ECHINODEKMATA. 



97 



ORDER I. ECHISTOIDEA. The animals included in this order 

 vary from the shape of a sphere or globe to that of a disk, and 

 they are all commonly known as " sea-urchins " or " sea-eggs." 

 They are all characterized by the fact that the body is encased 

 in a " test" or "shell" (Fig. 31, 2) composed of numerous cal- 

 careous plates mostly immovably jointed together so as to form 

 a kind of box. The intestine is convoluted, and there is a 

 distinct vent, or anal aperture. 



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 (1 a, 2 a) the plates are of large size, and are per- 



FIG. 31. Morphology of Echinoidea. 1. Portion of the test of a sea-urchin (Galwites) 

 enlarged, showing the ambulacral areas (6) and interambulacral areas (a). 2. Test of 

 the same, viewed from above: a Interambulacra ; b Ambulacra. 8. Genital disk of a 

 sea-urchin (Hemicidaris) enlarged: c Ocular plate; d Genital plate : e Anal aperture : 

 /Madrcporiform tubercle. 4. Spine of the same. (After Forbes.) 



forated by no apertures. These are termed the "interambu- 

 lacral areas." In the other five zones (1 5, 2 t>) the plates are 

 of small size, and are perforated by little apertures for the 

 emission of delicate locomotive suctorial tubes (the so-called 

 "ambularcal tube-feet"). These zones are therefore called 

 the " ambulacral areas." Besides these main rows of plates 

 which collectively make up the greater part of the test, there 

 are other plates placed in the leathery skin round the mouth 

 and vent. The most important of these form a kind of disk, 

 which is placed at the summit of the shell. This disk (Fig. 

 31,3) is composed of two sets of plates one called the "geni- 



