CHAPTER XVI 



ECHINODERMATA INTRODUCTION CLASSIFICATION ANATOMY 



OF A STAKFISH SYSTEMATIC ACCOUNT OF ASTEROIDEA 



The name Echinodermata ^ means literally " spiny -skinned," 

 and thus brings into prominence one very conspicuous feature 

 of most of the animals belonging to this phylum. All, it is true, 

 do not possess spines ; but with one or two doubtful exceptions, 

 all have calcareous plates embedded in the skin, and these plates, 

 in many cases, push out projections which raise the skin into 

 corresponding elevations, which are called the spines. The 

 spines are, like tlie other plates, inside the skin, and to s})euk of 

 an Echinoderm living in its shell, as we speak of a Snail, is a 

 serious error. The shell of a Mollusc is fundamentally a 

 secretion poured forth from the skin, and is thus entirely 

 external to the real living parts ; but the plates and spines of 

 an Echinoderm may be compared to our own bones, which are 

 embedded deeply in tlie flesh. Hence the name ossicle (little 

 bone) is used to designate these organs. 



Besides the possession of these spines, Echinoderms are 

 characterised by having their organisation -pervaded by a 

 fundamental radial symmetry. The principal organs of the 

 body are repeated and are arranged like the spokes of a wheel 

 round a central axis instead of being, as, for example, in 

 Chaetopoda, arranged behind one another in longitudinal series. 



In addition to these striking peculiarities, Echinoderms 

 possess a most interesting internal organisation, being in this 

 respect almost exactly intermediate between the Coelenterata, 



^ The name seems first to have been used by Klein in 1734, "Naturalis 

 dispositio Echinodermatum " (Danzig). Leuckart about 1850 first established 

 Ecliinodermata as a primary division of the animal kingdom. 



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