120 MARINE INVERTEBRATES 



open end and a crown of tentacles are the nutritive individuals. 

 Small, club-like dilations are immature zooids. The blastostyles, 

 or reproductive zooids, are long, cylindrical, mouthless, and cov- 

 ered. At maturity the cover is ruptured, and the medusae 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 medusae, or jellyfishes, the convex side of each saucer, or 

 swimming-bell, is called the ex-umbrella ; 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 planulm. 

 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 zoophyte stage beginning in the 

 autumn and the medusa stage in the spring. 



Some medusae, besides reproducing by means of eggs, multiply 

 by budding, small medusae 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 typo. In the sertularians 

 the zooids perish on the stem and have no medusa life, their 

 reproductive element giving rise to the hydroid form without 

 metamorphosis. The Trachylince, have no hydroid life, being 

 always free-swimming medusas ; others, the Siphonopkora, 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. 



