HYDRA AND CC2LENTERATES. 137 
(c) Two tubes of Galaxea skeleton with outline 
sketch of the polyps that built the skeleton as you 
think they may have appeared x 5. 
(2) Branches of Madrepora skeleton, natural size. 
(e) Longitudinal section of branch of Madrepora 
skeleton. 
Other Coral-like Animals. There are many other 
kinds of corals and coral-like animals. The red coral 
of commerce is produced by polyps which deposit the 
solid matter in such a way that no traces of septa show 
in the skeleton. Something like these red-coral polyps 
are those which deposit a horny substance instead of 
calcareous matter, forming the sea-fans, sea-pens, and 
other similar growths of tropical regions. 
Characteristics of the Celenterates. The Ccelen- 
terates may be said to be animals having hollow cylin- 
drical bodies provided with a single opening at one 
end. This opening is commonly surrounded by tenta- 
cles and these are provided with nettling-cells. 
Classes of Celenterates. Animals like the hydra, 
having the whole body-cavity used as a stomach, and 
bearing a hypostome at the mouth-end of the body are 
called Hydrozoa. 
Sea-anemones and coral polyps have the hypostome 
inverted to form a gullet. Radiating partitions or 
mesenteries are found in the body-cavity. These 
animals are called Actznozoa. 
Our largest jelly-fishes, briefly described on page 133, 
are called Scyphozoa. 
Laboratory Exercise for Review. Using specimens 
in numbered bottles or boxes, as indicated on page 
113, write in note-books, or on paper, as fully as you 
can concerning the topics indicated below. Specimens 
not previously seen by the pupil are especially valuable 
for this work.* 
*Of course all these activities cannot be inferred correctly from 
structure in every case, but the attempt to make the inference is valu- 
able, and success is more frequent than failure. 
