SEA-URCHINS. 



165 



Fig. 144. —Aristotle's lau- 

 tei'n of Arbacia. 



All these regular sea-uichiiis, as they are commonly called, have a highly complex 

 mouth apparatus. Each of the five pyramids consists of a hoUovir, wedge-shaped 

 alveolus, composed of four pieces, or rather of Wo lateral halves, each formed of a 

 superior and inferior portion ; and of a long, slender tooth, shaped somewhat like the 

 incisor of a rat or other rodent. The five alveoli, with their teeth, form a cone, and 

 the parts are united together by strong transverse muscular fibres and also by long 

 pieces applied radially to their upper edges. These radial rods are the rotulse. To 

 the inner end of each rotula is articulated a slender arcuated 

 rod, with a free forked, extremity. These are the radii. Thus 

 the entire apparatus, usually called ' Aristotle's lantern,' consists 

 of twenty principal parts, five teeth, five alveoli, five radii, and 

 five rotulfe, and as each alveolus consists of four, and each radius 

 of two parts, the total number of pieces is forty. When in 

 position, the alveoli and the teeth face the interambulacra, the 

 radii and rotulse the ambulacra. For the attachment of this 

 dentary apparatus to the test, the coronal plates of the margin 

 of the mouth are produced into five perpendicular perforated 

 processes, called the auricles. These usually arch over the am- 

 bulacra. From the auricles and interambulacral spaces at the 

 margin of the mouth arise a complex system of muscles, pro- 

 tractor, retractor, and oblique, inserted into various parts of the mouth apparatus, at 

 once attaching it firmly and regulating the movements of the parts. 



There is great variation in the size, shape, and surface of the spines of the Desmos- 

 ticha, but they are nevei- so delicate and silky as in the other orders of ecliinoids. 

 Usually certain large spines which form a continuous series from one end of an inter- 

 ambulacrum or ambulacrum to the other may be distinguished as primary spines, while 

 the smaller spines forming less complete series are known as secondary or tertiary. 

 The tubercles to which these spines are movably attached are in some cases marked 

 by a central pit, into which, and into a corresponding pit on the head of the spine, a 

 ligament of attachment is inserted. 



The radial ambulacral vessels reach the ambulacra from the circular canal around 

 the mouth by passing beneath the rotulse and through the arches of the auricles. 

 There are large ambulacral vesicles at the bases of the suckers, which are usually ex- 

 panded into a sucking disc at their tips, where they are strengthened by a calcareous 

 plate ; but in some genera the pedicels of the apical part of the test are flattened, 

 pectinated, and gill-like. 



The family Cidabid^ has a large number of small plates in the ambulacral areas, 

 and the pores are arranged in single pairs. The interambulacral regions are very 

 wide, with only a small number of tubercles, each of which is large and perforated, 

 and bears a massive solid spine. There arc no secondary spines, but the entire surface 

 of the test between the primary spines is filled in with small papillae, which extend 

 also over the oral membrane. The areas occupied by the oral and anal systems are 

 larger than in other regular sea-urchins ; the jaws are less complicated than in the 

 Echinidee and DiadematidaB, the teeth are gauge-shaped, and the auricles are inserted 

 in the interambulacral instead of the ambulacral areas. Processes developed from the 

 ambulacral plates form a sort of wall on each side of the ambulacral canal. The 

 ambulacral plates are continued on the peristome to the margin of the mouth, where 

 their edges overlap, producing a structure somewhat like that of the entire test of the 



