146 



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. 



174. Scyphozoa. — The jellyfishes of this class (Fig. 64) are very 

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



individuals 7J4 feet in diameter 

 and possessing tentacles over 100 

 feet in length when fully extended. 

 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 sec- 



B 



colonial marine 

 sp. X about 3. 



A 



Fig. od. Colonies of 

 hydroids. A, Pennaria 



Medusa buds are shown attached to the sides 

 of the polyps from which they have been tions by notcheS, in each of which 

 developed B Sertularia sp X 2. 5a ^g ^ j^. ^f ^^ -j^^^ j^^ ^^ . ^^^ 

 portion of a branch showing three pairs of . . ft- j > 



polyps retracted into the sessile hydro thecae. in many cases, in the abundance 



^ ^^' of fringed tentacles surrounding 



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. 



175. 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 (Fig. i65). The stomodeum, in turn, is fastened to the body wall 



by radially arranged membranes called mesefiteries. 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. 



