148 



METAZOAN PHYLA 



■Mesentery 



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. 185). The 

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



cavities of which communicate 

 1 /''v^ _..</ with the enteron. 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 stomodeum or 

 through pores in the wall of 

 the body. Acontia are believed 

 to serve as weapons of offense 

 and defense, while the tentacles 

 are the food-securing structures. 

 '^Basof/ p/afe gg^ anemones usually exist as 



Fig. 66.-Diagram to illustrate the forma- • ^ polyps, though grOUpS 

 tion of coral by a coral polyp. {From Thomson, '-^ & i J i^ ; & & i 



''Outlines of Zoology," after Pfurtscheller, by the may be formed by buddlUg. 



courtesy of D. Appieton & Company.) This Individuals may attain a diam- 



shows the formation of a basal plate and radial •' 



septa; it does not show the external wall or theca etcr of a foot Or more. 



which rises gradually with the basal plate and ^ ^^^^j animal, which is 



radial septa as the coral is deposited. 



usually an anthozoan polyp, 

 secretes lime under the basal disc and around the side of the body, form- 

 ing 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 hme are secreted alternating with these mesenteries on the 

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

 their arrangement (Fig. 66). Sohtary coral polyps exist which may be 

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

 large colonies. The colonial polyps average smaller than the sohtary 

 ones, the smallest not exceeding Ke 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. 67). 



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

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

 are often very brightly colored. Many jelly fishes are perfectly transparent 



