202 EC^HTNODERMS OF TUK BRITISH ISLES 



The (jenital organs lie in a ciiclc round the apical system ; 

 they are large, bush-shaped organs, from each of whieh a narrow 

 efferent duet issues, opening through the pore in the genital plate. 

 The regular Echinoids have 5 genital organs (or gonads), which 

 coalesce in some forms ; in the irregular Echinoids the posterior 

 gonad has disappeared, and the number of the gonads (and genital 

 pores) may further be reduced to three or even two. 



The watervascular ring lies on the upper side of the dental 

 apparatus. Polian vesicles are represented by some small 

 bushy outgrowths from the ring canal ; only in the family of the 

 Echinothurids they are large sausage-shaped sacs. The radial 

 water-vessels and the ampullae of the tube-feet lie wholly inside 

 the test, along each ambulacral area ; the same is the case 

 also with the nervous system. Axial organ and stone canal pass 

 directly through the body cavity from the oesophagus to the 

 apical system. 



The sea-urchins move slowly, especially b}^ means of the spines 

 on the oral side of the test, on which they walk almost as upon 

 stilts. The tube-feet are more used for climbing, e.g. on rock 

 walls or the walls of aquaria. Several forms make holes in tlie 

 rocks, mainly by means of their teeth. The irregular forms 

 are mostly burrowing animals, which live buried, as much as a 

 couple of decimetres deep, in the ground. Many species cover 

 themselves with small stones, shells, pieces of alga^, etc., which 

 they hold tightly with their tube-feet, probably with the object 

 of concealing themselves both from their enemies and from too 

 strong light. Some forms have been observed to lie concealed 

 during the day under stones or the like, coming out to feed 

 at night. The regular forms feed largely on algae, but also 

 take all sorts of animal food. The irregular Echinoids eat all 

 sorts of small bottom organisms, especialty mussels, snails, Fora- 

 minif era, which they are able to pick out from the bottom material 

 by means of their penicillate tube -feet ; or they fill their alimen- 

 tary canal with all sorts of bottom material, absorbing the organic 

 particles contained therein. About the growth and age only very 

 little is known. As a rule they appear to reach sexual maturity 

 in the course of one to two years. Echinus esculentus has been 

 observed to reach full size in the course of four years. Evidence 

 has been brought forward by Grieg tending to show that Stylo- 

 cidaris affinis takes at least two years in reaching maturity, 

 while e.g. Phormosoma j^lc^centa w^oulcl seem to take four to five 

 years in reaching full size. The power of regeneration is great ; 

 not only spines and the other external organs are easily 



