Glass 3. Echinoidea. 



137 



Movable calcareous spines articulate with, the smooth prominences 

 of the numerous tubercles mentioned aboye, and are attached to the 

 shell by muscle fibres. In the Eegular Urchins 

 the spines are usually of considerable importance; 

 in some^ they are very long and thick, and serve 

 as locomotor organs, accessory to the tube-feet ; in 

 irregular forms, on the contrary, they remain small 

 and thin, or even bristle-like. All the spines in 

 the same individual, however, are by no means of 

 equal size ; those with the large kind also exhibit 

 smaller, or even quite tiny, ones. The spines are 

 straight, and rounded in section; but some are 

 curved and flattened. Like the shell, the spines 

 are calcifications of the body-wall, and like the 

 shell also, they are covered with a soft superficial 

 layer, which is, however, often worn away from 

 the tip.* Stalked or sessile pedicellarise are 

 also articulated with the shells (c/. p. 124). 



In the Eegular Sea-urchins the tube- 

 feet are usually all alike ; at the end of each is 

 a sucker, supported by a perforated calcareous 

 plate ; occasionally those on the dorsal surface are 

 pointed and compressed. In many Irregular 

 ever, several forms occur: 1. true tube-feet with suckers; 2. a 

 similar kind, but with rounded ends ; 3. a brush-like form, with 

 numerous threads at the ends, modified as tactile organs, and occur- 

 ring near the mouth; 4. dermal branchiae, i.e., leaf -like 

 appendages, notched at the edges, present on the dorsal surface. 



The mouth in the regular and in some of the irregular forms, 

 is armed with a circle of five very powerful calcareous teeth, 

 supported on a somewhat complicated framework of calcareous pieces, 

 the so-called "Aristotle's lantern"; inmost of the irregular 

 forms, however, there are no teeth. The alimentary canal is 

 a long, cylindrical, much-coiled tube, occupying the greater portion 

 of the shell cavity. The position of the anus has been already 

 described. 



In tlie Regiilar UroUns the lantern is again suiTounded and supported "by a 

 calcareous ring provided with five upwardly directed processes, and connected to 

 the lower rim. of the shell. In these foi-ms the masticatory apparatus occupies a 

 considerable part of the whole cavity. The so-called siphon, or accessory 

 intestine, is a very peculiar structure occurring in most Echinoids. It consists of a 

 fine canal, running parallel to the gut for part of its length, and opening into it 

 at either end; in some forms this canal is wanting, and instead there is a 

 groove on the inner side of the gut. It is conceivable that the accessory intestine 

 arose by the constriction of a groove such as this. 



Fig. 91. 

 Dlagrammat c 

 longitudinal section 

 of the spine of a 

 Sea-urcliin. s shell, 

 fc tubercle, v wart- 

 like process of the 

 same, jj spine, m 

 mnsclfs. — Orig. 



Urchins, how- 



* In very large spines the soft covering may be present only at the base. 



