4 
CRETACEOUS ECIIINODERMATA 
clistinctions ; while the perforation of the tubercles is regarded to have a generic 
value, the distribution and size of the same, whether of equal or of different 
size on the various meridional zones of the test, etc., forms a sectional or family 
character. 
The apex, or apical apparatus, forms the centre, from wdiich the poriferous zones 
radiate ; it lies centrally or excentric in the upper surface, sometimes it is length- 
ened to a considerable extent, (as in EchitwcorysJ . This apical apparatus is 
composed of five, four, or three, perforated genital plates, rvliich externally alternate 
wdth five ocular plates. The right anterior genital plate is generally groAvn together 
wfith the madreporiform plate ; it is rarely firmly attached to any of the other 
genital or ocular plates. Accessory plates between the genital occur very rarely, as, 
for instance, in the In the centre of the genital plates (Cidahidje), ov 
only partially surrounded by them (some SalenidjeJ, lies the anus ; it is, like the 
apex, central, or entirely cxcentric, superiorly or inferiorly suh-tcrminal, or quite 
posterior, sometimes situated in a special furrow. The composition of the apical 
apparatus and the position of the anus are usually regarded as constant family 
characters, and when the latter is cxcentric, its situation very often forms a good 
generic character. Scarcely of less importance in point of classification is the 
position of tlie aperture, or the oral opening. It is almost always on the lower, 
flatter side, central or anteriorly excentric ; in the former case with a simple or an 
incised, round or decagonal peristome ; in the latter it is also round, pentagonal, or 
somewhat irregularly angular, or transversely ovate and bilabiate. 
Both the anal and oral openings, although often for brevity’s sake called anus and 
mouth, have, strictly speaking, not this signification. In the fresh animal they are 
covered with a soft or scalcy or scutigerous buccal membrane, and in this lies the 
true mouth or anus. In some cases this buccal membrane is supplied with special 
strong muscles, and eonsists of several epithelial and pigment layers ; in many 
CiDAiiiDjE the amhulacral pores extend on it. 
I have already noticed that the entire test, ineluding the spines or radioles, is 
covered by the epidermis, and that in consequence of this, its structure is intimately 
connected with the inner organisation of the animal. The only internal organ, 
however, which occasionally remains preserved in a fossil state, is the masticatory 
apparatus, although it is present only in a portion of the Eehinoids. It is best 
known under the name of Aristotle’s lanterne, a conical highly complicated appara- 
tus of solid plates and muscles and membranes. It is composed of five separate 
pyramids, each consisting of two clongately triangular, internally transversely 
striated, externally at the edge flattened, calcareous lamellm, enclosing a long tooth 
between these flattened edges, at the base wdth a few other shorter solid pieces, 
which serve for the attachment of muscles, and give, so to say, a steadiness to the 
operation of the whole apparatus. The ends of the five teeth pi'oti’ude a little from 
the mouth, which leads through an cesophagus to a short alimentary canal, generally 
with a thickened portion distinguished as stomach ; the alimentary canal has two 
( T4 ) 
