Ig2 LOW K II INVERTEBllATES. 



respond to tlio five larger plates, which are perforated with .1 larger aperttire for 

 the escape of the generative products, and are called genital jilates. One of these 

 genital plates, lying in what is recognizable as the right anterior inter-ambulacruni, 

 is larger than the others, and has a porous, convex surface. This is the madre- 

 poric body, and communicates with the water-system. Within the circle formed 

 by the genital and ocular plates are a number of small plates, of which one, the anal, 

 is larger than the others. The anus lies slightly out of the centre, between the anal 

 plate and the posterior margin of the anal area. The space around the mouth 

 (jieristome) is usually strengthened for some distance by irregidar oral plates ; and 

 ten rounded plates, sujijjorting as many suckers, and perforated by their canals, are 

 placed in ]iairs close to the lip. 



Each of the double series of coronal ])lates presents a zigzag suture 

 in the middle line. Each ambulacral plate is subdivided by a greater 

 or less series of sutures into a number of smaller plates, which are per- 

 forated by the pores of the suckers, and are called pore-plates. Tliese 

 are the jiriniitive ambulacral plates, and in the Cidaridai do not coalesce 

 into larger ambulacral ])lates, but sim])ly enlarge. 



Scattered over the body, especially near the mouth, are the pronged 

 pedicellariw ; and on most living echini, Ciilaris excejited, small button- 

 like bodies called s]ih;eridia are found. These are situated upon a short 

 stalk, and are thought to be possibly organs of taste. The sj^ines or 

 other appendages of the test are mounted upon tubercles, the size of 



which is proportioned to that of the sitines, so that the emiitv and 

 Fig. 141. — Pecli- ' ' . ' . ' • 



ceiiaria o£ se.v deiuided test of a sea-urchin, covered with pores .-ind tul)ercles, tells 



much resjiecting the affinities of its former habitant. 



In a large portion of the class, the regular urchins and the cake-urchins, the mouth 

 is furnished with five pyramids or jaws moved by powerful muscles, and in the regular 

 sea-urchins each jiyraniid is composed of eight pieces, making a total of forty pieces. 

 The digestive canal consists of a narrow gullet, a stomach of considerable length, 

 passing from left to right around the interior of the body, and then ttirning up and 

 curving back in the opposite direction ; and of a terminal intestine. The stomach 

 forms two series of loops, partly enclosing the ovaries, and is held in ]>lace by a broad, 

 thin membrane, the mesentery. The sexes are distinct, and there ai-e five ovaries or 

 spermaries, opening outward by the openings in the genital ]ilates. The single madre- 

 poric canal extends from the madreporic j)late to the circular vessel around the 

 mouth. 



The m.ajority of the Echinoidea undergo a metamorphosis, the early stages of which 

 are similar to those of the star-fish. The embryo sea-urchin is called a pluteus, and is 

 furnished with eight long arms supported by slender calcareous rods. These ai-ms 

 and rods are the locomotive apparatus of the young animal, which ])rogresses by open- 

 ing and closing them like an umbrella. The bod}' is also provided with a curiously- 

 curved band of vibratile cilia. Anything more unlike a sea-urchin than one of these 

 plutei, with its complex sprawling array of arms and bilaterally symmetrical, but not 

 radiated body, cannot well be imagined. 



In Stron(/i/locentrotns drobachiensis, the common urchin of the Atlantic coast, the 

 rudiments of the first tentacles apjiear in twenty-three days, by which time the pluteus 

 has acquired its complete external form, but the shajte of the larval digestive cavity is 

 concealed by the growing sea-urchin within. The body of the pluteus is gradually 



