166 C (ELEN TER AT ES. HYDRA-LIKE ANIMALS 
Figure 156. — Pennaria 
Tiarella. 
in this complicated series of 
changes are illustrated in Figure 
156. The hydroids and medusae 
show a form of reproduction 
called alternation of generations, 
that is, they reproduce alternately 
sexually and then asexually. 
146. Sea-anemone. — Sea- 
anemones are animals allied to 
the hydra. The interior of the 
body cavity is subdivided by 
many partitions which increase 
the digesting and absorbing sur¬ 
face. The sea-anemone repro¬ 
duces by eggs and sperms. 
The resulting embryo is free at 
first, but later becomes fixed to 
some object and develops into 
the sea-anemone. There is no 
medusa stage. 
147. Coral. — Geographies tell 
us of the many coral islands and 
reefs built up by the coral ani¬ 
mals. These animals are coelente- 
a, The hydroid colony; b, one 
of the female medusae, much 
enlarged; c, the egg of the 
medusa beginning to segment 
after it has been fertilized; 
d, e, f, further segmentation 
stages; g, the blastula stage; 
h, the free swimming larva 
(planula) ; i, /, and k show the 
gradual transformation of the 
rates, most of them closely allied 
to the sea-anemone, but the coral 
animal secretes about the body 
and along the partitions calcareous 
(kal-ka're-us, limy) skeletons 
which form the stone-like masses 
of the coral rock. The upper 
portion of the coral rocks is alive 
larva into a hydra-like colony. 
Branches grow on the stage shown in k until a colony like a results. This 
is the form that alternation of generations takes in this hydroid. (Arranged 
from a monograph on Pennaria by C. W. Hargitt.) 
