ECHINODERMATA. TOS 
This is by no means a true arrangement of these orders, but 
it is convenient to consider them in this sequence. 
ORDER I: EcuinorpEa.—The animals included in this order 
vary from the shape of a sphere or globe to that of a disc, and 
they are all commonly known as “sea-urchins” or “ sea-eggs.” 
They are all characterised by the fact that the body is encased 
in a “test” or “shell” (fig. 41, 2) composed of numerous cal- 
careous plates immovably jointed together so as to form a kind 
of box. The intestine is convoluted, and there is a distinct vent, 
or anal aperture. 
Fig. 41.—Morphology of Echinoidea. 1. Portion of the test of a sea-urchin (Galer- 
ites), enlarged, showing the ambulacral areas (4) and interambulacral areas (¢). 
2. Test of the same, viewed from above: @ Interambulacra ; 6 Ambulacra. 3- 
Genital disc of a sea-urchin (Hemicidaris) enlarged: c Ocular plate; ¢ Genital 
plate ; ¢ Anal apertute ; f Madreporiform tubercle. 4. Spine of thesame. (After 
Forbes.) 
The test of a sea-urchin, as just said, consists of many cal- 
careous plates accurately fitted together, and united by their 
edges. In all living forms the test is composed of ten zones 
of plates, each zone consisting of a double row. In five of 
these zones (I @, 2 a) the plates are of large size, and are per- 
forated by no apertures. These are termed the “ interambu- 
lacral areas.” In the other five zones (1 4, 2 4) the plates are 
of small size, and are perforated by little apertures for the 
emission of delicate locomotive suctorial tubes (the so-called 
