452 CHARACTERS OF INVERTEBRATE ANIMALS 



The most conspicuous feature of the upper surface of the 

 disc is found in the presence of a rounded plate in one of the 

 interradial areas. The meandering grooves which mark its sur- 

 face look like the markings upon certain corals (madrepores), 

 and have suggested the name of madreporic plate or madre- 

 porite. As there is only one of these structures, the radial 

 symmetry of the body is interfered with, and, strictly speaking, 

 it is bilaterally symmetrical, the median plane passing through 

 the plate and along one arm, which is generally considered to 

 be the front or anterior one. So that the Star- Fish really presents 

 bilateral symmetry masked by radial symmetry, so to speak. The 

 minute aperture of the intestine is placed near the central point 

 of the upper surface, but is not absolutely in the centre. 



The Star- Fish is firm to the touch, owing to the presence of 

 a well-developed system of hard calcareous plates, present in 

 the deeper part of the skin, and which are best seen in a dried 

 specimen. Those which support and strengthen the upper part 

 of the body are united together into an irregular net-work, but 

 in the neighbourhood of the ambulacra they are regular in shape 

 and have a definite arrangement. Of these plates the most con- 

 spicuous are a double series of ambulacral ossicles united together 

 so as to look, in the cross section of an arm, like an inverted V. 

 The edges of the ambulacra are fringed with spines, and it is 

 the presence of these in typical Echinoderms which has given 

 the name to the phylum (Gk. echinos, a hedgehog; derma, skin). 

 Spines, however, are much more numerous and conspicuous in 

 the members of some of the other sorts of Echinoderm, notably 

 in the Sea- Urchins, which is expressed in the name for " urchin ", 

 an old English word for a hedgehog. Among these spines, 

 especially in the neighbourhood of the mouth, are some which 

 have been peculiarly modified to form snapping jaw-like structures, 

 known as pedicellarice. One use of these is probably to assist in 

 cleaning the surface of the animal. 



Internal Structure (fig. 278). Several points of interest are 

 presented by the digestive organs. The mouth leads into a large 

 stomach with folded walls, capable of being protruded outside of 

 the body so as to engulf prey of comparatively large size. Star- 

 fishes are extremely rapacious animals, and are among the worst 

 enemies of the oyster. The stomach, after protrusion, is drawn 

 back into the body by special muscles. It is continued into a 



