ECHINODERMA. 3 
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 AcrrnogonipIAL, such as have not, ANACTINOGONIDIAL. 
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. No trace of this area or system 
of plates is to be found in Holothurians, which, therefore, are 
Non-Caticunatrr, 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 Srarozoa and Exev- 
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.—Kchinoderms 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. 
B2 
