270 



NATURAL HISTORY. 



remains of the food, and passing them on to those below. These, in their turn, close upon the 



particles, and pass them down the sides of the body until they can drop off into the water without 



becoming entangled among the tentacles and spines. 



Each of the narrow poriferous zones in the test of an Urchin is spoken of as " ambulacral," 



owing to its being pierced for the passage of the tentacles of the ambulacral system ; while the five 



broader zones which alter- 

 nate with them, and 

 bear larger tubercles, are 

 "interambulacral." Each 

 zone, whether ambulacral 



B iBf ^ A^W^^MSS^S&^^i^Sm^^^ or interambulacral, con- 



sists of a double series of 

 alternating plates, as is 

 well shown in Fig. 14, 

 A, B. All the zones con- 

 verge towards the summit 

 of the test, where, in 

 the regular Urchins 

 (Desmosticha*), the anal 

 opening is situated. It 

 occupies a more or less 

 excentric position within 



Fig. 14. GENERAL VIEW OF THE TEST OF AN URCHIN (Echinus microstoma), FROM a gp ace which is knOWn 



ABOVE. NATURAL SIZE. (After Wyuille Thomson.) 



jm T nf* nfM'iTyFOCij i 111 ci is 



A, Most of the spines have been removed. The madreporite is seen on the genital plate lying N.E. of the centre. ' 



B, Portions of the ambulacral and intcrambulncral zones in the test of Toxopneustesvariegatus. hirtnwd. (After w V, n ll v . nr -nflvtinllv fillpil 

 Agassiz ) c, Head of Pedicellaria with the valves open. (After Erdl.) D, Pedicellanu with the valves closed. WilOliy Ul pdl Uldaij . 



up by minute limestone 



plates. The periproct is separated from the apices of the ambulacral and interambulacral zones 

 by two rings of larger plates alternately arranged (Fig. 14, A). Those of the inner ring, which 

 terminate the interambulacral zones, are pierced by the ducts of the genital glands. One of them, 

 that occupying a N.E. position in the figure, is pierced by the water-pores, and thus represents the 

 madreporite which is at the upper extremity of the water-tube of the Starfish (Fig. 1, m). 

 The plates of the outer ring are pierced by the unpaired tentacles, which terminate the water-vascular 

 trunks, and represent the " ocular tentacles " at the ends of the Starfish arms. 



In the regular Urchins (Desmosticlia) the mouth and anus are at opposite poles of the vertical 

 axis of the shell ; but either one or both may be more or less excentric. In the Clypeastrida% (Cake- 

 urchins) the anus is near the margin of 

 the dorsal surface, while in Spatangus 

 (Heart-urchin or Sea-bun) and its allies 

 the anus is marginal, or even on the 

 under-surface of the test, in which the 

 mouth may also occupy a more or less 

 excentric position. 



In the Desmosticha and Clypeastrida 

 the mouth is provided with a very com- B A 



plicated masticating apparatus, which Fig. 15. A, INTERNAL VIEW OF THE TEST OF Echinus microstoma, 

 attains its highest development in the SHOWINO THE DE * TAL PYBAMID OR ? 3 ^?*? t %?" 



CENTRE OF THE RING OF AURICULA, NAT. SIZE ; B, THE DENTAL 



former group. It consists of twenty PYRAMID. (After Wyville Thomson.) 

 principal pieces arranged into a five- 

 sided conical mass, which was aptly compared by Aristotle to a lantern (Fig. 15, A, B). In the 

 centre of the whole are five teeth working in bony sockets, or pyramids, that are connected by 

 muscles with one another, with the interior of the test, and with the arched auriculae already 



* Greek, desmos, a band ; stickos, a row. 



J Latin, Clypeus, a shield ; Greek, aster, a star ; eidos, form. 



t Greek, peri, round about. 

 Greek, spatanyes. 



