144 



METAZOAN PHYLA 



dilation and contraction which forces water out through a central opening 

 in it with force sufficient to drive the animal through the water. Though 

 a system of radial canals is developed in hydrozoan jellyfishes, these 

 canals remain, generally speaking, few in number and unbranched. 

 The hydrozoan jellyfishes are relatively small, most of them being less 

 than an inch in diameter and the giants among them reaching a diameter of 

 only 15 inches. They have a marginal row of tentacles and no tentacles 

 around the mouth or, at most, a limited number. 



172. Scyphozoa. — The jellyfishes of this class (Fig. 60) are very 

 large as compared to the hydrozoan jellyfishes. There are records of 



individuals 12 feet in diameter anfl 

 possessing tentacles 100 feet in 

 length. Bulky as such individuals 

 are, they consist almost entirely of 

 water and when dried form only a 

 thin film. There are in some cases 

 scattered ameboid cells in the 

 mesoglea but these are not con- 

 sidered as forming a third layer. 

 These jellyfishes differ from the 

 hydrozoan jellyfishes in not hav- 

 ing a velum; in having a com- 

 plexly branched system of radial 

 canals; in the fact that the mar- 

 gin of the bell is divided into 

 sections by notches, in each of 



A B Bd 



Fm. 59. — Colonics of colonial marine 

 hydroids. A, Pennaria sp. X about 3. 

 Medusa buds are shown attached to the sides 



of the polyps from which they have been which is a pair of marginal lappets; 



developed B Sertularia^ sp X 2. Ba ^^^ ^^ ^^ ■ ^^ ^ 



portion of a branch showmg three pairs of ' . 



polyps retracted into the sessile hydrothecae. dance of fringed tentacles SUr- 



^ '^^' rounding the mouth. Many of 



these forms exist only as jellyfishes, generation after generation, but in 

 some this type alternates with a modified type of polyp. 



173. Anthozoa. — Among the Anthozoa are the sea anemones, which 

 are polyps in which there extends downward from the margin of the 

 mouth into the enteron a tubular membrane forming a stomodeum, or 

 gullet (Pig. 61). The stomodeum, in turn, is fastened to the body wall 

 by radially arranged membranes called mesenteries. These divide the 

 enteron into a number of chambers which may be entered from below. 

 Between these mesenteries are shorter ones running inward from the 

 body wall and not meeting the gullet; thus recesses are produced on the 

 outer wall of the chambers. Since these incomplete mesenteries may 

 vary in length, they produce recesses of several degrees of depth and of 

 varying breadth. Openings in the upper part of a mesentery, putting 

 two chambers into communication, are called ostia. 



