146 



METAZOAN PHYLA 



Anthozoan polyps are much firmer in texture than are those of 

 Hydrozoa and Scyphozoa, and the skin, though soft, is tough. Bands of 

 contractile fibers lie on the surface of the mesenteries and by their 

 contraction enable the animal to protect itself by drawing the body 

 down into a compact mass with the mouth and tentacles completely 

 hidden from view. In some cases cells exist among these contractile 

 fibers, but these are not considered to form a mesoderm (Sec. 183). The 

 upper surface of the polyp is covered with many hollow tentacles the 



^ cavities of which communicate 



'^-"" ■• ^ ''' "■ '' y -^■^ -.' with the enteron. These ten- 



tacles can be extended by water 

 being forced into them and 

 retracted by its withdrawal. 

 Meser?fery Nematocysts are found on the 

 tentacles and also on the acontia, 

 which are threadlike structures 

 attached to the base of the 

 mesenteries and capable of being 

 protruded through the stomo- 

 deum or through pores in the 

 wall of the body. Acontia are 

 believed to serve as weapons of 

 offense and defense, while the 



StomodeufV 



septan? 



^ Baser/ p/afe 



Fig. 62. — Diagram to illustrate the forma- 

 tion of coral by a coral polyp. {From Thomson, 



''Outlines of Zoology," after Pfurtscheller, hy the tcntaclcS are the food-SeCUring 



courtesy of D. Appleton & Company.) This , , q 



shows the formation of a basal plate and radial StrUCtUreS. bca anemoneS 



septa; it does not show the external wall or theca usually exist as single polyps, 



which rises gradually with the basal plate and ,11 i c 1 



radial septa as the coral is deposited. though groups may be formed 



by budding. Individuals may 

 attain a diameter of a foot or more. 



A coral animal, which is usually an anthozoan polyp, secretes lime 

 under the basal disc and around the side of the body, forming a cup. 

 The mesenteries extending inward from the outer wall of the body 

 are continued across the basal wall and tend to meet at the center. 

 Ridges of lime are secreted corresponding to these mesenteries on the 

 basal wall and, when the coral polyp is removed, indicate the plan of 

 their arrangement (Fig. 62). Solitary coral polyps exist which may be 

 several inches in diameter, but very frequently coral animals live in 

 large colonies. The colonial polyps average smaller than the solitary 

 ones, the smallest not exceeding }{q inch in breadth. In the case of 

 the sea fans and sea pens a very large colony of exceedingly minute 

 polyps builds a skeleton of characteristic shape which suggests the com- 

 mon name (Fig. 63). 



174. Color. — Hydroid colonies are generally whitish in color, though 

 they may show a slightly brown or yellow tint. Anthozoan polyps 



