of 

 how many layers is it composed? Focus up and do\vn so as to 

 obtain views (optical sections) at various levels. Is the tentacle 



31 



