ECH1N0DERMA. 6 



exhibited by the generative organs as well as by the systems just 

 mentioned ; but in the Holothurians it is not so. In them the 

 reproductive organ (gonad) is either single and on the left side of 

 the oro-anal axis of the body, or double and right and left of it. 



I have suggested that this distinction between the Holothurians 

 and all other known Echinoderms should be marked by the use of 

 distinctive terms. Such as have a radial disposition of the gonads 

 may be called Actixogoxidial, such as have not, Axactinogoxidial. 



Calyx. — This is not the only point in which the Holothurians 

 differ from all other Echinoderms ; the forms best known to us, such as 

 the Starfish and the Sea-Urchin, are characterized by the possession 

 of a continuous skeleton, whereas in most Holothurians the skeletal 

 structures are nothing more than scattered spicules, and even 

 where plates are formed the skeleton is not continuous over the 

 whole of the body. Of the parts of which this skeleton is com- 

 posed there is one set of plates to which great, but probably not 

 undue, attention has been directed. In the middle of the surface 

 of the body opposite to that in which the mouth is placed there is, 

 typically, developed a central plate, around which appear two or 

 more circlets of plates, set radially or interradially *. These com- 

 bined form what is known as the calyx (cup) or calycinal area. 

 It undergoes very considerable changes, modifications, or reductions, 

 but it seems to have had a very profound influence on the organi- 

 zation of most Echinoderms. ISTo trace of this area or system 

 of plates is to be found in Holothurians, which, therefore, are 

 Non-Caliculate, as compared with all other Echinoderms, which 

 are Caliculate. 



Divisions of Caliculata. — The caliculate, actinogonidial Echino- 

 derma break up into two divisions : some are capable of continued 

 movement and have organs which may rightly be called locomotor ; 

 others are permanently or temporarily fixed by means of a stalk 

 which is developed on the aboral surface of the body. These fixed 

 and free forms may be known respectively as Statozoa and Eleu- 



THEROZOA. 



The Statozoa were much more abundant in earlier stages of the 

 world's history than they are now, when only one division, that of 

 Crinoids, remains, and is, in our seas, represented by only one genus 

 and but few species. 



The great majority of the forms to be described in the succeeding 

 pages are eleutherozoic — that is, are Sea-Urchins, Heart-Urchins, 

 Starfishes, or Brittle-stars. 



Habits and Mode of Life of Echinoderms. — Echinoderms are, 

 almost without exception, confined to the sea, being rarely found 

 in brackish water. They are known from all depths, are common 



* The nomenclature of these plates has been, apparently, the sport of 

 writers on Echinoderm Morphology ; there is no need to burden the memory 

 of the student with them, for the obviously best course is to speak of them 

 quite objectively as first radial circlet, second radial circlet, first interradial 

 circlet, and so on. 



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