10 TERMINOLOGY. 



All the Echinoidea have the mouth situated at the under side of the body. The surface 

 in which this opening is placed is called the base ; that region of the test opposite to the 

 base is the upper or dorsal surface. 



The most convex part of the margin, border, or sides, between the base and the upper 

 surface, is called the circumference, — the ambitus of some authors. It is round, flat, 

 convex, angular, or carinated, according to the general form and thickness of the test. 



The length, or antero-posterior diameter, is the distance between the anterior and 

 posterior regions, and corresponds to the middle line of the body. 



The breadth, or transverse diameter, is the distance between the greatest lateral con- 

 vexity of the circumference, in the direction of a line which will cut the line of length at 

 right angles. 



The height is the distance between the most convex part of the upper surface and a 

 plane on which the base of the test can rest. The apical disc is generally situated at the 

 vertex, but it is not always so. The height has reference to the highest point of the test, 

 quite irrespective of any other consideration. 



The test has invariably two openings, the one for the mouth, the other for the anus. 



The mouth-opening is always situated at the under surface ; to its circumference is 

 attached the buccal membrane, and through its central aperture, when they exist, 

 protrude the five jaws. (PI. IV, fig. lb; PL XI, fig. 1 .) The buccal, like the anal mem- 

 brane in many families, is clothed with numerous small plates. In Cidaris, Rabdocidaris, 

 Goniocidaris, Diplocidaris, and probably in other Cidarida, the mouth-opening is central, 

 circular, or slightly pentagonal (PI. I, figs. 2 a, 4<a ; PI. II, figs. 1 a, 2 b) ; but in Hemi- 

 cidaris, Diadema, Hemipedina, Pedina, Echinus, and other Echinid^e, the mouth opening 

 is more or less decagonal, its margin being divided by notches {cntailles) into ten lobes. 

 (PI. Ill, figs. 2 b, 6 b; PI. IV, fig. 2 b, e ; PI. V, fig. 1 e; PI. VI, figs. 2 b, 5 c.) The lobes 

 are, in general, unequal in size ; those which correspond to the base of the ambulacra 

 are the largest, and are called the ambulacral lobes ; those corresponding to the base of the 

 inter-ambulacra are the inter -ambulacral lobes. The margin of the mouth-opening is called 

 the peristome ; to it the buccal membrane, which closes the base of the test, is attached. 



The mouth-opening is central, and is armed with jaws, in the Cidarid^e, Echinid^e, 

 Salenid,e, Galeritid,e, and Clypeasterid.e. It is more or less excentral and edentu- 

 lous in the Echinonid^e, CoLLYRrrmiE, Echinolampid^e, EcHiNocoRYDyE, and Spatan- 

 gid^e. In these families it is round, oval, or pentagonal ; sometimes its margin is ray- 

 like, or surrounded by five prominent lobes ; in others it is distinctly bilabiate. 



The anal opening is always in the upper surface, directly opposite to the mouth, 

 in the centre of the genital and ocular plates, and is either central or sub-central in the 

 Cidarid,e, Echinidte, and Salenice. (PL III, figs. 1 e, 3 c; PL IV, figs. \g, 2/.) In 

 the other families its position varies much ; sometimes it opens on the upper surface, as in 

 some GALERrriDiE and Cassiuuliu^e ; sometimes it opens near the margin, or is supra- 

 marginal, marginal, or infra-marginal ; often it opens at the base, between the mouth and 



