22 ECHINOIDEA. 



2nd. The visceral cavity, containing the organs of digestion, respiration, circulation, 

 and generation, is formed entirely by the interior of the test. 



3rd. The external surface of the test is covered with spines, which are moveal^ly 

 articulated, with the tubercles seen on the surface. 



The normal position of the hody. — In describing the different parts of the test of the 

 Echinoidea, it is assumed that an urchin, the common purple heart-urchin, Sjjatangus 

 purpureus, Miiller,^ for example, is placed before the observer. The side with the single 

 ambulacrum lodged in the anteal sulcus, and the mouth in that third of the base, is the 

 anterior ret/ion. The side having the single inter-ambulacrum in the middle and the vent- 

 opening in the upper part of the border is the posterior region. The four other ambu- 

 lacra are disposed in pairs, and correspond to the right and left sides of the observer's 

 body ; there is, therefore, a right antero-lateral and a right postero-lateral, a left antero- 

 lateral and left postero-lateral, ambulacral area. The four other inter-ambulacra, besides 

 the single one in which the vent is situated, are likewise disposed in pairs, two of 

 these, with the single ambulacrum, forming the anterior part, the other pair, with the 

 pairs of ambulacra, the sides, and the single inter-ambulacrum the posterior part of 

 the test. 



All Echinoidea3 have the mouth situated at the under side of the body ; the surface in 

 which the opening is placed is the base, that region of the test opposite the base is the 

 up2)er or dorsal surface. 



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

 surface, is the circumference, or 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 

 convexity of the circumference in the direction of a line cutting the line of length at 

 right angles. 



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

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

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

 test, quite irrespective of any other consideration. The test has invariably two openings, 

 one for the mouth, the other for the vent. 



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

 attached the buccal membrane, and through the central aperture thereof protrudes the 

 five jaws (PL V, fig. 1 ; PI. VII, fig. 1) ; when they exist, the buccal, like the anal mem- 

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



In the Cidaris, Rabdocidaris, Goniocidaris, Diplocidaris, and probably in all other 



' Tlie common Chalk-urchin Miraster cor-anginum, Klein, -will answer equally well. 



