114 MARINE INVERTEBRATES 



tinct divisions or chambers. The number of divisions varies 

 with the different species and also with the age of the animal. 

 Other partitions start from the outer sac, and extend toward the 

 central axis, but do not unite with it. These partitions, called 

 mesenteries, are always in definite multiples, varying in different 

 species, new divisions growing between the first partitions in 

 regular order. On the inner edge of these 

 partitions the eggs of the animal are 

 formed, which, when mature, drop into 

 the chambers and pass through openings 

 into the inner sac, or digestive cavity, and 

 out of the mouth into the water. 



The animals are classed according as the 

 eggs are formed on all or on special par- 

 titions, those being of the highest order 

 Diagram of radiate structure, where a limitation and constancy of func- 

 tion is maintained. The upper surface 



of the body has hollow tentacles, each one of which opens into 

 one of the chambers and extends outward. All parts of the 

 animal communicate, and whatever enters the mouth circulates 

 through the whole structure; and when assimilation is com- 

 pleted the residue returns by the same road and is expelled 

 through the mouth. This structure is common to all polyps; 

 but there are great differences in their texture, some being soft 

 and some horny, while others deposit a calcareous skeleton 

 (corals). Some grow in colonies, like the hydroids and corals, 

 and are stationary, others are free and independent ; some have 

 but few, others have many tentacles ; and they differ widely in 

 size, form, and color. 



Hydroids, sea-anemones, corals, sea-fans, starfishes, and sea- 

 urchins are different examples of the radiate structure. They are 

 not, however, all of them polyps. 



The Ccelenterata are divided into four classes : Hydrozoa, which 

 include the colonies of zoophytes which resemble seaweeds, the 

 small jellyfishes which are born of these colonies, and the mille- 

 pores, which are colonies of zoophytes which secrete a stony 

 instead of a horny skeleton, yet differ in some respects from other 



