162 LOWER INVERTEBRATES. 



respond to the five larger plates, which arc perforated with a larger aperture for 

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

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

 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 

 (peristome) is usually strengthened for some distance by irregular oral plates ; and 

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

 placed in pairs close to the lij). 



Each of the double series of coronal plates 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-])lates. These 

 are the primitive ambulacral plates, and in the Cidarida3 do not coalesce 

 into larger ambulacral plates, but simjjly enlarge. 



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

 jiediccllarije ; and on most living echini, Cidans excepted, small button- 

 like bodies called s]ihaBridia are found. These are situated upon a short 

 stalk, and are thought to be possibly organs of taste. The spines or 

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

 which is proportioned to that of the spines, so that the empty and 

 ceiiariaof sea- denuded test of a sea-urchin, covered with pores and tubercles, tells 

 much respecting the affinities, of its former habitant. 

 In a large joortion of the class, tlic regular urchins and the cake-urchips, the mouth 

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

 sea-urchins each pyramid 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 bod}-, and then turning up and 

 cui"ving 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 place by a broad, 

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

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

 poric canal extends from the madreporic plate to the circular vessel around the 

 mouth. 



The majority 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 arms 

 and rods are the locomotive apparatus of the young animal, which progresses liy open- 

 ing and closing them like an umbrella. The body 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 sjrmmetrical, but not 

 radiated body, cannot well be imagined. 



In Strongylocentrotus drobachiensis, the common urchin of the Atlantic coast, the 

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

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

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



