120 MARINE INVERTEBRATES 
open end and a crown of tentacles are the nutritive individuals. 
Small, club-like dilations are immature zodids. The blastostyles, 
or reproductive zodids, are long, cylindrical, mouthless, and cov- 
ered. At maturity the cover is ruptured, and the meduse have 
the appearance of a pile of thin saucers attached by the middle 
of the convex side. When at length these saucers are set free 
as little medusa, or jellyfishes, the convex side of each saucer, or 
swimming-bell, is called the ex-wmbrella ; the concave, under side, 
the subumbrella. From the center of the subumbrella projects 
the manubrium, or stomach of the animal. At the free end of the 
manubrium is a four-cornered mouth. From the attached end 
of the manubrium four tubes or canals diverge, and, extend- 
ing through the animal, open into a circular canal which runs 
around the margin of the umbrella. When the medusa is as 
above described, it has reached the highest point in its 
development. 
When the medusa has matured, it lays eggs, known as planule. 
These are spherical bodies covered with cilia (hairs), by means of 
which they swim about for a time; but they finally attach them- 
selves to some object, there to grow and develop into hydroid 
colonies. The cycle of life is thus completed. This process is 
known as alternation of generation, or metagenesis, one life-history 
containing two quite different forms of being. The term of life 
of an individual is one year, the zodphyte stage beginning in the 
autumn and the medusa stage in the spring. 
Some meduse, besides reproducing by means of eggs, multiply 
by budding, small meduse growing on the manubrium or on the 
margin of the umbrella. Sarsia and Lizzia sometimes increase 
by budding. 
The Hydrozoa are not all of the above type. In the sertularians 
the zodids perish on the stem and have no medusa life, their 
reproductive element giving rise to the hydroid’*form without 
metamorphosis. The Zrachyline have no hydroid life, being 
always free-swimming meduse; others, the Siphonophora, live a 
hydroid life which is unattached, the colony floating on the ocean ; 
the millepores secrete calcareous skeletons and always remain 
fixed, reproducing by budding. 
