ECHINODERMATA. 17 



relation of the organs of digestion, generation, and vision, to each other, imparts great 

 value to the position of the anal opening; and hence it forms the basis of the subdivision 

 of this order into two sections, which are thus defined : 



Echinoidea endocydica. 



A. Test circular, spheroidal, more or less depressed, rarely oblong ; mouth in the centre 



of the base. Anus in the centre of the upper surface, directly opposite to the 

 mouth, surrounded by the five perforated genital plates, and having external to 

 them the five ocular plates. Mouth always armed with five powerful calcareous 

 jaws, formed of many elements, disposed in a vertical direction. 



Echinoidea exocyclica. 



B. Test sometimes circular and hemispherical ; oftener oblong, pentagonal, depressed, 



clypeiform or discoidal ; mouth central or ex-central. Anus external to the circle 

 of genital and ocular plates, never opposite the mouth, but situated in different 

 positions in relation to that opening : four of the genital plates are generally 

 perforated. The mouth is sometimes armed with five jaws, but it is oftener 

 edentulous. The elements of the lantern are disposed in a more or less horizontal 

 direction. 



The structure of the ambulacral areas, and the poriferous zones, which are in relation 

 with the organs of respiration and locomotion, afford good characters of secondary im- 

 portance, for grouping the genera into natural families, especially when taken in connection 

 with the position of the anal opening, which varies in its relation to that of the mouth 

 opening in difierent families. 



The form, number, and arrangement of the tubercles, and the spines which are jointed 

 with them ; the miliary granulation ; the bands of microscopic granules forming the fas- 

 cioles, which have determinate and permanent positions on the test ; added to the size and 

 number of the elements of the apical disc, and the position of the anus, afford collectively 

 good characters for defining the genera. 



The minute details in the structure of the plates ; the form, and size, and number of 

 the tubercles on each of them, — the form and arrangement of the pores in the zones ; their 

 proximity or remoteness from each other ; the general outline of the test, which has only 

 certain limits of variation ; the character of the sculpture on the plates ; the form of the 

 areolas; the presence or absence, the size and distribution of the granules forming the areolar 

 or scrobicular circle ; the completeness or incompleteness of the same. The length of the 

 spines, their form and sculpture, are all points which afford good specific characters, as 

 they are persistent details which are developed on every considerable fragment of the test 

 and spines of Echinoidea. 



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