(JENEBA AND GROUPS OF THE ECHINOIDEA. 295 



EXPLANATION OF TERMS. 



The following explanation of the anatomical terms employed in the classifi- 

 cation is intentionally brief, and further information regarding the anatomy 

 of the Echinoidea can be obtained in the Text-books of Zoology and Oompara- 

 tive Anatomy, in ' The Eeyision of the Echini ' and the Report on the ' Chal- 

 lenger ' Echini, by A. Agassiz, in Loven's ' Etudes sur les Echinoidees,' in 

 ' Pourtalesia,' and in 'The Echini described by Linnaeus,' by the same author, 

 in works by Perrier, Koehler and Ludwig, and by various authors in the Journal 

 of the Linnean Society, the Quarterly Journal of the Greological Society, and 

 the Quart. Journ. Micros. Sci., during the last decade. 



The Test. — The individual Echinoid — its covering of calcareous plates and 

 spines, and its internal and external soft structures and organs. The term, 

 when employed in the description of fossil forms, especially refers to the 

 denuded animal, and in the recent forms it is also thus applied ; but the 

 shape when the spines remain must be considered, as well as when denuded. 

 The plates of the test are numerous and are in certain regions or sys- 

 tems. The upper surface of a test is abaciinal or dorsal, it has the plates 

 of the dorso-central or afical system at the apex or elsewhere ; the under 

 or opposite surface is actinal, and, except in one family, the peristome 

 opens out there ; the eqtiatorial circumference is the amhitios or margin. 

 There are five ambulacral areas, composed of plates reaching in 

 vertical rows from the Eadial plates of the Borso-central or Apical system 

 over the ambitus to the peristome and sometimes beyond, to the true mouth ; 

 and five interradial areas, consisting of plates in rows placed between the 

 ambulacra and reaching from the Basal plates of the Apical system to the 

 peristome or further. In rare instances, the poster o -lateral interradia join 

 abactinally at the median line of the dorsum of the test, and a similar 

 union may occur actinally. 



The peristome and the periproct are at the opposite poles of the test in 

 some Orders of Echinoidea, and such are called Endocyclica or Begulares. 

 In other orders the periproct is beyond the Borso-central system and is 

 somewhei'e in the median line of the posterior interradiiim, either dor- 

 sally or actiually ; such forms are called Exocyclica or Irregulares. The 

 orientation of the Exocyclica is not difficult, the periproct indicating the 

 posterior region of the test ; furthermore, the madreporite is, in the majority 

 of cases, in the right anterior Basal plate of the Apical System and often 

 extends to the centre of the system or posteriorly and even more or less 

 beyond. The orientation of one of the Endocyclica is evident when the 

 apical system is preserved, and it may be taken that the madreporite is 

 mainly in the right antero-Iateral Basal plate. Then the Badial plate in 

 front of, or anterior to, and on the left hand of the madreporite-bearing 

 basal is the anterior Badial plate, and the ambulacrum associated with it 

 is the anterior one. In the event of the loss of the apical system, the 

 determination of the axes of the test may usually be settled by employing 

 Loven's method (Etudes, pp. xi et seq.). 



The margin of the peristome formed by ambulacral and interradial plates 

 has a membranous tissue connected with it and is more or less covered with 



