142 LIFE BY THE SEASHORE. 



no mastication, and so we find that the lantern and its 

 supports have disappeared. The position of the anus at 

 the posterior end, instead of at the top of the shell as in 

 EC) Linus } is probably an adaptation to life in a burrow ; for 

 as the urchin's food to a large extent must come from 

 above, it is desirable that waste material should not be 

 deposited where it might mingle again with the food. 



What effect has this more or less sedentary life had upon 

 the ambulacral system ? In the first place it is obvious that 

 this has at least very largely lost its locomotor functions. 

 The feet have now no suckers; they are not, as in the 

 common urchin, arranged so as to make locomotion in every 

 direction possible, and indeed the shape of the test would 

 render this impossible in any case. The tube-feet now act 

 largely as tentacles, and also possess, as in the regular 

 urchins, some respiratory function. We have noticed that 

 they seem not to be continuous over the whole test, but 

 form a petaloid area on the dorsal surface, and a similar 

 but less well developed area about the mouth on the ventral 

 surface. Careful examination of the interior will, however, 

 show you that the radial canals are continuous internally, 

 and that the upper and lower petaloid areas are connected 

 by regions in which a few small scattered tube-feet occur. 

 In the dry shell on the dorsal surface, to the posterior side 

 of four pores which you will find near the upper end of 

 the groove, you will be able with the aid of a lens to 

 discover the madreporite, or rose-plate, which has remained 

 in its primitive position, while the anus has moved back- 

 wards. Thus we see that the ambulacral system is con- 

 structed on the same plan in Echinocardium as in Echinus ; 

 but in the former certain of the tube-feet have, as it were, 

 been accentuated, at the expense of others which are now 

 only very slightly developed. It is interesting to note 

 that the irregularity which manifests itself in the external 

 appearance of the urchin is also apparent internally in the 

 reproductive organs, of which there are now four only 

 instead of five. The four pores spoken of above are the 

 four genital pores (cf. the five of Echinus). 



This description of Echinocardium will not be found very 

 readily intelligible unless it is studied with the help of 

 actual specimens, but dried specimens at least are so extra- 



