CHAPTER V 



TYPE III. ECHINODERMATA 



The Echinodermata (Gr. i%lvo<;, hedgehog, sea urchin, and 

 Bep/xa, skin) include those animals commonly known as sea 

 lilies or stone lilies, starfishes, brittle or serpent stars, basket 

 fishes, sea urchins, sand dollars, and sea cucumbers. Their 

 position in the system of classification is very doubtful. During 

 the earlier part of the nineteenth century they were placed 

 under one type with the Ccelentcrata, and this type was called 

 the Radiata, because the animals composing it exhibited in most 

 cases a radial symmetry. Later, when the anatomy of this 

 group was better known, it became evident that the Echinoder- 

 mata and the Ccelenterata possessed very few characteristics in 

 common, and the further study of echinoderm embryology has 

 led to the present tendency to place this type much higher in 

 the scale of animal life, in fact only a little below some of the 

 groups closely related to the Vertebrata, or backboned animals. 

 On the other hand, since the position of these animals is still 

 very doubtful, it appears quite as well to take them up at this 

 point as at some other. 



The Echinodermata are exclusively marine animals, and, like 

 all the types, exhibit a verv great diversity of external form. In 

 general, however, they have a more or less pronounced, super- 

 ficial, radial symmetry, though in most cases there is bilateral 

 symmetry as well. The parts of the body are usually arranged 

 on a plan of five, so that we can distinguish five radii and the 

 same number of spaces between them, — the interradii; these 

 parts are very apparent in such animals as the starfishes, and 

 are present, though somewhat obscured at times, in all. Along 

 each radius are usually two or more rows of soft tubelike pro- 

 cesses, terminating in sucking discs ; these are the tube feet or 



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